| The present study takes a spatiotemporal filtering perspective on one of the most prominent organizational principles of population activity in visual cortex, viz. columns or feature maps. Functional maps have been demonstrated in V1 for stimulus position, orientation, direction of motion and spatial frequency, as well as for ocular dominance, although not all maps are present in all species examined. The existence of such maps and the observed topological relationships between them has led to the proposal that they could serve as the basis for a spatial feature code. In this scheme, a specific combination of stimulus features would elicit a pattern of population activity that is predicted from the intersections of the independent feature maps. However, several properties of neural receptive fields in primary visual cortex make this scenario unlikely. While neural responses to stimulus position, orientation and eye of stimulation are indeed independent of each other, the same is not the case for orientation, direction of motion and spatial frequency. Spatiotemporal frequency theory that models neurons not as feature detectors but as frequency filters having bandpass properties in the two spatial and the temporal dimension, predicts several critical interdependencies between the neural selectivity for these three features. Here we demonstrate for the first time, using optical imaging of intrinsic signals, the population level consequences of spatiotemporal filtering that force us to revise our current notions of what is mapped in primary visual cortex. We also construct a simple receptive field model to show that such filtering can indeed account for the neural behavior we observe. Finally, we investigate the related question of the mapping of spatial frequency preference and show that this map, at least in the ferret, is dependent upon the maps of orientation and visual space. Thus the body of work presented in this thesis, taken together, argues against the view of V1 as consisting of several overlapping feature maps (of orientation, direction and spatial/temporal frequency) and in favor of preferred position in 3D frequency space being the most prominent functional receptive field property mapped in primary visual cortex. |
