This proposal addresses the way the visual system processes complex shape. We focus on two intermediate visual areas, V2 and V4, located in the ventral processing stream immediately beyond primary visual cortex (area VI). These areas serve as the major input stages for higher-order shape processing areas in the temporal cortex. We propose neurophysiological experiments to investigate the way that shape is represented in these areas and the way that attention modulates these representations. Shape is difficult to describe and parameterize, so previous neurophysiological studies of shape processing have utilized simple, regular shapes that are experimentally convenient. However, intermediate shape processing is highly nonlinear, so results obtained with reduced stimulus sets may not generalize to other stimuli. We therefore propose to use both complex, natural stimuli and simpler stimuli such as gratings. To facilitate this, we are developing novel nonlinear regression algorithms to estimate the stimulus-response mapping functions of neurons in V2 and V4. The underlying shape dimensions represented therein can then be determined by applying visualization algorithms (developed in our laboratory) to the stimulus-response mapping functions estimated for single neurons. In another series of experiments we plan to investigate how extrastriate visual areas integrate information from earlier sensory areas. Finally, we propose to examine how visual attention affects shape representations in V2 and V4. We will accomplish this by quantifying the effects of selective attention to a specific shape (feature attention) and attention directed toward a specific location in space (spatial attention) on neuronal tuning curves. Successful completion of these projects will provide critical information to aid in development of quantitative computational models of shape processing in intermediate vision.
