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Abstract:
Blasdel and Salama's sensory maps of orientation-selective edge detectors in the monkey striate cortex can be reduced to an idealized scheme in which orientation hypercolumns of the d- and l-type occur in alternating sequence (Fig. 1). This scheme resolves the apparent contradiction between linear and circular arrangements of successive edge directions in earlier accounts. The actual configuration of hypercolumns is in register with two possible templates for the self-organization of orientation selectivity: the isometric cytochrome oxidase blobs of the colour system, and the anisometric slabs of the ocular dominance system. The centers of the hypercolumns coincide with the blobs. Simulation of cortical self-organization shows this co-incidence even in the absence of template-specific interactions. However, blobs and slabs are symmetrical to these centers, and therefore no templates for the asymmetrical distribution of preferred orientation in the hypercolumns. The present simulation derives the pre-natal formation of an initial scheme from a hypothetical gradient of nervous activity. Post-natal formation, or maturation, of this scheme is achieved by visual experience. Simulation of corresponding interactions between simultaneously activated neurons illustrates both the gain in orientation selectivity (Figs. 2 and 3), and the optimization of farfield diversity and nearfield conformity (Figs. 4 and 5). The results are compatible with the actual distribution of blob-centered d- and l-hypercolumns, iso-orientation modules and orientation fractures in the monkey. A surprisingly similar distribution of blobless d- and l-hypercolumns is expected in the absence of the colour system. Applied to the apparently blobless cortex of the cat, the scheme explains the modulation of deoxyglucose uptake along the iso-orientation bands in a report of Löwel, Freeman, and Singer.
