Abstract not found

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

Abstract not found

Most models of disparity selectivity consider only the spatial properties of binocular cells. However, the temporal response is an integral component of real neurons ’ activities, and time-varying stimuli are often used in the experiments of disparity tuning. To understand the temporal dimension of V1 disparity representation, we incorporate a specific temporal response function into the disparity energy model and demonstrate that the binocular interaction of complex cells is separable into a Gabor disparity function and a positive time function. We then investigate how the model simple and complex cells respond to widely used time-varying stimuli, including motion-in-depth patterns, drifting gratings, moving bars, moving random-dot stereograms, and dynamic random-dot stereograms. It is found that both model simple and complex cells show more reliable disparity tuning to time-varying stimuli than to static stimuli, but similarities in the disparity tuning between simple and complex cells depend on the stimulus. Specifically, the disparity tuning curves of the two cell types are similar to each other for either drifting sinusoidal gratings or moving bars. In contrast, when the stimuli are dynamic random-dot stereograms, the disparity tuning of simple cells is highly variable, whereas the tuning of complex cells remains reliable. Moreover, cells with similar motion preferences in the two eyes cannot be truly tuned to motion in depth regardless of the stimulus types. These simulation results are consistent with a large body of extant physiological data, and provide some specific, testable predictions.

In macaque monkeys, the age at which neurons in the primary visual cortex (V1) become sensitive to interocular image disparities, a prerequisite for stereopsis, is a matter of conjecture. To resolve this fundamental issue in binocular vision development, we measured the responsiveness of individual V1 neurons in anesthetized and paralyzed infant monkeys as a function of the relative, interocular, spatial phase of dichoptic sine-wave gratings. We found that an adult-like proportion of units were sensitive to interocular image disparity as early as the sixth postnatal day, several weeks before the onset age for stereopsis in monkeys. The ocular dominance distributions of cells in infant monkeys were also indistinguishable from those of adults. Thus, at or only a few days after birth, V1 neurons are capable of combining neural signals from the two eyes as in adults and are sensitive to interocular image disparities. How-Our ability to generate a robust, three-dimensional percept of the world based on a pair of two-dimensional retinal images (stereopsis) requires an array of neurons in the visual cortex that can detect interocular image disparities (Marr and Poggio, 1979). In the primary visual cortex (V1) of mature cats and monkeys, signals from the two eyes are linearly combined (Ohzawa and Freeman, 1986a,b; Ohzawa et al., 1996; Smith et al., 1996a,b) and interocular differences in receptive-field position and/or structure (phase) are thought to provide critical disparity cues for both stereopsis and fusional eye movements

this article, we compare the predictions of two hybrid models: one in which position and phase shifts are constrained to produce subregion correspondence and an "unconstrained hybrid " model in which position and phase shifts are uncorrelated. We show that data on binocular response properties of individual simple cells are equally consistent with both of the hybrid models. However, data on the distribution of preferred disparities for binocular cells in cat areas 17 and 18 strongly favor the subregion correspondence model. In addition, although both hybrid models are equally consistent with observed relationships between interocular phase shift and preferred orientation, only the subregion correspondence model allows this relationship to emerge from a developmental process in which binocular RF organization has no explicit dependence on preferred orientation. Finally, we show that both hybrid models are equally consistent with existing joint measurements of interocular phase and position shifts (as determined relative to reference cells). Additional such measurements, involving groups of three or more simultaneously measured cells, are required to definitively decide between the models.

The shape of the contour separating two regions strongly influences judgments of which region is “figure ” and which is “ground.” Convexity and other figure–ground cues are generally assumed to indicate only which region is nearer, but nothing about how much the regions are separated in depth. To determine the depth information conveyed by convexity, we examined natural scenes and found that depth steps across surfaces with convex silhouettes are likely to be larger than steps across surfaces with concave silhouettes. In a psychophysical experiment, we found that humans exploit this correlation. For a given binocular disparity, observers perceived more depth when the near surface’s silhouette was convex rather than concave. We estimated the depth distributions observers used in making those judgments: they were similar to the natural-scene distributions. Our findings show that convexity should be reclassified as a metric depth cue. They also suggest that the dichotomy between metric and nonmetric depth cues is false and that the depth information provided many cues should be evaluated with respect to natural-scene statistics. Finally, the findings provide an explanation for why figure–ground cues modulate the responses of disparity-sensitive cells in visual cortex.

Abstract not found

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this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact