QUANTITATIVE STUDIES OF CORTICAL VISUAL PROCESSING

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Our objective in this research is to reveal the neural circuitry generating novel response properties in the primate primary visual cortex (area V1). The experiments are designed to uncover the role cortical dynamics, suppressive signals, receptive field non-linearities, and functional intra- cortical connectivity play in the generation of neural response selectivity to visual stimulus attributes: orientation, direction of motion, spatial frequency, length, and width. Dynamics of Tuning: We will measure the dynamics of tuning selectivity in the different V1 layers using reverse correlation techniques. A recent improvement to this method (the inclusion of "blank" images) provides the means to determine the dynamics of excitation and inhibition, and to establish their contribution to the cell's response. Using this methodology, we will characterize the dynamics of tuning to orientation, direction of motion, spatial frequency, length, and width. The hypothesis that sharp tuning selectivity is present from the very beginning of the response will be tested. Role of Suppression: We will test the hypothesis that suppressive signals are involved in shaping the tuning selectivity of neural responses. To facilitate the detection of suppressive signals in extracellular recordings, we will measure tuning curves on top of a conditioning stimulus (two-dimensional noise) whose purpose is to elevate the activity of the cortex above the usually low spontaneous rates. We will seek evidence of inhibition: regions in the tuning curve where responses are depressed below the baseline activity. Spatio-temporal Linearity: We will test the hypothesis that simple cells behave as a linear spatio-temporal filter followed by a static non-Linearity. We will test the hypothesis that simple method that avoids the problems associated with the contribution of the output non-Linearity to the response. A goal of this experiment is to determine if quasi-linear mechanisms are sufficient to account for the tuning properties of simple cells. Functional connectivity: We will begin to explore the pattern of cortical interactions in the dimensions of space, orientation and spatial frequency. To achieve this goal, we will record simultaneously from pairs of neurons when their responses are elevated by the use of a conditioning stimulus. These experiments are designed to study, from multiple conceptual angles, the neural mechanisms generating cortical selectivity.. Taken together, these findings will advance our understanding of the computations and strategies used by V1 to process thalamic input.