A MODEL FOR THE COORDINATED DEVELOPMENT OF COLUMNAR SYSTEMS IN PRIMATE STRIATE CORTEX

The existence of patchy regions in primate striate cortex in which orientation selectivity is reduced, and which lie in the centers of ocular dominance stripes is well established (Hubel and Livingstone 1981). Analysis of functional maps obtained with voltage sensitive dyes (Blasdel and Salama 1986) has suggested that regions where the spatial rate of change of orientation preference is high, tend to be aligned either along the centers of ocular dominance stripes, or to intersect stripe borders at right angles. In this paper I present results from a developmental model which show that a tendency for orientation selectivity to develop more slowly in the centers of ocular dominance stripes would lead to the observed relationships between the layout of ocular dominance and the map of orientation gradient. This occurs despite the fact that there is no direct connection between the measures of preferred orientation (from which the gradient map is derived) and orientation selectivity (which is independent of preferred orientation). I also show that in both the monkey and the model, orientation singularities have an irregular distribution, but tend to be concentrated in the centers of the ocular dominance stripes. The average density of singularities is about 3/lambda(theta)2, where lambda(theta) is the period of the orientation columns. The results are based on an elaboration of previous models (Swindale 1980, 1982) which show how, given initially disordered starting conditions, lateral interactions that are short-range excitatory and long-range inhibitory can lead to the development of patterns of orientation or ocular dominance that resemble those found in monkey striate cortex. To explain the coordinated development of the two kinds of column, it is proposed that there is an additional tendency in development for the rate of increase in orientation selectivity to be reduced in the centers of emerging ocular dominance stripes. This might come about if a single factor modulates plasticity in each cell, or column of cells. Thus plasticity may be turned off first in regions in the centers of ocular dominance stripes where relatively extreme and therefore stable ocular dominance values are achieved early in development. Consequently it will be hard for cells in these columns to modify other properties such as orientation preference or selectivity.