The goal of this research proposal is to understand the role of primary visual cortex (V1) and second visual cortical area (V2) in real and illusory brightness perception. In the previous support cycle, we investigated the organization of real and illusory contour processing domains in V1 and V2 of the macaque monkey visual cortex. We found that signals from V1 and V2 are similar during the processing of real contours, but complementary during that of illusory contours (Ramsden et al., 2001). These findings may suggest that the encoding of contour identity is dependent on the coordinated activation of multiple cortical areas. In this support cycle, we propose to examine whether the processing of real and illusory brightness bears any parallels with that of real and illusory contour processing. Our perception of surface brightness is determined both by absolute luminance as well as by luminance contrast. Recently, studies using simultaneous contrast stimuli established that a brightness response to luminance modulation of flanking regions can be observed in Area 17 of the cat (Rossi et. al., 1996; Rossi & Paradiso, 1999). Although this demonstrated that brightness percepts due to surface luminance contrast can be implemented as early as primary visual cortex, it remained unclear whether contrast borders also contribute to this brightness response. Here, we propose to examine the neural processing of a border-induced brightness percept, the Craik-O'Brien-Cornsweet illusion. Using electrophysiological and optical imaging methods, we aim to examine the responses of V1 and V2 in both the cat and monkey to real and border-induced brightness stimuli.
We aim to localize cortical compartments in V1 and V2 involved in real and illusory brightness processing. In addition, we will determine, by studying single-unit responses, the phase relationships of real and illusory responses, and the spatial extent of illusory response in the cortex. By using different illusory brightness conditions, we aim to identify surface and border contributions to the neural encoding of brightness perception.
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