Psychology

Abstract: Despite the generally accepted notion that humans are very good and fast at recognizing familiar individuals based on their face, to date the actual speed at which this fundamental brain function can be achieved remains largely unknown. Here, two groups of participants were required to respond by finger-lift when presented with either a photograph of a personally familiar face (classmate), or an unfamiliar face, respectively. This speeded manual Go/No-go categorization task revealed that personally familiar faces could be categorized as early as 380ms, about 80ms faster than when unfamiliar face categorization was required. When response times were averaged across all eight stimulus presentations, we found that minimum RTs for both familiar and unfamiliar face decisions were substantially lower (310ms and 370ms). Analyses confirmed that stimulus repetition enhanced the speed with which faces were categorized irrespective of familiarity, and that repetition did not affect the observed benefit for familiar over unfamiliar face decisions. These data, representing the elapsed time from stimulus onset to motor output, put constraints on the time course of familiar face recognition in the human brain, which can be tracked more precisely by high-temporal resolution electrophysiological measures.

Abstract: Acquired prosopagnosia (AP) is characterized by impaired recognition of individual faces following brain damage. The nature of the functional impairment(s) underlying AP remains debated. Recent studies have demonstrated deficient processing of diagnostic information in the region of the eyes (Caldara et al., 2005); other studies suggest that patients fail to judge relative distances between facial features (Barton et al., 2002). We hypothesized that these apparently different observations are related to a common cause. More precisely, we suggest that AP arises due to an impairment of a process that reduces uncertainty about the nature/location of the diagnostic cues for face individualization: the ability to perceive multiple elements of a face as a single global representation (holistic processing). Being impaired at processing individual faces holistically, prosopagnosic patients would tend to perform relatively worse for processing facial areas containing multiple elements (i.e., the eyes), and for elements that are widely spaced apart. Here we tested PS, a single case of AP, at matching unfamiliar faces differing either with respect to local features or inter-feature distances, over the upper and lower areas of the face. A pilot study and Experiment 1 confirmed that PS was extremely poor at using information encompassing the eyes, but was also deficient at perceiving relative distances between features. When uncertainty about the location and nature of the diagnostic cue was removed in Experiment 2, PS' performance remained below normal range, but she improved substantially. Most interestingly, her pattern of performance across the different conditions appeared qualitatively identical to that of normal controls. In line with previous observations of PS and other cases of prosopagnosia, our findings indicate that the reduced reliance on the area of the eyes and on relative distances between features in AP may have a common underlying cause-the disruption of holistic processing of the individual face.

Abstract: Neuroimaging studies of humans have provided inconsistent evidence with respect to the response properties of the fusiform face area (FFA). It has been claimed that neural populations within this region are sensitive to subtle differences between individual faces only when they are perceived as distinct identities [P. Rotshtein et al. (2005) Nature Neuroscience, 8, 107-113]. However, sensitivity to subtle changes of identity was found in previous studies using unfamiliar faces, for which categorical perception is less pronounced. Using functional magnetic resonance adaptation and morph continua of personally familiar faces, we investigated sensitivity to subtle changes between faces that were located either on the same or opposite sides of a categorical perceptual boundary. We found no evidence for categorical perception within the FFA, which exhibited reliable sensitivity to subtle changes of face identity whether these were perceived as distinct identities, or not. On the contrary, both the posterior superior temporal sulcus and prefrontal cortex exhibited categorical perception, as subtle changes between faces perceived as different identities yielded larger release from adaptation than those perceived as the same identity. These observations suggest that, whereas the FFA discriminates subtle physical changes of personally familiar faces, other regions encode faces in a categorical fashion.

Abstract: Previous studies recording eye gaze during face perception have rendered somewhat inconclusive findings with respect to fixation differences between familiar and unfamiliar faces. This can be attributed to a number of factors that differ across studies: the type and extent of familiarity with the faces presented, the definition of areas of interest subject to analyses, as well as a lack of consideration for the time course of scan patterns. Here we sought to address these issues by recording fixations in a recognition task with personally familiar and unfamiliar faces. After a first common fixation on a central superior location of the face in between features, suggesting initial holistic encoding, and a subsequent left eye bias, local features were focused and explored more for familiar than unfamiliar faces. Although the number of fixations did not differ for un-/familiar faces, the locations of fixations began to differ before familiarity decisions were provided. This suggests that in the context of familiarity decisions without time constraints, differences in processing familiar and unfamiliar faces arise relatively early â?? immediately upon initiation of the first fixation to identity-specific information â?? and that the local features of familiar faces are processed more than those of unfamiliar faces.

