Student Theses and Dissertations

Date of Award

2015

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Freiwald Laboratory

Abstract

Temporal and spatial context play a key role in vision as a whole, and in face perception specifically. However, little is known about the neurophysiological mechanisms by which contextual cues exert their effects. Anatomically distinct face patches in the macaque brain analyze facial form, and studies of the activity within these patches have begun to clarify the neural machinery that underlies facial perception. This system provides a uniquely valuable opportunity to study how context affects the perception of form. We used functional magnetic resonance imaging (fMRI) to investigate the brain activity of macaque monkeys while they viewed faces placed in either temporal or spatial context. Facial motion transmits rich and ethologically vital information, but the way that the brain interprets such natural temporal context is poorly understood. Facial motion activates the face patches and surrounding areas, yet it is not known whether this motion is processed by its own specialized neural machinery, and if so, what that machinery’s organization might be. To address these questions, we monitored the brain activity of macaque monkeys while they viewed low- and high-level motion and form stimuli. We found that, beyond classical motion areas and the known face patch system, moving faces recruited a heretofore-unrecognized face patch. Although all face patches displayed distinctive selectivity for face motion over object motion, only two face patches preferred naturally moving faces, while three others preferred randomized, rapidly varying sequences of facial form. This functional divide was anatomically specific, segregating dorsal from ventral face patches, thereby revealing a new organizational principle of the macaque face-processing system. Like facial motion, bodies can provide valuable social context, revealing emotion and identity. Little is known about the joint processing of faces and bodies, even though there is reason to believe that their neural representations are intertwined. To identify interaction between the neural representations of face and body, we monitored the brain activity of the same monkeys while they viewed pictures of whole monkeys, isolated monkey heads, and isolated monkey bodies. We found that certain areas, including anterior face patches, responded more to whole monkeys than would be predicted by summing the separate responses to isolated heads and isolated bodies. The supralinear response was specific to viewing the conjunction of head and body; heads placed atop nonbody objects did not evoke this activity signature. However, a supralinear context response was elicited by pixelated, ambiguous faces presented on bodies. The size of this response suggests that the supralinear signal in this case did not result from the disambiguation of the ambiguous faces. These studies of contextually evoked activity within the macaque face processing system deepen our understanding of the cortical organization of both visual context and face processing, and identify promising sites for future research into the mechanisms underlying these critical aspects of perception.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

Available for download on Saturday, April 22, 2017

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