Topography and functional organization of extrastriate areas V3 and V4
Fan, Reuben Hsing
Identification of homologous primate visual areas and common organizational strategies among visual areas are essential to determining if a basic plan for primate visual cortex and visual areas exists. In this dissertation, we argue that V3 should be considered a homologous area among primates and that the study of V4 modularity provides important insight into how visual areas organize and process information. Because galagos are part of the prosimian radiation of surviving primates, cortical areas that bear strong resemblances in prosimians and other primates provide a strong argument for their conserved existence. Using intrinsic signal optical imaging (IOI) and 100 electrode array, our data support the presence of V3 in galago visual cortex, thus supporting its inclusion as a homologous primate visual area. It is also critical to study how visual areas organize and process information in order to identify possible themes across visual areas. Using IOI in awake macaques, we uniquely examined organizational features and properties of highly foveal representations in macaque V4. We found that bands of color and orientation modules in foveal V4 repeat visuotopic positions with similar cortical magnification factors, analogous to the modality specific representations of the visual field in V2 stripes. These visuotopic relationships could be a common strategy used by all visual areas to efficiently process information. We also investigated the size and spatial frequency preferences for color and orientation selective domains in foveal V4. For size, we found that surround suppression in orientation domains tended to be greater than color domains inV4. We also discovered a bimodal Gaussian distribution in our optical imaging signal in response to changing spatial frequency for both color and orientation selective domains of V4. While the source for this bimodal finding is currently unclear, it is clear that our modular level study of the foveal representation in V4 has revealed organizational properties and response properties that will impact our understanding of how a visual area organizes and processes information.