ABSTRACT: 214 words
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Martin J. Pickering
University of Edinburgh
Department of Psychology
7 George Square
Edinburgh EH8 9JZ
United Kingdom
Email: Martin.Pickering@ed.ac.uk
http://www.psy.ed.ac.uk/Staff/academics.html#PickeringMartin

Simon Garrod
University of Glasgow
Department of Psychology
58 Hillhead Street
Glasgow G12 8QT
United Kingdom
Email: simon@psy.gla.ac.uk
http://staff.psy.gla.ac.uk/~simon
 
 

Traditional mechanistic accounts of language processing derive almost entirely from the study of monologue. By contrast we propose a mechanistic account of dialogue. The account assumes that, in dialogue, production and comprehension become tightly coupled and this coupling leads to the automatic alignment of linguistic representations at many levels. The interactive alignment process greatly simplifies production and comprehension in dialogue. It supports a simple interactive inference mechanism that enables dialogue participants to keep track of their common ground. It enables the development of local dialogue routines that greatly simplify language processing. Finally, it explains the origins of self-monitoring in production.

Traditional mechanistic accounts of language processing derive almost entirely from the study of monologue. Yet, the most natural and basic form of language use is dialogue. As a result, these accounts offer limited and inadequate theories of the mechanisms that underlie language processing in general. We propose a mechanistic account of dialogue and use it to derive a number of predictions about basic language processes. The account assumes that, in dialogue, production and comprehension become tightly coupled and this coupling leads to the automatic alignment of linguistic representations at many levels. This interactive alignment process greatly simplifies production and comprehension in dialogue as compared to monologue. After considering the evidence for the interactive alignment model, we concentrate on three aspects of processing that follow from it. First, it supports a simple interactive inference mechanism that enables dialogue participants to keep track of their common ground without having to explicitly model their partner’s beliefs. Second, it enables the development of local dialogue routines that greatly simplify language processing. Third, the alignment model makes interesting predictions about the nature of self-monitoring during production. Finally, we consider the need for a grammatical framework that can account for language in dialogue which is sufficiently flexible to deal with language fragments yet precise enough to support a coherent semantics.

Keywords: dialogue, language processing, common ground,  routines, language production,  monitoring.
 
 

Psycholinguistics aims to describe the psychological processes underlying language use. The most natural and basic form of language use is dialogue: Every language user, including young children and illiterate adults, can hold a conversation, yet reading, writing, preparing speeches and even listening to speeches are far from universal skills. Therefore a central goal of psycholinguistics should be to provide an account of the basic processing mechanisms that are employed during natural dialogue.

Currently, there is no such account. Existing mechanistic accounts are concerned with the comprehension and production of isolated words or sentences, or with the processing of texts in situations where no interaction is possible, such as in reading. In other words, they rely almost entirely on monologue. Thus, theories of basic mechanisms depend on the study of a derivative form of language processing. We argue that such theories are limited and inadequate accounts of the general mechanisms that underlie processing. In contrast, this paper outlines a mechanistic theory of language processing that is based on dialogue, but which applies to monologue as a special case.
 
 

Contrary to many people’s intuitions, the same is true of dialogue. For instance, Branigan, Pickering, and Cleland (2000) showed effects of the priming of syntactic structure during language production in dialogue that were exactly comparable to the priming shown in isolated sentence production (Bock, 1986b) or sentence recall (Potter & Lombardi, 1998). In Branigan et al.’s study, the degree of control of independent and dependent variables was no different from in Bock’s study, even though the experiment involved two participants engaged in a dialogue rather than one participant producing sentences in isolation. Similar control is exercised in studies by Clark and colleagues (e.g., Brennan & Clark, 1996; Brennan & Schober, 2001;Wilkes-Gibbs & Clark, 1992; also Horton & Keysar, 1996). Well-controlled studies of language production in dialogue may require some ingenuity, but such experimental ingenuity has always been a strength of psychology.

The theoretical reason is that psycholinguistics has derived most of its predictions from generative linguistics, and generative linguistics has developed theories of isolated, decontextualized sentences that are used in texts or speeches — in other words, in monologue. In contrast, dialogue is inherently interactive and contextualized: Each interlocutor both speaks and comprehends during the course of the interaction; each interrupts both others and herself; on occasion two or more speakers collaborate in producing the same sentence (Coates, 1990). So it is not surprising that generative linguists commonly view dialogue as being of marginal grammaticality, contaminated by theoretically uninteresting complexities. Dialogue sits ill with the competence/performance distinction assumed by most generative linguistics (Chomsky, 1965), because it is hard to determine whether a particular utterance is "well-formed" or not (or even whether that notion is relevant to dialogue). Thus, linguistics has tended to concentrate on developing generative grammars and related theories for isolated sentences; and psycholinguistics has tended to develop processing theories that draw upon the rules and representations assumed by generative linguistics. So far as most psycholinguists have thought about dialogue, they have tended to assume that the results of experiments on monologue can be applied to the understanding of dialogue, and that it is more profitable to study monologue because it is "cleaner" and less complex than dialogue. Indeed, they have commonly assumed that dialogue simply involves chunks of monologue stuck together.

The main advocate of the experimental study of dialogue is Herbert Clark. However, his primary focus is on the nature of the strategies employed by the interlocutors rather than basic processing mechanisms. Clark (1996) contrasts the "language-as-product" and "language-as-action" traditions. The language-as-product tradition is derived from the integration of information-processing psychology with generative grammar and focuses on mechanistic accounts of how people compute different levels of representation. This tradition has typically employed experimental paradigms and decontextualized language; in our terms, monologue. In contrast, the language-as-action tradition emphasizes that utterances are interpreted with respect to a particular context and takes into account the goals and intentions of the participants. This tradition has typically considered processing in dialogue using apparently natural tasks (e.g., Clark, 1992; Fussell & Krauss, 1992). Whereas psycholinguistic accounts in the language-as-product tradition are admirably well-specified, they are almost entirely decontextualized and, quite possibly, ecologically invalid. On the other hand, accounts in the language-as-action tradition rarely make contact with the basic processes of production or comprehension, but rather present analyses of psycholinguistic processes (e.g., the formulation and use of common ground; Clark, 1985,1996; Clark & Marshall, 1981) that are closely akin to rational analyses or ideal-observer models (Anderson, 1990; Chater & Oaksford, 1999; Legge, Klitz, & Tjan, 1997).

This dichotomy is a reasonable historical characterization. Almost all mechanistic theories happen to be theories of the processing of monologue; and theories of dialogue are almost entirely couched in intentional non-mechanistic terms. But this need not be. The goals of the language-as-product tradition are valid and important; but researchers concerned with mechanisms should investigate the use of contextualized language in dialogue.

In this paper we propose a mechanistic account of dialogue and use it to derive a number of predictions about basic language processing. The account assumes that in dialogue, production and comprehension become tightly coupled in a way that leads to the automatic alignment of linguistic representations at many levels. This process greatly simplifies production and comprehension in dialogue as compared to monologue. Sections 2 and 3 introduce the interactive alignment model and discuss the evidence for it. Sections 4, 5, and 6 then consider three aspects of processing that follow from alignment: interactive inference, establishment of routines, and self-monitoring. Finally, Section 7 considers how to formulate a linguistic framework that is consistent with this account.
 
 

Table 1 shows a transcript of a conversation between two players in a co-operative maze game (Garrod & Anderson, 1987). In this extract one player A is trying to describe his position to his partner B who is viewing the same maze on a computer screen in another room.

 

Table 1. Example dialogue taken from Garrod and Anderson (1987).

1-----B: .... Tell me where you are?
2-----A: Ehm : Oh God (laughs)
3-----B: (laughs)
4-----A: Right : two along from the bottom one up:*
5-----B: Two along from the bottom, which side?
6-----A: The left : going from left to right in the second box.
7-----B: You're in the second box.
8-----A: One up :(1 sec.) I take it we've got identical mazes?
9-----B: Yeah well : right, starting from the left, you're one along:
10----A: Uh-huh:
11----B: and one up?
12----A: Yeah, and I'm trying to get to ...

[ 28 utterances later ]

41----B: You are starting from the left, you're one along, one up?(2 sec.)
42----A: Two along : I'm not in the first box, I'm in the second box:
43----B: You're two along:
44----A: Two up (1 sec.) counting the : if you take : the first box as being one up :
45----B: (2 sec.) Uh-huh :
46----A: Well : I'm two along, two up: (1.5 sec.)
47----B: Two up ? :
48----A: Yeah (1 sec.) so I can move down one:
49----B: Yeah I see where you are:

* The position being described in the utterances shown in bold is highlighted with an arrow in Figure 1. Colons mark noticeable pauses of less than 1 second.

