Psychology

Abstract: In two replication studies we examined response bias and dependencies in voluntary decisions. We trained a linear classifier to predict �¢??spontaneous decisions�¢?? and in the second study �¢??hidden intentions�¢?? from responses in preceding trials and achieved comparable prediction accuracies as reported for multivariate pattern classification based on voxel activities in frontopolar cortex. We discuss implications of our findings and suggest ways to improve classification analyses of fMRI BOLD signals that may help to reduce effects of response dependencies between trials.

Abstract: It is shown that existing processing schemes of 3D motion perception such as interocular velocity difference, changing disparity over time, as well as joint encoding of motion and disparity do not offer a general solution to the inverse optics problem of local binocular 3D motion. Instead we suggest that local velocity constraints in combination with binocular disparity and other depth cues provide a more flexible framework for the solution of the inverse problem. In the context of the aperture problem we derive predictions from two plausible default strategies: (1) the vector normal prefers slow motion in 3D whereas (2) the cyclopean average is based on slow motion in 2D. Predicting perceived motion directions for ambiguous line motion provides an opportunity to distinguish between these strategies of 3D motion processing. Our theoretical results suggest that velocity constraints and disparity from feature tracking are needed to solve the inverse problem of 3D motion perception. It seems plausible that motion and disparity input is processed in parallel and integrated late in the visual processing hierarchy.

Abstract: Motion-induced blindness (MIB) describes the occasional disappearance of salient visual objects in the presence of moving features (Y. S. Bonneh, A. Cooperman, & D. Sagi, 2001). Here we test whether motion adaptation and the ensuing motion aftereffect (MAE) are sufficient to trigger disappearance of salient targets. In three experiments, observers adapted to either rotating or static stimuli. Immediately afterwards, a static test pattern was presented consisting of a mask with texture elements and three superimposed target dots in a triangular arrangement. Observers reported dot disappearance and reappearance. The results clearly show that illusory motion in a static test pattern, following motion adaptation, promotes the disappearance of target dots. Furthermore, disappearance is modulated by the depth relationship between test pattern and targets, increasing for targets placed stereoscopically behind the test pattern. We conclude that MIB is influenced by perceived relative motion between depth-segregated features.

Abstract: When counterphase spatio-temporal flicker is presented to the left and right eye continuous directional motion can be perceived. Here we investigate whether this type of dichoptic motion can be observed at different depth planes. Four observers indicated direction of motion for dichoptic motion stimuli, presented in a context containing crossed and uncrossed disparity information in different conditions. Our results show that despite the presence of disparity cues in the stimulus, discrimination of motion direction remained maximal at interocular phase offsets that correspond to binocular motion perception at zero disparity. This constraint brings into question perception of dichoptic motion as the result of an early binocular motion system. We compared our results with predictions of a computational stereo-motion model (Qian, 1994; Qian & Andersen, 1997). In contrast to our empirical results, simulations of cell activation in this hybrid energy model predicts maximal activation at non-zero disparities. It is concluded that perception of dichoptic motion is a by-product of early interocular combination at low contrasts rather than the result of a dedicated stereo-motion system.

Abstract: Psychophysical studies on 3-D motion perception have shown that perceived trajectory angles of a small target traveling in depth are systematically biased. Here predictions from Bayesian models are investigated that extend existing models of motion-first and stereo-first processing. These statistical models are based on stochastic representations of monocular velocity and binocular disparity input in a binocular viewing geometry. The assumption of noise in these inputs together with a plausible prior for 3-D motion leads to testable predictions of perceived trajectory angle and velocity. Results from two experiments are reported suggesting that disparity rather than motion processing introduces perceptual bias. Keywords: velocity difference, disparity change, motion in depth, uncertainty, stereo-motion

Abstract: We used the Pulfrich effect to investigate perception of motion in depth. Independent manipulation of spatial and temporal frequency content in stereoscopic motion stimuli revealed the tuning characteristics of motion-in-depth perception. Sensitivity to interocular phase difference between sinusoidally oscillating sine-wave gratings was measured in four observers who judged direction of motion in depth. Discrimination thresholds in terms of interocular phase difference were determined to investigate spatial and temporal tuning characteristics of a system that is based on interocular phase difference, interocular delay, binocular disparity and velocity difference. Temporal frequency tuning of interocular phase difference thresholds was band-pass and relatively dependent on spatial frequency variation. These results together with evidence from two control experiments support the idea that sensitivity to direction of motion in depth is limited by a stereo-motion system that monitors binocular horizontal disparity and motion rather than interocular phase difference, interocular delay, or interocular velocity difference.

Abstract: An efficient graph-theoretical decomposition technique is introduced treating inconsistencies in behavioral data as systematic adaptations rather than random errors. This technique, which is known as ear decomposition, reduces inconsistencies in any binary data set to a basis of directed cycles. Such a basis characterizes the data set in terms of inconsistencies and its size offers an improved measure of internal consistency. In two examples it is illustrated how different implementations of the ear decomposition technique can help to identify choices that are critical for violations of transitivity.

Abstract: A long-term sensory memory is believed to account for spatial frequency discrimination when reference and test stimuli are separated by long intervals. We test an alternative proposal: that discrimination is determined by the range of test stimuli, through their entrainment of criterion-setting processes. Experiments 1 and 2 show that the 50% point of the psychometric function is largely determined by the midpoint of the stimulus range, not by the reference stimulus. Experiment 3 shows that discrimination of spatial frequencies is similarly affected by orthogonal contextual stimuli and parallel contextual stimuli and that these effects can be explained by criterion-setting processes. These findings support the hypothesis that discrimination over long intervals is explained by the operation of criterion-setting processes rather than by long-term sensory retention of a neural representation of the stimulus.