Gary G. Blasdel

Singularities in Primate Orientation Maps

We report the results of an analysis of orientation maps in primate striate cortex with focus on singularities and their distribution. Data were obtained from squirrel monkeys and macaque monkeys of different ages. We find that approximately 80 of singularities that are nearest neighbors have the opposite sign and that the spatial distribution of singularities differs significantly from a random distribution of points. We do not find evidence for consistent geometric patterns that singularities may form across the cortex. Except for a different overall alignment of orientation bands and different periods of repetition, maps obtained from different animals and different ages are found similar with respect to the measures used. Orientation maps are then compared with two different pattern models that are currently discussed in the literature: bandpass-filtered white noise, which accounts very well for the overall map structure, and the field analogy model, which specifies the orientation map by the location of singularities and their properties. The bandpass-filtered noise approach to orientation patterns correctly predicts the sign correlations between singularities and accounts for the deviations in the spatial distribution of singularities away from a random dot pattern. The field analogy model can account for the structure of certain local patches of the orientation map but not for the whole map. Neither of the models is completely satisfactory, and the structure of the orientation map remains to be fully explained.

Wavelength-dependent differences between optically determined functional maps from macaque striate cortex.

This study investigates the role of wavelength in determining the source and dynamic range of activity-driven reflectance changes in macaque striate cortex. By using short (600 nm) and long (720 nm) wavelengths to map ocular dominance, orientation, and position from the same region of cortex on alternate trials, we isolated wavelength-dependent differences in the contributions of different tissue compartments. In agreement with previous reports, 600-nm illumination was found to produce optical signals that were more than twice the size of those obtained with 720-nm illumination. In addition, 600- and 720-nm images were found to correlate everywhere except in regions occluded by blood vessels, where the images obtained at 600 nm correlated with the overlying vasculature. Since the 720-nm images do not correlate with the vasculature, this difference suggests that differential images obtained under 600-nm illumination are disproportionately sensitive to vascular events (e.g., changes in blood flow, volume, etc.). This finding is supported by the absorption spectra of hemoglobin and its derivatives, which absorb 600-nm light 4-1000 times more strongly than 720-nm light. Hence, for the 40% of cortex covered by blood vessels larger than 50 microns, images obtained at 600 nm are dominated by the vascular compartment to the exclusion of signals from the neural compartment below.

1994 Special Issue: Putative strategies of scene segmentation in monkey visual cortex

The process of visual feature abstraction begins in the retina and proceeds at each subsequent level of visual processing. In this article we review recent advance in the functional mapping of striate cortex in primates and explore ways in which the observed maps may facilitate tasks of scene segmentation in subsequent cortical areas. In so doing we also explore the possible origins of these maps from simple self-organizing processes, based on Hebbian learning, and find that within this framework, the observed maps can be interpreted as the neighborhood preserving projections of five-dimensional feature space onto the two available dimensions of striate cortex.