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Figure 1. Schematic representation of the maze being described in the conversation shown in Table 1. The arrow points to the position being described by the utterances marked in bold in the table.

At first glance the language looks disorganized. According to standard linguistics, many of the utterances are not grammatical sentences (e.g., only one of the first six contains a verb). There are occasions when production of a sentence is shared between speakers, as in (7-8) and (43-44). It often seems that the speakers do not know how to say what they want to say. For instance, A describes the same position quite differently in (4), "two along from the bottom one up," and (46), "two along, two up."

In fact the sequence is quite orderly so long as we assume that dialogue is a joint activity (Clark, 1996; Clark & Wilkes-Gibbs, 1986). In other words, it involves cooperation between interlocutors in a way that allows them to sufficiently understand the meaning of the dialogue as a whole; and this meaning results from these joint processes. In Lewis’s (1969) terms, dialogue is a game of cooperation, where both participants "win" if both understand the dialogue, and neither "wins" if one or both do not understand.

Conversational analysts argue that dialogue turns are linked across interlocutors (Schegloff & Sacks, 1973). A question, such as (1) "Tell me where you are?", calls for an answer, such as (3) "Two along from the bottom and one up." Even a statement like (4) "Right, two along from the bottom two up," cannot stand alone. It requires either an affirmation or some form of query, such as (5) "Two along from the bottom, which side?" (Linnell, 1998). This means that production and comprehension processes become coupled. B produces a question and expects an answer of a particular type; A hears the question and has to produce an answer of that type. For example, after saying "Tell me where you are?" in (1), B has to understand "two along from the bottom one up" in (4) as a reference to A’s position on the maze; any other interpretation is ruled out. Furthermore, the meaning of what is being communicated depends on the interlocutors’ agreement or consensus rather than on dictionary meanings (Brennan & Clark, 1996) and is subject to negotiation (Linnell, 1998, p.74). Take for example utterances (4-11) in the fragment shown above. In utterance (4), A describes his position as "Two along from the bottom and one up," but the final interpretation is only established at the end of the first exchange when consensus is reached on a rather different description by B (9-11) "You're one along […] one up?" These examples demonstrate that dialogue is far more coordinated than it might initially appear.

At this point, we should distinguish two notions of coordination that became rather confused in the literature. According to one notion (Clark, 1985), interlocutors are coordinated in a successful dialogue just as participants in any successful joint activity are coordinated (e.g., ballroom dancers, lumberjacks using a two-handed saw). According to the other notion, coordination occurs when interlocutors share the same representation at some level (Branigan et al., 2000; Garrod & Anderson, 1987).To remove this confusion, we refer to the first notion as coordination and the second as alignment. Dialogue is a coordinated behavior (just like ballroom dancing). However, the linguistic representations that underlie coordinated dialogue come to be aligned, as we claim below.

We now argue three points: (1) that alignment of situation models (Zwaan & Radvansky, 1998) is basic to successful dialogue and brings about many desirable properties (e.g., understanding of how the interlocutor is likely to react to information); (2) that the way that alignment of situation models is achieved is by a primitive mechanism that produces alignment at other levels of representation (lexical, syntactic, etc.) and by interconnections between the levels; and (3) that another primitive mechanism allows interlocutors to repair representations interactively. This process of alignment is essentially resource-free and does not depend on modeling the interlocutor’s mental state. More sophisticated strategies for such "other modeling" are of course used on occasion, but do not form part of the basic process of alignment. On this basis, we propose a general interactive alignment account of dialogue.

 

2.1 Alignment of situation models is central to successful dialogue

Alignment of situation models is not necessary in principle for successful communication. It would be possible to communicate successfully by representing one’s interlocutor’s situation model, even if that model were not the same as one’s own. For instance, one player could represent the maze according to a figure scheme but know that their partner represented it according to a path scheme, and vice versa. But this would be wildly inefficient as it would require maintaining two underlying representations, one for producing one’s own utterances and the other for comprehending one’s interlocutor’s utterances. Even though communication might work in such cases, it is unclear whether we would claim that the people understood the same thing. More critically, it would be computationally very costly to have fundamentally different representations. In contrast, if the interlocutors’ representations are basically the same, there is no need for listener modelling.

Under some circumstances storing the fact that one’s interlocutors represent the situation differently than oneself is necessary (e.g., in deception, or when trying to communicate to one interlocutor information that one wants to conceal from another). But even in such cases, many aspects of the representation will be shared (e.g., I might lie about my location, but would still use a figural representation to do so if that was what you were using). Additionally, it is clearly tricky to perform such acts of deception or concealment (Clark & Schaefer, 1987). These involve sophisticated strategies that do not form part of the basic process of alignment, and are costly, because they require the speaker to concurrently develop two representations.

Of course, interlocutors need not entirely align their situation models. In any conversation where information is conveyed, the interlocutors must have somewhat different models, at least before the end of the conversation. In cases of partial misunderstanding, conceptual models will not be entirely aligned. In (unresolved) arguments, interlocutors have representations that cannot be identical. But they must have the same understanding of what they are discussing in order to disagree about a particular aspect of it (e.g., Sacks, 1987). For instance, if two people are arguing the merits of the Conservative vs. the Labour parties for the U.K. government, they must agree about who the names refer to, roughly what the politics of the two parties are, and so on, so that they can disagree on their evaluations. In Lewis’s terms, such interlocutors are playing a game of cooperation with respect to the situation model (e.g., they succeed insofar as their words refer to the same entities), even though they may not play such a game at other "higher" levels (e.g., in relation to the argument itself). Therefore, we assume that successful dialogue involves approximate alignment at the level of the situation model at least.

 

2.2 Achieving alignment of situation models

This was pointed out by Garrod and Anderson (1987) in relation to their principle of output/input coordination. They noted that in the maze game task speakers closely matched the lexical, semantic, and pragmatic choices that they had just encountered as listeners. In other words, their outputs tended to match their inputs. As the interaction proceeded, the two interlocutors therefore came to align the representations used for generating output with the representations used for interpreting input. This points to a mechanism in which the combined system (i.e., the interacting dyad) is completely stable only if both subsystems (i.e., speaker A’s representation system and speaker B’s representation system) are aligned. In other words, the dyad is only in equilibrium when what A says is consistent with B’s currently active lexical, semantic, and pragmatic representation of the dialogue and vice versa (see Garrod & Clark, 1993). Thus, because the two parties to a dialogue produce aligned language, the underlying linguistic representations also tend to become aligned. We shall argue that alignment is more general than Garrod and Anderson suggested. First, it occurs at other levels (e.g., the syntactic). More fundamentally, alignment at one level "percolates" through to produce alignment at other levels, and that this process leads to alignment at the level of the situation model (see Section 2.2.2). This process is essentially resource-free and automatic.

Hence, we suggest that in dialogue representations become aligned at many linguistic levels, such as the semantic, lexical, and syntactic, and that this leads to alignment at the critical level of the situation model. We shall argue that such aligned processing has far reaching implications for everything from lexical and syntactic analysis to discourse inference in dialogue. We also make it clear that dialogue involves more sophisticated strategies as well as alignment, but these strategies are not primitive mechanisms (and we return to them in Section 4.2).

2.2.1 Evidence for alignment at other levels

Dialogue transcripts are full of repeated linguistic elements and structures indicating alignment at various levels (Aijmer, 1996; Schenkein, 1980; Tannen, 1989). Alignment of lexical processing during dialogue was specifically demonstrated by Garrod and Anderson (1987) as in the extended example in Table 1 (see also Garrod & Clark, 1993; Garrod & Doherty, 1994), and by Clark and colleagues (Brennan & Clark, 1996; Wilkes-Gibbs & Clark, 1992). These latter studies show that interlocutors tend to develop the same set of referring expressions to refer to particular objects and that the expressions become shorter and more similar on repetition with the same interlocutor and are modified if the interlocutor changes.

Levelt and Kelter (1982) found that speakers tended to reply to "What time do you close?" or "At what time do you close" (in Dutch) with a congruent answer (e.g., "Five o’clock" or "At five o’clock"). This alignment may be syntactic (repetition of phrasal categories) or lexical (repetition of at). Branigan et al. (2000) found clear evidence for syntactic alignment in dialogue. Participants took it in turns to describe pictures to each other (and to find the appropriate picture in an array). One speaker was actually a confederate of the experimenter and produced scripted responses, such as "the cowboy offering the banana to the robber" or "the cowboy offering the robber the banana." The syntactic structure of the confederate’s description strongly influenced the syntactic structure of the experimental subject’s description. Their work extends "syntactic priming" work to dialogue: Bock (1986b) showed that speakers tended to repeat syntactic form under circumstances in which alternative non-syntactic explanations could be excluded (Bock, 1989; Bock & Loebell, 1990; Bock, Loebell, & Morey, 1992; Hartsuiker & Westerberg, 2000; Pickering & Branigan, 1998; Potter & Lombardi, 1998; Smith & Wheeldon, 2001).

