Functional organization of visual cortex in bush babies and owl monkeys revealed by optical imaging of intrinsic signals

Abstract

Considerable debate exists over how the cortex is able to represent an accurate, detailed map of visual space while simultaneously allowing different cells to be organized to analyze different stimulus attributes. A proposed conceptual model was that visual cortex is composed of iterated functional modules that together represent visual space seamlessly (Hubel and Wiesel, 1977). Concerns have been raised about the existence of modules and the degree to which functional modules are similar across species. In this thesis I used optical imaging to examine the functional organization of visual cortex of two distantly related nocturnal primates, the prosimian bush baby and the New World owl monkey. I was able to demonstrate in two visual cortical areas, the primary visual area (V1) and the middle temporal visual area (MT) that optical imaging could be used as a high-resolution tool, but that optical activity likely reflects both spiking activity and subthreshold neural changes. I also showed that the organization of orientation preference in owl monkey V1 and bush baby MT was quite similar to that found in other primates. In V1, CO blobs and orientation pinwheels showed a 1:2 relationship. In MT, each orientation preference domain showed compartments that represent different directions of motion. In V2 the organization of orientation preference of owl monkeys and bush babies, however, differed. In bush baby V2, orientation was represented continuously while in owl monkey V2 was divided into four functional compartments with only two compartments showing evidence of high orientation selectivity. In owl monkey V3, zones of high and low orientation selectivity that correlated with zones of CO dark and light patches, respectively were identified. In bush baby V1, investigations of spatial frequency preference showed that this property was organized continuously across the cortex. Finally, I showed that no 1:1 relationships exist between functional maps regardless of eccentricity suggesting that, although each attribute appears to be represented at each visuotopic location, no common modular units exist in visual cortex.