Neurons in the visual cortex of monkeys respond selectively to the disparity between the images in the two eyes. Recent recordings have shown that some of the disparity-selective neurons in the primary visual cortex and the posterior parietal cortex are modulated by the distance of fixation. A population of such gain-modulated, disparity-selective neurons forms a set of basis functions of horizontal disparity and distance of fixation that can be used as an intermediate representation for computing egocentric distance. This distributed representation is consistent with psychophysical studies of human depth perception; in contrast, neurons explicitly tuned to distance are not consistent with how we perceive distance. In a population model that includes noise in the firing rates of neurons, the perceived distance is

The perception of transparency in binocular vision presents a challenge for any model of stereopsis. We investigate here how well human observers cope with stereo transparency by comparing their e#ciency between transparent and opaque depth judgments. In two experiments, the e#ciency measure was computed relative to an ideal observer to take into account the larger correspondence ambiguity in the transparent condition. We found that thresholds for human and ideal observers were consistently higher in the transparent condition than in the opaque condition, across a range of dot densities (Experiment 1) and disparity ratios (Experiment 2). E#ciencies (the ratio of human to ideal performance) were approximately equivalent for the opaque and transparent conditions across all stimulus conditions. Therefore, the cost for stereoscopic transparency can be accounted for by the greater correspondence problem in that condition. Indeed, the fact that e#ciencies were very low, around 1%, and decreased with increasing dot density demonstrates that human observers use far less information than is available to perform the task. This account contrasts with previous interpretations for the cost in stereoscopic transparency in terms of inhibitory interactions specific to transparent configurations. We relate our findings to a previous and comparable study of motion e#ciency, and discuss our findings in terms of a physiologically plausible model.

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : xiii I Introduction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 1 A. Spatial representations and sensori-motor coordination : : : : : : : : : 1 B. The posterior parietal cortex : : : : : : : : : : : : : : : : : : : : : : : 2 C. Neural code for spatial representations : : : : : : : : : : : : : : : : : : 4 1. Dynamic remapping : : : : : : : : : : : : : : : : : : : : : : : : : : 4 2. Gain modulation : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 3. The Zipser and Andersen Network : : : : : : : : : : : : : : : : : : 6 D. Parallel vectorial representations : : : : : : : : : : : : : : : : : : : : : 9 E. Thesis Outline : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 10 1. Hierarchy in spatial representations : : : : : : : : : : : : : : : : : 10 2. A basis function approach for spatial representation : : : : : : : : 11 II Egocentric spatial representation in early vision : :...

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