If syntactic alignment is due, in part, to the interactional nature of dialogue, then the degree of syntactic alignment should reflect the nature of the interaction between speaker and listener. As Clark and Schaeffer (1987; see also Schober & Clark, 1989; Wilkes-Gibbs & Clark, 1992) have demonstrated, there are basic differences between addressees and other listeners. So we might expect stronger alignment for addressees than for other listeners. To test for this, Branigan, Pickering, and Cleland (2001) had two speakers take it in turns to describe cards to a third person, so the two speakers overheard but did not speak to each other. Priming occurred under these conditions, but it was weaker than when two speakers simply responded to each other. Hence, syntactic alignment is affected by speaker participation in dialogue.

Alignment also occurs at the level of articulation. It has long been known that as speakers repeat expressions, articulation becomes increasingly reduced (i.e., the expressions are shortened and become more difficult to recognize when heard in isolation; Fowler & Housum, 1987). However, Bard et al. (2000) found that reduction was just as extreme when the repetition was by a different speaker in the dialogue as it was when the repetition was by the original speaker. In other words, whatever is happening to the speaker’s articulatory representations is also happening to their interlocutor’s. There is also evidence that interlocutors align accent and speech rate (Giles, Coupland, & Coupland, 1992; Giles & Powesland, 1975).

Finally, there is some evidence for alignment in comprehension. Levelt and Kelter (1982, Experiment 6) found that people judged question-answer pairs involving repeated form as more natural than pairs that did not; and that the ratings of naturalness were highest for the cases where there was the strongest tendency to repeat form. This suggests that speakers prefer their interlocutors to respond with an aligned form. Garrod and Anderson (1987) also found that players in the maze game would query descriptions from an interlocutor that did not match their own previous descriptions (see Section 4 below). In other words, the listener is guided and constrained by her own recent contributions as a speaker. However, there is still little evidence concerned with the "microstructure" of comprehension in dialogue (as opposed to gross findings such as that comprehension is easier for interlocutors than overhearers; Schober & Clark, 1989).

2.2.2 Alignment at one level leads to alignment at another

So far, we have concluded that successful dialogue leads to the development of both aligned situation models and aligned representations at all other linguistic levels. There are good reasons to believe that this is not coincidental, but rather that aligned representations at one level lead to aligned representations at other levels.

Consider the following two examples of influences between levels. First, Garrod and Anderson (1987) found that once a word had been introduced with a particular interpretation it was not normally used with any other interpretation in a particular stretch of maze-game dialogue. For instance, the word row could refer either to an ordered set of levels (e.g., with descriptions containing an ordinal like "I’m on the fourth row") or to unordered sets of levels (e.g., with descriptions that do not contain ordinals like "I’m on the bottom row").² Speakers who had adopted one of these uses of row and needed to refer to the other would introduce a new term, such as line or level. Thus, they would talk of the fourth row and the bottom line, but not the fourth row and the bottom row. In other words, aligned use of a word seemed to go with a specific aligned interpretation of that word. Restricting usage in this way allows dialogue participants to assume quite specific unambiguous interpretations for expressions. Furthermore, if a new expression is introduced they can assume that it would have a different interpretation from a previous expression, even if the two expressions are "dictionary synonyms." This process leads to the development of a lexicon of expressions relevant to the dialogue (see Section 5). What interlocutors are doing is acquiring new senses for words or expressions. To do this, they use the principle of contrast just like children acquiring language (e.g., E.V. Clark, 1993).

Second, it has been shown repeatedly that priming at one level can lead to more priming at other levels. Specifically, syntactic alignment (or "syntactic priming") is enhanced when more lexical items are shared. In Branigan et al.’s (2000) study, the stooge produced a description using a particular verb (e.g., the nun giving the book to the clown). Some experimental subjects then produced a description using the same verb; whereas other subjects produced a description using a different verb. Syntactic alignment was considerably enhanced if the verb was repeated. Thus, interlocutors do not align representations at different linguistic levels independently. Likewise, Cleland and Pickering (2001) found people tended to produce noun phrases like the sheep that’s red as opposed to the red sheepmore often after hearing the goat that’s red than after the book that’s red. This demonstrates that semantic relations between lexical items enhance syntactic priming.

These effects can be modeled in terms of a lexical representation outlined in Pickering and Branigan (1998). A node representing a word (i.e., its lemma; Levelt, Roelofs, & Meyer, 1999; cf. Kempen & Huijbers, 1983) is connected to nodes that specify its syntactic properties. So, the node for give is connected to a node specifying that it can be used with a noun phrase and a prepositional phrase. Processing giving the book to the clown activates both of these nodes and therefore makes them both more likely to be employed subsequently. However, it also strengthens the link between these nodes, on the Hebbian principle that coactivation strengthens association. Thus, the tendency to align at one level, such as the syntactic, is enhanced by alignment at another level, such as the lexical. Cleland and Pickering’s finding demonstrates that exact repetition at one level is not necessary: the closer the relationship at one level (e.g., the semantic), the stronger the tendency to align at the other (e.g., the syntactic). Note that we can make use of this tendency to determine which specific levels are linked.
 
 

Aligned language processing is difficult to accommodate within a one-way communication model of the kind assumed by current theories of production and comprehension. In such models, the transfer of information between producers and comprehenders takes place via decoupled production and comprehension processes. The speaker (or writer) formulates an utterance on the basis of her representation of the situation. In turn, the listener (or reader) infers what the speaker (or writer) intended on the basis of his autonomous representation of the situation. So, from a processing point of view, speakers and listeners act in isolation. The only link between the two is in the information conveyed by the utterances themselves (Cherry, 1956); see Fig. 2. In this autonomous transmission account, only the final representation (i.e., sound) is "accessible." Each act of transmission is treated as a discrete stage, with a particular unit being encoded into sound by the speaker, being transmitted as sound, and then being decoded by the listener. Levels of linguistic representation are constructed during encoding and decoding, but there is no particular association between the levels of representation used by the speaker and listener. Thus, there is no reason why those intermediate levels should be retained (as demonstrated in explicit memory tests with monologue; Jarvella, 1979; Johnson-Laird & Stevenson, 1970; Sachs, 1967, as well as implicit memory tests; Garrod & Trabasso, 1973; Garnham, Oakhill, & Cain, 1998). It is also unclear why alignment should necessarily occur at other levels of representation beside the situation model.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Consider, however, what happens in an extended dialogue (see e.g. Clark, 1996; Garrod, 1999). In formulating an utterance the speaker is guided by what has just been said to her and in comprehending the utterance the listener is constrained by what he has just said, as in the example dialogue in Table 1. As the dialogue proceeds, the communicators come to operate according to the interactive alignment model (see Fig. 3). This model assumes that interlocutors are continuously aligning their representations at different linguistic levels. Each level of representation is causally implicated in the process of communication and these intermediate representations are retained implicitly. Because alignment at one level leads to alignment at others, the interlocutors come to align their situation models and hence are able to understand each other.
 
 
 
 
 
 

This situation of course only occurs when interlocutors take turns in contributing — in other words, during dialogue. In monologue (including writing), the speaker’s and the listener’s representations can rapidly diverge (or never align at all). The listener then has to draw inferences on the basis of his knowledge about the speaker, and the speaker has to infer what the listener has inferred (or simply assume that the listener has inferred correctly). Of course, either party could easily be wrong, and these inferences will often be costly. In monologue, the automatic mechanisms of alignment are not present (the consequences for written production are demonstrated in Traxler & Gernsbacher, 1992, 1993). It is only when regular feedback occurs that the interlocutors can control the alignment process.

On the autonomous transmission account, the processes employed in production and comprehension need not draw upon the same representations (see Fig. 2). By contrast, the interactive alignment model assumes that the processor draws upon the same representations (see Fig. 3). This parity means that a representation that has just been constructed for the purposes of comprehension can then be used for production (or vice versa). This straightforwardly explains, for instance, why we can complete one another’s utterances (and get the syntax, semantics, and phonology correct; see Section 7.1).

The notion of parity of representation is controversial but has been advocated by a wide range of researchers working in very different domains (Calvert et al., 1997; Lahiri & Marslen-Wilson, 1991; Liberman & Whalen, 2000; MacKay, 1987; Mattingly & Liberman, 1988). It is also increasingly advocated as a means of explaining perception/action interactions outside language (Hommel, Müsseler, Aschersleben, & Prinz, in press). Note that only the representations must be the same. The processes leading to those representations need not be related (e.g., there is no need for the mapping between representations to be simply reversed in production and comprehension).