Abstract: Prosopagnosia is an impairment at individualizing faces that classically follows brain damage. Several studies have reported observations supporting an impairment of holistic/conï¬?gural face processing in acquired prosopagnosia. However, this issue may require more compelling evidence as the cases reported were generally patients suffering from integrative visual agnosia, and the sensitivity of the paradigms used to measure holistic/conï¬?gural face processing in normal individuals remains unclear. Here we tested a well-characterized case of acquired prosopagnosia (PS) with no object recognition impairment, in ï¬?ve behavioral experiments (whole/part and composite face paradigms with unfamiliar faces). In all experiments, for normal observers we found that processing of a given facial feature was affected by the location and identity of the other features in a whole face conï¬?guration. In contrast, the patientâ??s results over these experiments indicate that she encodes local facial information independently of the other features embedded in the whole facial context. These observations and a survey of the literature indicate that abnormal holistic processing of the individual face may be a characteristic hallmark of prosopagnosia following brain damage, perhaps with various degrees of severity.

Abstract: Event-related potential (ERP) studies have shown that sensitivity to individual faces emerges as early as â?¼160 ms in the human occipitotemporal cortex (N170). Here we tested whether this effect generalizes across changes in viewpoint. We recorded ERPs during an unfamiliar individual face adaptation paradigm. Participants were presented ï¬?rst with an adapting face (â?¼3000 ms) rotated 30�° in depth, followed by a second face (200 ms) in a frontal view of either the same or a different identity. The N170 amplitude at right occipitotemporal sites to the second stimulus was reduced for repeated as compared to different faces. A bilateral adaptation effect emerged after 250 ms following stimulus onset. These observations indicate that individual face representations activated as early as 160 ms after stimulus onset in the right hemisphere show a substantial degree of generalization across viewpoints.

Abstract: Selective impairment of face recognition following brain damage, as in acquired prosopagnosia, may cause a dramatic loss of diagnosticity of the eye area of the face and an increased reliance on the mouth for identification (Caldara et al., 2005). To clarify the nature of this phenomenon, we measured eye fixation patterns in a case of pure prosopagnosia (PS, Rossion et al., 2003) during her identification of photographs of personally familiar faces (27 children of her kindergarten). Her age-matched colleague served as a control. Consistent with previous evidence, the normal control identified the faces within two fixations located just below the eyes (central upper nose). This pattern (location and duration) of fixations remained unchanged even by increasing difficulty by presenting anti-caricatures of the faces. In contrast, the great majority of the patientâ??s fixations, irrespective of her accuracy, were located on the mouth. Overall, these observations confirm the abnormally reduced processing of the upper area of the face in acquired prosopagnosia. Most importantly, the prosopagnosic patient also fixated the area of the eyes spontaneously in between the first and last fixation, ruling out alternative accounts of her behaviour such as, for example, avoidance or failure to orient attention to the eyes, as observed in autistic or bilateral amygdala patients. Rather, they reinforce our proposal of a high-level perceptual account (Caldara et al., 2005), according to which acquired prosopagnosic patients have lost the ability to represent multiple elements of an individual face as a perceptual unit (holistic face perception). To identify a given face, they focus very precisely on local features rather than seeing the whole of a face fromits diagnostic centre (i.e. just below the eyes). The upper area of the face is particularly less attended to and less relevant for the prosopagnosic patient because it contains multiple features that require normal holistic perception in order to be the most diagnostic region. Consequently, prosopagnosic patients develop a more robust representation of the mouth, a relatively isolated feature in the face that may contain more information than any single element of the upper face area, and is thus sampled repeatedly for resolving ambiguity in the process of identification.

Abstract: Prosopagnosia is a deficit in recognizing familiar faces following brain damage whose study offers invaluable clues to understand the neural and functional aspects of normal face processing. Most studies of prosopagnosia have focused on processing of unfamiliar faces. As processing of personally familiar faces is substantially different from unfamiliar or famous face processing, these approaches lack ecological and thus informative value with reference to the underlying cause(s) of prosopagnosia. Here we present the first systematic investigation of familiar face processing in PS, a well-known case of acquired prosopagnosia with a deficit restricted to faces (Rossion et al., 2003; Schiltz et al., 2006). Due to largely preserved perceptual functions and an excellent memory, PS has been able to compensate for her deficits and has been pursuing her profession in a kindergarten. We conducted an exhaustive set of behavioral experiments with photographs of the 26 children of 3 to 4 years old that were in contact with PS and her 2 colleagues during the whole year. Tasks included familiar/unfamiliar face decisions (whole or half face, or face parts), and forced choice matching to improve the patient's performance. In striking contrast to controls, PS was most efficient in recognizing familiar faces by using the mouth relative to the eyes of the face, in line with our previous observation using the Bubbles methods (Caldara et al., 2005). Moreover, despite being able to perceive metric distances between features, several experiments (whole-part interference, face composite effect, assessment of ratio of face relations) showed that PS does not process individual faces holistically. These results suggest that the ability to integrate facial features into a holistic individual representation centered on the eyes of the face is a critical component of a normal face recognition system that can be selectively disrupted following brain damage.