|Ph.D Thesis||Department of Biomedical Engineering|
|Supervisor:||Assoc. Prof. Gur Moshe|
How is visual information encoded into responses of cortical neurons? This thesis addresses this fundamental question by studying the neuronal activity in primary visual cortex (V1) of alert and behaving monkeys. The goal was to examine stimulus-response relationships in the situation similar to "natural vision". We recorded eye movements and extracellular responses from single cells in V1 of macaques performing a fixation task, while presenting an extensive array of visual stimuli. The data were examined with techniques from systems analysis, statistics, and computer simulations. The major findings are: (1) Fixational eye movements strongly affect the firing of V1 neurons, in a way predictable from receptive field properties combined with information about stimulus-RF spatiotemporal interactions imposed by these movements. (2) Most neurons had two "activating regions" (AR), one responsive to increments of the luminance and other responsive to decrements. The majority (complex cells) had overlapping increment and decrement ARs, while a smaller group (simple cells) had separate ARs. Spatial mapping distinguishes between these cell types on the basis of quantitative overlap index, provided that eye movements are taken into account. (3) Spatial simple/complex dichotomy is not equivalent to "linear/nonlinear" classification by modulation to drifting gratings, since many complex cells respond with significant pseudolinear component at the grating temporal frequency. (4) Depending on grating spatial and temporal attributes, complex cells display a variety of behaviors ranging from nonlinear unmodulated firing, frequency doubling and subharmonic modulation to pseudolinear modulation. Thus, stimulus attributes are encoded in the harmonic content of the response and not only in its amplitude. Existing cortical models do not account for response diversity exhibited by complex cells. These findings place physiological constraints on models of visual processing, and suggest that complex cells represent basic functional units by which primate V1 extracts the information from visual scene.