To summarize, the interactive alignment model differs from autonomous transmission account with respect to three sets of assumptions: (1) There is priming at different levels of linguistic representation and priming of the links between the different levels; (2) there is parity between the representations used for production and comprehension processes, so representations activated in comprehension will affect production processes and vice versa; (3) priming is enhanced during dialogue because of the cumulative and reciprocal nature of the production and comprehension processes.
 
 

Alignment between interlocutors has traditionally been thought to arise from the establishment of common, mutual, or joint knowledge (Lewis, 1969; McCarthy, 1990; Schiffer, 1972). An influential example of this approach is Clark and Marshall’s (1981) argument that successful reference depends on the speaker and the listener inferring mutual knowledge about the circumstances surrounding the reference. Thus, for a female speaker to be certain that a male listener understands what is meant by "the movie at the Roxy," she needs to know what he knows and what he knows that she knows, and so forth. Likewise, for him to be certain about what she means by "the movie at the Roxy," he needs to know what she knows and what she knows that he knows, and so forth. However, there is no foolproof procedure for establishing mutual knowledge, because it requires formulating recursive models of interlocutors’ beliefs (see Clark, 1996, chapt. 4; Halpern & Moses, 1990; Lewis, 1969). Therefore, Clark and Marshall (1981) suggested that interlocutors instead infer what Stalnaker (1978) called the common ground. This involves what can reasonably be assumed to be known to both of them on the basis of the evidence at hand (e.g., if both know that they come from the same city they can assume a degree of common knowledge about that city; if both admire the same view and it is apparent to both that they do so, they can infer a common perspective).

To represent common ground directly requires the interlocutor to retain a very complex model that specifies both her own knowledge and the knowledge that she assumes to be shared with her partner. To do this, she has to keep track of the knowledge state of her interlocutor in a way that is separate from her own knowledge state. This is a very stringent requirement for routine communication, in part because she has to make sure that this model is constantly updated appropriately (e.g., Halpern & Moses, 1990). So it is perhaps not surprising that studies of both production and comprehension in situations where there is no direct interaction (i.e., situations which do not allow feedback) indicate that language users do not always take common ground into account in producing or interpreting references.

For example, Horton and Keysar (1996) found that speakers under time pressure did not produce descriptions that took advantage of what they knew about the listener’s view of the relevant scene. In other words, the descriptions were formulated with respect to the speaker’s current knowledge of the scene rather than with respect to the speaker and listener’s common ground. Keysar, Barr, Balin, and Paek (1998) found that in visually searching for a referent for a description listeners are just as likely to initially look at things that are not part of the common ground as things that are. In a similar vein, Brown and Dell (1987) showed that apparent listener-directed ellipsis was not modulated by information about the common ground between speaker and listener, but rather was determined by the accessibility of the information for the speaker alone (but see Schober & Brennan, 2001, for reservations).

Even in fully interactive dialogue it is difficult to find evidence for direct listener modeling. For example, it was originally thought that articulation reduction might reflect the speaker’s sensitivity to the listener’s current knowledge (Lindblom, 1990). However, Bard et al. (2000) found that the same level of articulation reduction occurred even after the speaker encountered a new interlocutor. In other words, degree of reduction seemed to be based only on whether the reference was given information for the speaker and not on whether it was part of the common ground. Additionally, speakers will sometimes use definite descriptions (to mark the referent as given information; Haviland & Clark, 1974) when the referent is visible to them, even when they know it is not available to their interlocutor (Anderson & Boyle, 1994).

Nevertheless, under certain circumstances interlocutors do engage in strategic inference relating to common ground. As Horton and Keysar (1996) found, with less time pressure speakers often do take account of common ground in formulating their utterances, and as Keysar et al. (1998) argued in their perspective adjustment model, listeners can at a later monitoring stage take account of common ground in comprehension under circumstances in which speaker/listener perspectives are radically different (see also Brennan & Clark, 1996; Schober & Brennan, 2001).

Taken together, these results suggest that performing inferences about common ground is an optional strategy that interlocutors employ only when resources allow (see Clark & Wasow, 1998, p 202 and section 4.2). Critically, such strategies need not always be used, and most "simple" (e.g., dyadic, non-didactic, non-deceptive) conversation works without them most of the time.

4.1.1 Interactive alignment produces an implicit common ground

In Section 2 we argued that dialogue participants automatically build up aligned representations, including aligned situation models. Whereas Clark and Marshall (1981) argued that interlocutors need to infer common ground explicitly, we argue that this is not necessary because the shared information in the aligned representations already constitutes an implicit common ground. We call it implicit common ground because it reflects information that is in both interlocutors’ situation models, but does not derive from communicators explicitly modelling their partner’s beliefs.

Of course, the process of alignment does not always lead to appropriately aligned representations. Under these circumstances the implicit common ground is faulty because it does not reflect shared knowledge (i.e., knowledge that they both actually have irrespective of whether each knows the other has it). So how can the interlocutors rectify the situation? We argue that they employ an interactive inference mechanism to ensure that the implicit common ground truly reflects shared knowledge. The mechanism relies on two processes: (1) monitoring the input in relation to one’s own representation, and (2) an interactive repair process aimed at re-establishing alignment under circumstances where utterances cannot be resolved against this representation.

Consider again the example in Table 1. Throughout this section of dialogue A and B assume subtly different interpretations for two along. A interprets two along by counting the boxes on the maze, whereas B is counting the links between the boxes (see Fig. 1). This temporary misalignment arises because the two speakers represent the meaning of expressions like two along differently in this context.

Therefore, the players engage in an interactive repair process. If the input that they receive cannot be resolved in relation to their current representation, then they reformulate the information at their next turn. The reformulation can be a simple repetition with rising intonation (as in 7), a repetition with an additional query (as in 5), or a more radical restatement (as when A reformulates "two along" as "second box" in 6). Such reformulation throws the problem back to the interlocutor, and the cycle continues until the misalignment has been resolved (for discussion of such embedded repairs see Jefferson, 1987). In fact, the misalignment is only satisfactorily resolved at a much later point (43-44) when A is able to complete B’s utterance without further challenge.

This account is consistent with the findings that speakers regularly adapt their utterances according to whether the information can be accessed within their own situational model rather than via a direct assessment of common ground. However, because access is from aligned representations which reflect the implicit common ground these adaptations will normally be helpful incidentally for the listener (for further discussion see Brown & Dell, 1987). For example, when speakers reduce the articulation of easily accessible repeated expressions, those expressions tend to be more recognisable to the listener as well (Bard et al., 2000), as most of the time the listener has heard the earlier uses of the expression. Whereas in the autonomous transmission account discourse inference processes are internalised in the minds of the speaker and listener, in the interactive alignment account the process can be externalised in the interchange between the two dialogue participants supported by the process of aligning output and input.

The interactive inference mechanism is basic because it only relies on the speaker monitoring the conversation in relation to her own knowledge of the situation. Of course there will be occasions when a more complicated and strategic assessment of common ground may be necessary, most obviously when interlocutors are not well aligned in some critical respect. In such cases, the listener may have to draw inferences about the speaker (e.g., "She has referred to John; does she mean John Smith or John Brown? She knows both, but thinks I don’t know Brown, hence she probably means Smith."). Other important cases are when one speaker is trying to deceive the other in some way or to conceal information (e.g., Clark & Schaefer, 1987), or when people decide deliberately not to align at some level (e.g., by refusing to use the same expression as one’s interlocutor because it has a political implication that one finds distasteful; Jefferson, 1987). Such cases may involve complex (and probably conscious) reasoning, and there may be great differences between people’s abilities (e.g., between those with and without an adequate "theory of mind"; Baron-Cohen, Tager-Flusberg, & Cohen, 2000). For example, Garrod and Clark (1993) found that younger children could not circumvent the automatic alignment process. Seven year old maze-game players failed to introduce new description schemes when they should have done so, because they could not overcome the pressure to align their description with the previous one from the interlocutor. By contrast, older children and adults were twice as likely to introduce a new description scheme when they had been unable to understand their partner’s previous description. Whereas the older children could adopt a strategy of non-alignment when appropriate, the younger children seemed unable to do so. Our claim is that these strategic processes are overlaid on the basic interactive alignment mechanism.  However, such strategies are clearly costly in terms of processing resources and may be beyond the abilities of less skilled language users.

