Representation of Three-Dimensional Space in Primate Visual Cortex

Author: Jeo, Richard M.

Year: 1998

Degree: Dissertation (Ph.D.)

Advisor: Allman, John Morgan

Committee Members: Allman, John Morgan; Konishi, Masakazu; Andersen, Richard A.; Laurent, Gilles J.

Option: Biology

DOI: 10.7907/4xqy-wf39

Abstract

Primates can accurately judge the size of objects from a wide range of viewing distances. Size perception depends on both the size of the image projected onto the retina as well as the perceived distance to the object. Size constancy is the ability to compensate for changes in retinal image size as viewing distance changes. This work describes experiments that examine the neural basis of size constancy in the primate visual system.

Viewing distance and gaze angle modulate neural responses in dorsal cortical areas (the "where" pathway, leading to parietal cortex); these areas are known to be involved in visuospatial coding. However, lesions to dorsal areas do not disrupt size constancy judgments in monkeys, suggesting that the visuospatial information utilized for size constancy is encoded in other areas. Area V4 is at an intermediate level in the ventral visual cortical pathway (the "what" pathway, leading to inferotemporal cortex) and contains a large proportion of size-selective neurons. We were thus interested in examining the representation of size and distance in primate area V4.

We found that over half of the neurons recorded from areas V1, V2, and V4 showed significant changes in response with viewing distance and observed that distance interacts with retinal size information by modulating the gain of the neural response. Thus, object distance is represented in ventral cortical visual areas suggesting that spatial processing is not confined to dorsal cortical areas. Another focus was to explore the influence of various cues to distance. We measured size and distance tuning under restricted viewing conditions that removed or reduced cues to distance. Results from these experiments indicate that a combination of retinal and extraretinal cues contribute to distance modulation in V1, V2 and V4.

The encoding of both retinal size and distance information by ventral cortical neurons 1s consistent with these neurons serving as a basis for size constancy. We speculate that like retinotopic maps, an abstract representation of three-dimensional space is a characteristic feature of all visual cortical areas.

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