The overall goal of this project is to understand the function of cortical circuitry in macaque monkey primary visual cortex Vi, and how it contributes to visual perception. Experiments are designed to test models of the cortex. The crucial question in most models is, what is the nature of cortico-cortical interaction? I will pursue the following aims. 1. To measure the dependence of steady-state orientation selectivity on contrast. We have preliminary data that indicate there is some effect of contrast on selectivity for sine-grating. Also contrast invariance (or its absence) is crucially important for theories of orientation selectivity. Furthermore, I will measure orientation selectivity for flashed bars and edges on the same cells as for sine gratings, and also measure effects of contrast with these stimuli.2. To measure the dependence of orientation selectivity dynamics on contrast and context. Measuring orientation-tuning dynamics as a function of contrast will provide further strong constraints on models of orientation selectivity. Also, we will measure orientation dynamics for contours embedded (vs. not embedded) in visual objects, to probe whether perception-related signals are measurable in Vi with this technique. The dynamics of responses to step increases in contrast at preferred orientations will also be measured and compared with the reverse correlation results.3. To measure cross-orientation inhibition in plaids (and other 2D patterns). Another way to test whether or not there is recurrent interaction between orientation-tuned elements is to use a plaid stimulus (Wallach, 1935; Adelson and Movshon, 1982). In the particular plaid patterns I will use, the two 1-D components are near each other in orientation (say 20-300), but differ in temporal frequency.4. To examine the effect of random or natural-image backgrounds or other context on orientation selectivity. I want to measure tuning curves of stimuli placed on modulated backgrounds that will tend to elevate the cell's spike rate and get it away from threshold. There is the added question; will natural image backgrounds have a different effect from that of random noise? Steady state orientation tuning curves, as well as step responses, will be measured on these different backgrounds and compared with the dynamics from reverse correlation experiments on the same neurons.
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