The strategies discussed above relate specifically to alignment (either avoiding it or achieving it explicitly), but of course many aspects of dialogue serve far more complicated functions. Thus, a speaker can attempt to produce a particular emotional reaction in the listener by an utterance, or persuade the listener to act in a particular way or to think in depth about an issue (e.g., in expert-novice interactions; see Isaacs & Clark, 1987). Likewise, the speaker can draw complex inferences about the mental state of the listener and can try to probe this state by interrogation. Thus, it is important to stress that we are proposing interactive alignment as the primitive mechanism underlying dialogue, not a replacement for the more complicated strategies that conversationalists may employ on occasion.

However, we claim that normal conversation does not routinely require modeling the interlocutor’s mind. Instead, the overlap between interlocutors’ representations is sufficiently great that a specific contribution by the speaker will either trigger appropriate changes in the listener’s representation, or will bring about the process of interactive repair. Hence, the listener will retain an appropriate model of the speaker’s mind, because, in all essential respects, it is the listener’s representation as well.

Processing monologue is quite different in this respect. Without automatic alignment and interactive repair the listener can only resort to costly bridging inferences whenever she fails to understand anything. And, to ensure success, the speaker will have to design what he says according to what he knows about the audience (see Clark & Murphy, 1982). In other words, he will have to model the audience’s state of knowledge.
 
 

The process of alignment means that interlocutors draw upon representations that have been developed during the dialogue. Thus it is not always necessary to construct representations that are used in production or comprehension from scratch. This perspective radically changes our accounts of language processing in dialogue. One particularly important implication is that interlocutors develop and use routines (set expressions) during a particular interaction. Most of this section addresses the implications of this perspective for language production, where they are perhaps most profound. We then turn more briefly to language comprehension.

The seminal account of language production is Levelt's (1989) book Speaking, which has the informative subtitle From intention to articulation. Chapter by chapter, Levelt describes the stages involved in the process of language production, starting with the conceptualization of the message, through the process of formulating the utterance as a series of linguistic representations (representing grammatical functions, syntactic structure, phonology, metrical structure, etc.), through to articulation. The core assumption is that the speaker necessarily goes through all of these stages in a fixed order. The same assumption is common to more specific models of word production (e.g., Levelt et al., 1999) and sentence production (e.g., Bock & Levelt, 1994; Garrett, 1980). Experimental research is used to back up this assumption. In most experiments concerned with understanding the mechanisms underlying language production, the speaker is required to construct the word or utterance from scratch, or from a pre-linguistic level at least. For example, a common method is picture description (e.g., Bock, 1986b; Schriefers, Meyer, & Levelt, 1990). These experiments therefore employ methods that reinforce the ideomotor tradition of action research that underlies Levelt’s framework (see Hommel et al., in press).

It appears to be universally agreed that this exhaustive process is logically necessary because speakers have to articulate the words. Indeed, a common claim in work on language production is that, while comprehenders can sometimes "short-circuit" the comprehension process by taking into account the prior context (e.g., guessing thematic roles without actually parsing), producers always have to go through each step from beginning to end. To quote Bock and Huitema (1999 p.385):

Repetition of an utterance may seem unnatural or uncertainly related to normal processing, but in fact, as we have noted, normal dialogue is highly repetitive (e.g., Tannen, 1989). This is of course different from carefully crafted monologue where — depending to some extent on the genre — repetition is regarded as an indication of poor style (see Amis, 1997, pp. 246-250). In our example dialogue 82% of the 127 words are repetitions; in this paragraph only 25% of the 125 words are repetitions. (Ironically, we — the authors — have avoided repetition even when writing about it.) In fact, the assumption that repetition is unusual or special is a bias probably engendered by psychologists’ tendency to spend much of their time reading formal prose and designing experiments using decontextualized "laboratory" paradigms like picture naming.

So it is possible that people can short-circuit parts of the production process just as they may be able to short-circuit comprehension. Moreover, this may be a normal process that occurs when engaged in dialogue. We strongly suspect (see below) that phrases (for instance) are not simply inserted as a whole, but that the true picture is rather more complicated. But it is critical to make the logical point that the stages of production are not set in stone, as previous theories have assumed.

A routine is an expression that is "fixed" to a relatively great extent. First, the expression has a much higher frequency than the frequency of its component words would lead us to expect (e.g., Aijmer, 1996). (In computational linguistics this corresponds to having what is called a high "mutual information" content; Charniak, 1993.) Second, it has a particular analysis at each level of linguistic representation. Thus, it has a particular meaning, a particular syntactic analysis, a particular pragmatic use, and often particular phonological characteristics (e.g., a fixed intonation). Extreme examples of routines include repetitive conversational patterns such as How do you do? and Thank you very much. Routines are highly frequent in dialogue: Aijmer estimates that up to 70% of words in the London-Lund speech corpus occur as part of recurrent word combinations (see Altenberg, 1990). However, different expressions can be routines to different degrees, so actual estimates of their frequency are somewhat arbitrary. Some routines are idioms, but not all (e.g., I love you is a routine with a literal interpretation in the best relationships; see Nunberg, Sag, & Wasow, 1994; Wray & Perkins, 2001)

Most discussion of routines focuses on phrases whose status as a routine is pretty stable. Although long-term routines are important, we also claim that routines are set up "on the fly" during dialogue. In other words, if an interlocutor uses an expression in a particular way, it may become a routine for the purposes of that conversation alone. We call this process routinization. Here we consider why routines emerge and why they are useful. The next section considers how they are produced (in contrast to non-routines). This, we argue, leads to a need for a radical reformulation of accounts of sentence production. Finally, we consider how the comprehension of routines causes us to reformulate accounts of comprehension.

5.2.1 Why do routines occur?

Most stretches of dialogue are about restricted topics and therefore have quite a limited vocabulary. Hence, it is not surprising that routinization occurs in dialogue. But monologue can also be about restricted topics, and yet all indications suggest it is much less repetitive and routinization is much less common. The more interesting explanation for routinization in dialogue is that it is due to interactive alignment. A repeated expression (with the same analysis and interpretation) is of course aligned at most linguistic levels. Thus, if interlocutors share highly activated semantic representations (what they want to talk about), lexical representations (what lexical items are activated), and syntactic representations (what constructions are highlighted), they are likely to use the same expressions, in the same way, to refer to the same things. The contrast with most types of monologue occurs (in part, at least) because the producer of a monologue has no-one to align her representations with (see Section 2). The use of routines contributes enormously to the fluency of dialogue in comparison to most monologue — interlocutors have a smaller space of alternatives to consider and have ready access to particular words, grammatical constructions, and concepts.

Consider the production of expressions that keep being repeated in a dialogue, such as "the previous administration" in a political discussion. When first used, this expression is presumably constructed by accessing the meaning of "previous" and combining it with the meaning of "administration." The speaker may well have decided "I want to refer to the Conservative Government, but want to stress that they are no longer in charge, etc. so I’ll use a circumlocution." He will construct this expression by selecting the words and the construction carefully. Likewise, the listener will analyze the expression and consider alternative interpretations. Both interlocutors are therefore making important choices about alternative forms and interpretations. But if the expression is repeatedly used, the interlocutors do not have to consider alternatives to the same extent. For example, they do not have to consider that the expression might have other interpretations, or that "administration" is ambiguous (e.g., it could refer to a type of work). Instead, they treat the expression as a kind of name that refers to the last Conservative government. Similar processes presumably occur when producing expressions that are already frozen (Pinker & Birdsong, 1979; see also Aijmer, 1996). Generally, the argument is that people can "shortcircuit" production in dialogue, by removing or drastically reducing the choices that otherwise occur during production (e.g., deciding which synonym to use, or whether to use an active or a passive).

Why might this happen? The obvious explanation is that routines are in general easier to produce than non-routines. Experimental work on this is lacking, but an elegant series of field studies by Kuiper (1996) suggest that this explanation is correct. He investigated the language of sports commentators and auctioneers, who are required to speak extremely quickly and fluently. For example, radio horse-racing commentators have to produce a time-locked and accurate monologue in response to rapidly changing events. This monologue is highly repetitive and stylized, but quite remarkably fluent. He argued that the commentators achieve this by storing routines, which can consist of entirely fixed expressions (e.g., They are coming round the bend) or expressions with an empty slot that has to be filled (e.g., X is in the lead), in long-term memory, and then access these routines, as a whole, when needed. Processing load is thereby greatly reduced in comparison to non-routine production. Of course, this reduction in load is only possible because particular routines are stored; and these routines are stored because the commentators repeatedly produce the same small set of expressions in their career.

Below, we challenge his assumption that routines are accessed "as a whole," and argue instead that some linguistic processing is involved. But we propose a weaker version of his claims, namely that routines are accessed telegraphically, in a way that is very different from standard assumptions about language production (as in, e.g., Levelt, 1989). Moreover, we argue that not all routines are learned over a long period, but that they can instead emerge "on the fly," as an effect of alignment during dialogue.

5.2.2 Massive priming in language production

Contrary to Kuiper (1996), some compositional processes take place in routines, as we know from the production of idiom blends (e.g., That’s the way the cookie bounces) (Cutting & Bock, 1997). However, there are good reasons to assume that production of idioms and other routines may be highly telegraphic. The normal process of constructing complex expressions involves a large number of lexical, syntactic, and semantic choices (why choose one word or form rather than another, for instance). In contrast, when a routine is used, most of these choices are not necessary. For example, speakers do not consider the possibility of passivizing an idiom that is normally active (e.g., The bucket was kicked), so there is no stage of selection between active and passive. Likewise, they do not consider replacing a word with a synonym (e.g., kick the pail), as the meaning would not be preserved. Similarly, a speech act like I name this ship X is fixed, insofar as particular illocutionary force depends on the exact form of words (cf. I give this ship the name X). Also, flat intonation suggests that no choices are made about stress placement (Kuiper, 1996).

Let us expand this by extending some of the work of Potter and Lombardi to dialogue (Lombardi & Potter, 1992; Potter & Lombardi, 1990, 1998). They address the question of how people recall sentences (see also Bock, 1986b, 1996). Recall differs from dialogue in that (1) the same sentences are perceived and produced; and (2) there is only one participant, acting as both comprehender and producer. Potter and Lombardi had experimental subjects read and then recall sentences whilst performing concurrent tasks. They found that a "lure" word sometimes intruded into the recalled sentence, indicating that subjects did not always store the surface form of the sentence; that these lure words caused the surface syntax of the sentence to change if they intruded and did not fit with the sentence that was read; and that other clauses could syntactically prime the target sentence so that it was sometimes misremembered as having the form of the prime sentence. They argued that people did not remember the surface form of the sentence but rather remembered its meaning and had the lexical items and syntactic constructions primed during encoding. Recall therefore involved converting the meaning into the surface form using the activation of lexical items and syntax to cause a particular form to be regenerated. In normal sentence recall, this is likely to be the form of the original sentence.

This suggests that language production can be greatly enhanced by the prior activation of relevant linguistic representations (in this case, lexical and syntactic representations). In dialogue, speakers do not normally aim simply to repeat their interlocutors’ utterances. However, production will be greatly enhanced by the fact that previous utterances will activate their syntactic and lexical representations. Hence, they will tend to repeat syntactic and lexical forms, and therefore to align with their interlocutors. These arguments suggest why sentence recall might actually present a reasonable analogue to production in naturalistic dialogue; and why it is probably a better analogue than, for instance, isolated picture description. In both sentence recall and production in dialogue, very much less choice needs to be made than in monologue. The decisions that occur in language production (e.g., choice of word or structure) are to a considerable extent driven by the context and do not need to be a burden for the speaker. Thus, they are at least partly stimulus-driven rather than entirely internally generated, in contrast to ideomotor accounts like Levelt (1989).

However, our account differs from Potter and Lombardi’s in one respect. They assume no particular links between the activation of syntactic information, lexical information, and the message. In other words, the reason that we tend to repeat accurately is that the appropriate message is activated, the appropriate words are, and the appropriate syntax is. But we have already argued that alignment at one level leads to more alignment at other levels (e.g., syntactic priming is enhanced by lexical overlap; Branigan et al., 2000), and that this is due to Hebbian learning. The alignment model assumes interrelations between all levels, so that a meaning, for instance, is activated at the same time as a word. This explains why people not only repeat words but also repeat their senses in a dialogue (Garrod & Anderson, 1987). In other words, what actually occurs in dialogue is lots of lexical, syntactic, and semantic activation of various tokens at each level, and activation of particular links between the levels. This leads to a great deal of alignment, and hence the production of routines. It also means that the production of a word or utterance in dialogue is only distantly related to the production of a word or utterance in isolation.

Kuiper (1996) assumes that most routines are stored after repeated use, in a way that is not directly related to dialogue. However, he considers an example of how an auctioneer creates a "temporary formula" by repeating a phrase (p. 62). He regards this case as exceptional and does not employ it as part of his general argument. In contrast, we assume that the construction of temporary formulae is the norm in dialogue. Many studies show how new descriptions become established for the dialogue (e.g., Brennan & Clark, 1996; Clark & Wilkes-Gibbs, 1986; Garrod & Anderson, 1987). In general it is striking how quickly a novel expression can be regarded as entirely normal, whether it is a genuine neologism or a novel way of referring to an object (Gerrig & Bortfeld, 1999).

In situations in which a community of speakers regularly discuss the same topic we might expect the transient routines that they establish to eventually become fixed within that community. In fact, Garrod and Doherty (1994) demonstrated that an experimentally established community of maze-game players quickly converged on a common description scheme. They also found that the scheme established by the community of players was used more consistently than schemes adopted by isolated pairs of players over the same period. This result points to the interesting possibility that the interactive alignment process can be responsible for fixing routines in the language or dialect spoken by a community of speakers (see Clark, 1998).

5.2.3 Producing words and sentences

Most models of word production assume that the apparent fluency of production hides a number of stages that lead from conceptual activation to articulation. In Levelt et al. (1999) a lexical entry consists of sets of nodes at different levels (or strata): a semantic representation, a syntactic (or lemma) representation, a phonological representation, a phonetic representation, and so on. Each level is connected to the one after it, so that the activation of a semantic representation (e.g., for cat) leads to the activation of its syntactic representation (the "cat" lemma plus syntactic information specifying that it is a singular count noun), which in turn leads to the access of the phonological representation /k//a//t/. Evidence for the sequential nature of activation comes from time-course data (Schriefers et al., 1990; van Turrennout, Hagoort, & Brown, 1998), "tip-of-the-tongue" data (Vigliocco, Antonini, & Garrett, 1997), and so on. Alternative accounts question the specific levels assumed by Levelt et al. and the mechanisms of activation but do not question the assumption that earlier levels become activated before later ones (Caramazza, 1997; Dell, 1986). Notice that the data used to derive these accounts is almost entirely based on paradigms that require generation from scratch (e.g., picture naming) or from linguistic information with a very indirect relationship to the actual act of production required (e.g., responding with the object of a definition).

We do not contend that the dialogical perspective leads us to a radically different view of word production. More specifically, we have no reason to doubt that the same levels of representation are accessed in the same order during production in dialogue (though this question has not been addressed by mainstream psycholinguistic research). For example, Potter and Lombardi’s data suggest that even in repetition of a word, it is likely that lexical access occurs (and that there is no direct access of the word-form, for example). However, contextual activation is likely to have some effects on the time-course of production, particularly in relation to the decisions at different stages in the production process. For example, a choice between two synonyms might normally involve some processing difficulty, but if one has been established in the dialogue (e.g., by lexical entrainment), no meaningful process of selection is needed.

The situation is very different with isolated sentence production. Models of production assume that a speaker initially constructs a message, then converts this message into a syntactic representation, then into a phonological representation, and then into sound (Bock & Levelt, 1994; Garrett, 1980; Levelt, 1989). Normally, they also assume that the syntactic level involves at least two stages: a functional representation, and a constituent-structure representation. It is accepted that cascading may happen, so that the complete message does not need to be computed before syntactic encoding can begin (e.g., Meyer, 1996). But ordering is assumed, so that, for instance, a word cannot be uttered until it is assigned a functional role and a position within a syntactic representation.

However, we propose that it may be possible to break this rigid order of sentence production, and instead to build a sentence "around" a particular phrase if that phrase has been focused in the dialogue. In accord with this, context can affect sentence formulation in monologue, so that a focused phrase is produced first (Bock, 1986a; Prat-Sala & Branigan, 2000). Prat-Sala and Branigan, in particular, found effects of focus on word order that were not due to differences in grammatical function. Hence it may be possible to utter a phrase before assigning it a grammatical function. For instance, in That picture, I think you like, and That picture, I think pleases you, the meaning of That picture does not vary but its grammatical function (subject or object) does vary. Assuming that production is at least partially incremental, people can therefore utter That picture before assigning it a grammatical function. This would of course not be possible within traditional models where phonological representations and acoustic form cannot be constructed before grammatical function is assigned (e.g., Bock & Levelt, 1994). So the effects of strong context, in either dialogue or monologue, may be to change the process of sentence production quite radically.

The vast literature on lexical comprehension is almost entirely concerned with monologue (e.g., reading words in sentential or discourse contexts) or isolated words. But the alignment model suggests that lexical comprehension in dialogue is very different from monologue. A major consequence of alignment at a lexical level is that local context becomes central. Listeners, just like speakers, should be able to select words from a set that have been central to that dialogue — a "dialogue lexicon."

One of the most universally accepted phenomena in experimental psychology, enshrined in all classic models (e.g., Morton, 1969), is the word frequency effect: More frequent words are understood (and produced) faster than less frequent words. Of course, processing is affected by repetition but this is normally regarded as only modulating the underlying frequency effect. However, in dialogue, local context is so central that the frequency of an expression (or, e.g., its age of acquisition) should become far less important. To a large extent, frequency is replaced by accessibility with respect to the dialogue context. In contrast, the analogous context in monologue does not lead to alignment and there is a strong tendency to avoid repetition in many genres (e.g., formal writing) so the value of local context will be much less. Frequency is central to comprehension of monologue because it is what people fall back on if they have no strong context. So a prediction of our account is that frequency effects will be dramatically reduced in dialogue.

With respect to lexical ambiguity, we predict that context will have a very strong role, so that effects of meaning frequency can be overridden. Most current theories of lexical ambiguity resolution follow Swinney (1979) in assuming that multiple meanings of an ambiguous word are accessed in a bottom-up manner, largely irrespective of context. Similarly, differences in frequency do not affect access, unless perhaps one meaning is highly infrequent (see Balota, Paul, & Spieler, 1999; Moss & Gaskell, 1999, for discussion). But in dialogue, only the contextually relevant meaning may be activated (or, in a modular account, the irrelevant meaning may always be suppressed rapidly). Hence, an interlocutor will straightforwardly adopt the appropriate meaning. An implication is that dialogue context should allow "subordinate bias effect" to be overridden (Duffy, Morris, & Rayner, 1988). According to Duffy et al., context can support the less frequent meaning and make it as accessible as the more frequent meaning, but it cannot cause the less frequent meaning to become more accessible than the more frequent meaning (Binder & Rayner, 1998; cf. Kellas & Vu, 1999; Rayner, Pacht, & Duffy, 1994). Although this may be true for reading (and monologue processing generally), it may not hold for dialogue.

The comprehension of routines is in a sense like lexical comprehension, in that their "frequency" and interpretation is set by the dialogue. However, this effect is in fact so strong that it appears to occur in monologue comprehension as well. A great deal of work is concerned with the comprehension of novel compounds in isolation (e.g., Murphy, 1988; Wisniewski, 1996), and the interpretations assigned depend on specific aspects of the words combined. Strong discourse contexts appear to enable direct access to infrequent interpretations of compound nouns such as baseball smile in reference to the smile of a boy given a baseball (Gerrig & Bortfeld, 1999). This would indicate that people can also "shortcircuit" the normal access to the individual nouns in a compound when there is a restricted meaning available from the immediate context.
 
 

The autonomous transmission model assumes that the speakers construct a message, formulate an utterance as a series of linguistic representations and then articulate it as sound; and the listener then hears the message, converts it into linguistic representations and then comprehends it. The interlocutors (ideally) end up with the same semantic representation, and alignment at other levels is a derivative process (if it ever occurs at all). In contrast, Fig. 3 proposes that interlocutors align themselves at different levels simultaneously via a basic automatic mechanism, and the parity assumption insures that the same representations are used in production and comprehension. Self-monitoring uses the same mechanism of alignment, but — as it were — within the speaker.

All models assume that speakers monitor their own output, so that, for instance, they are able to interrupt their productions in order to change what they say (Hartsuiker & Kolk, 2001; Levelt, 1983; Levelt, 1989). This can occur either before or after they start to produce a word. According to Levelt, speakers monitor their own productions by using the comprehension system (cf. Postma, 2000, for discussion of alternatives). They can monitor their actual outputs, in which case comprehension proceeds in an essentially normal way. According to a model that only contained this outer loop, monitoring would fit straightforwardly into the autonomous transmission model shown in Fig. 2. The only difference would be that both interlocutors were the same person. However, Wheeldon and Levelt (1995) assume the existence of an inner loop as well, that acts upon the phonological representation (Wheeldon & Levelt, 1995). Additionally, Levelt assumes that monitoring can occur within conceptualization, in order to make so called "appropriateness repairs," for instance. Now it is impossible to include "inner" monitoring straightforwardly within the autonomous transmission model, because the monitor has access to a representation that the interlocutor does not have access to. From another perspective, it is unclear how the inner loop or the loop within the conceptualizer should have developed, given that they bear no relationship to any process involved in comprehending one’s interlocutor. The postulation of a monitor that uses the comprehension system is parsimonious (and it is easy to see how it could have evolved), but the postulation of special loops that correspond to no aspect of comprehension is not.

In contrast, the existence of the inner loop and the loop within the conceptualizer fits straightforwardly into the alignment model. Interlocutors have access to each others' semantic and phonological representations. Hence a speaker also has access to her own representations at these levels. Given that such levels are available, there is no reason why the speaker should not automatically monitor at these levels. We propose that the speaker performs monitoring at these different levels in a way that leads to self-alignment. When the speaker produces an error at (say) the syntactic level (e.g., by selecting the wrong lemma), the result is a lack of alignment between the intended representation and the representation available to the monitor. Indeed, this misalignment will propagate to other levels of representation (e.g., the semantic level, because the wrong meaning is accessed). At this point, the speaker can initiate a self-correction, which involves a repair process essentially similar to the repair process used during interaction (see Section 4).

The alignment model makes the very interesting prediction that monitoring can occur at any level of linguistic representation that can be aligned. For example, we predict the existence of syntactic monitoring. Consider the misassignment of syntactic gender and its subsequent detection. Speakers clearly can begin to say Le tête and then correct to La tête. Obviously this detection could occur via the outer loop or via the phonological loop, and determining which level any error occurs at is clearly hard. But an important prediction of this account is that monitoring (and the correction of errors) can occur at the syntactic level (e.g., correcting gender, count/mass errors, errors of auxiliary selection, or errors of subcategorization), and at other levels as well. One reason for suspecting that this might be correct is that "other monitoring" (i.e., detecting errors in others’ speech) appears faster for phonological than syntactic errors (Oomen & Postma, in press). If self-monitoring of syntax occurred via the phonological loop, we would predict that it would be slow in comparison to self-monitoring of phonological errors. But we know of no evidence for this claim.

More generally, the existence of monitoring appears to be a consequence of dialogue. In dialogue, interlocutors have to switch between being speaker and listener rapidly and repeatedly, and interlocutors have to be able to listen and plan their next utterance at the same time (otherwise the lack of pauses, for instance, could not be explained). The obvious way in which this can occur is for interlocutors to be listening at all times, with that listening involving aligning one's representations with the input. If interlocutor A is speaking, then B is listening to A and thus aligning with A. But if A is speaking, then A listens to herself through monitoring and thus aligns with herself. In other words, monitoring is a by-product of a language processing system that is sufficiently flexible to allow comprehension and production to occur to some extent simultaneously in dialogue. This means, for example, that monitoring should be hard during periods of overlapping speech.
 
 

In the interactive alignment model, the mechanism that allows the joint construction of utterances (Clark & Wilkes-Gibbs, 1986) does not need to be stipulated but instead falls out of the alignment of representations. Because they share their interlocutor’s representation, speakers do not need to construct entire utterances from scratch if their interlocutor has done some of the work already. Instead, they can straightforwardly complete each others’ utterances. Thus, A produces an utterance (e.g., a question, the start of a sentence, or whatever); B aligns her representations with A’s; B then begins her utterance using these representations. Hence, dialogue turns need not be complete units at any standard syntactic or semantic level, but the combination of adjacent turns does lead to a complete unit. This account might not be controversial for the situation model, but is not normally assumed for other levels of representation.

For example, if A begins a sentence Mary donated ..., B can complete with the book to John. The utterance is a complete sentence, but produced by a joint act (see e.g., 43 & 44 in Table 1). Contrast two accounts of the process. The orthodox (monological) view is that B first parses A's utterance and assigns it a semantic interpretation. This assumes that the parser can interpret an ungrammatical input (though how this can be done is rarely specified). Then B must access the syntax and semantics (at least) associated with Mary donated but must suppress production of these words. B must also generate the appropriate grammatical frame for the sentence. Next B must "fill in" this frame by accessing and producing the book to John. A will in turn have to interpret B's "degenerate" utterance, presumably by integrating the two previous utterances into one complete sentence, via some special mechanisms that allows her to integrate fragments from herself and another speaker. If things are this complicated it is unclear why interruptions should occur at all,  ³ why they can occur so rapidly, or why producing language in such contexts is not manifestly harder, say, than monologue. It also predicts that elliptical responses to questions should be harder than non-elliptical ones. This is clearly incorrect (e.g., Clark, 1979 showed that full responses are complex and have special implicatures).

Contrast this with the straightforward account in which B, as listener, activates the same representations as A. These representations can be used in production in just the same way as in comprehension. On this account, dialogue should be more straightforward that monologue because B can make direct use of what A has already done. Thus, we predict that it should be as easy to complete someone else’s sentence as one’s own, but that completing should in general be easier than repeating. Similarly, interlocutors can finish off each others’ words (e.g., if one speaker has difficulty completing a word). This requires shared phonological representations. One prediction is that speech errors could be induced through perception as well as production (e.g., if interlocutor B finishes off A’s tongue twister, then B should be liable to produce errors).

As we have noted, one reason why theories of language processing are almost entirely based on monologue is that psycholinguistics derives many of its assumptions from linguistics, and linguistic theory is predominantly based on monologue. Thus, ideally, the development of a psycholinguistic theory of dialogue requires the corresponding development of a linguistic theory of dialogue. Such a theory has not developed, presumably because syntacticians focus on the grammaticality of sentences produced in isolation and assume that language produced in dialogue is so full of "performance errors" that it cannot be used to determine "competence" (Chomsky, 1965). Most theories accept that a few dialogue phenomena do need to be explained. For instance, "binding" theory (Chomsky, 1981) can be evoked to explain why himself is coreferential with John in Q: Who does John love? A: Himself. It is clearly impossible to have an adequate linguistic theory of such utterances if the question and the answer are considered separately — what is needed is an account of why the pair is grammatical. But such phenomena are regarded as peripheral to the study of syntax, which focuses on sentences produced by a single speaker (though cf. Ginzburg & Sag, in press). In contrast, we assume that question-answer pairs are actually a particularly orderly aspect of dialogue. In general, we need a linguistic account of well-formed dialogue utterances, and this account cannot be derived straightforwardly from linguistic theories based on monologue or citation speech.

Of course, some dialogue utterances are ill-formed, as for example when a speaker simply stops mid-utterance (Levelt, 1983). One reasonable assumption is that a well-formed dialogue turn must be a constituent, with a semantic interpretation. Its meaning can be combined with the meanings of other participants’ turns in a compositional manner, so that the interpretations of Mary donated and a book to John can be combined into a sentence meaning. This combination should be possible, as discussed in the last section, by linguistic mechanisms. For example, after A’s fragment, both interlocutors can construct a semantic representation in which that gave is the predicate and Mary is the agent, but which requires two further arguments to become a complete proposition.⁴ After B produces a book, the representation can be updated to include book as the patient. After B produces to John, the representation becomes a complete proposition in which John is added as the beneficiary. Notice that the sentence is comprehended incrementally. Contrast this with a traditional linguistic approach in which analyses and interpretations would have to be assigned to the two fragments individually, before they could be combined by inference over elliptical representations containing much missing information (e.g., Mary donated is assigned a semantics like "Mary gave someone something" and a book to John is assigned "something was done involving a book and an act directed to Sue," and these are combined by problem-solving). As such "bridging" inferences generally cause processing difficulty (Haviland & Clark, 1974), the traditional account would again counterfactually predict that dialogue would be difficult.

The best-known linguistic approach that accords with the demands of incrementality is Combinatorial Categorial Grammar (Steedman, 2000; cf. Ades & Steedman, 1982; Phillips, in press; Pickering & Barry, 1993). It allows most (but not all) fragments to be constituents, and therefore a plausible candidate for analyzing the syntax of dialogue. It also allows incremental processing, so that a fragment like Mary donated has the category S/PP/NP, which indicates that it is a sentence missing a noun phrase followed by a prepositional phrase. After a book is added, the fragment has category S/PP (a sentence missing a prepositional phrase); after to John, it has category S (a sentence). This approach can deal with "well-behaved" dialogues like question-answer pairs and of course monologue. It also provides a natural account of routines, because these may be constituents within flexible categorial grammar but not traditional linguistics (e.g., He’s overtaking; Kuiper, 1996). Such linguistic proposals have already had some impact on psycholingustic accounts concerned primarily with monologue comprehension (e.g., Altmann & Steedman, 1988; Pickering & Barry, 1991).

Our account predicts that incremental interpretation should occur in dialogue to the same extent that it does in monologue, and that there should be incremental integration of the contributions of different participants. So participants should notice that a piece of dialogue can become anomalous during a joint utterance (e.g., A: you donated; B: an atom). They can then immediately seek to correct it.

The shift from the model in Fig. 2 to the alignment model in Fig. 3 is compatible with a shift from a Chomskyean "transformational" theory, with a central generative syntactic component and peripheral semantic and phonological systems that are purely "interpretative," to a theory with multiple generative components (see Jackendoff, 1997, 1999). In Chomskyean approaches (whether Standard Theory, Government and Binding Theory, or Minimalism), syntax creates sentence structure, and sound and meaning are "read off" this structure (Chomsky, 1965, 1981, 1995). In contrast, many alternative linguistic theories now assume that phonology and semantics have complex combinatorial structures of their own (see Jackendoff, 1999 for references). On this account, phonological, syntactic, and semantic formation rules generate phonological, syntactic, and semantic structures respectively, and are brought into correspondence by "interface rules" which encode the relationship between sound and syntax, or syntax and meaning.

In processing terms, this notion of multiple generative components fits straightforwardly with Fig. 3. During dialogue, the interlocutors tend to align at all different levels of representation. For example, during production, the processor will tend to employ the same phonological, syntactic, and semantic rules where possible. On this account, language production and comprehension involve mapping between the different levels by drawing on the appropriate rules. In contrast, it is much more difficult to see why alignment should occur at phonological and semantic levels if no generative component underlies these levels. Moreover, the correspondence between the Chomskyean architectures and models of production and comprehension has always been difficult to sustain (e.g., Bock et al., 1992; Fodor, Bever, & Garrett, 1974; Pickering & Barry, 1991). Thus, we see the integration of a framework incorporating multiple generative components with a grammar that has a flexible approach to constituency as forming the linguistic basis for a psycholinguistic account of dialogue.
 
 

In this paper we have argued that dialogue is the primary setting for language use and hence that dialogue processing represents the basic form of language processing. However, people also engage in monologue. When writing and reading letters, newspaper articles, or books people produce and comprehend language without being able to interact directly with any interlocutor. Even face-to-face group discussions can involve language processing of a monologue kind. Fay, Garrod and Carletta (2000) found that in large groups there is little evidence of alignment between consecutive contributors to the discussion. Their results are consistent with a transition from a dialogue style of language processing in small groups to a monologue style in large groups. But, what exactly is the difference and how do dialogue mechanisms relate to the processing of monologue?

Monologue offers is no opportunity for inter-speaker interaction. Hence, in production, the speaker will have to formulate everything on her own, and receive no help in decisions about, for example, choice of word or syntactic form. In comprehension, the listener will have to bring to bear appropriate inference skills. For example, he will have to draw bridging inferences to help understand what the writer or speaker had really meant with a definite reference (Garrod & Sanford, 1977; Haviland & Clark, 1974). In monologue there is no opportunity to call on aligned linguistic representations and no opportunity to resolve ambiguities using interactive alignment. Instead people will have to fall back on the frequency of words, syntactic forms, and meanings in making production or comprehension decisions. In effect, language processing will move from a predominantly top-down organisation built on the scaffolding of alignment to a predominantly bottom-up organisation determined by the only information available — the utterance itself in comprehension, or the speaker’s intentions in production.

As a result, language users will need to develop a whole range of elaborate strategies to become competent processors of monologue. Otherwise, they are much more likely to go off track during comprehension and production. Even after these strategies have been developed, people still find monologue far more difficult than dialogue. We believe that a true understanding of both dialogue and monologue requires an understanding of the basic processing skills that have developed, primarily, to deal with dialogue.

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2. Notice that the description "fourth row" is ambiguous in relation to the maze (it could be near the top or near the bottom). Successful interpretation depends on an agreed ordering function for rows, as a result of which "row" refers to the ordered set of rows. On the other hand, the description "bottom row" is self-contained and unambiguous, with "row" denoting an unordered set of rows. [return]

3. Estimates from small group dialogues indicate that as many as 31% of turns are interrupted by the listener (Fay, Garrod, & Carletta, 2000).[return]

4. This can be represented more formally as lx lygave (mary, x, y), which is a function that takes two further arguments to become a proposition, and where the arguments are represented in the order agent, patient, beneficiary. In our example, the complete proposition is gave (mary, book, john).[return]
 
 

We wish to thank Ellen Bard, Holly Branigan, Nick Chater, Herb Clark, Rob Hartsuiker, Tony Sanford, Philippe Schyns, Patrick Sturt and members of the psycholinguistic groups at the universities of Edinburgh and Glasgow for valuable comments, criticisms and helpful suggestions on earlier versions of this paper.