A Comparison between a Neural Network Model for the Formation of Brain Maps and Experimental Data

G.G. Blasdel Harvard Medical School Harvard University Boston, MA 02115 Recently, high resolution images of the simultaneous representation of orientation preference, orientation selectivity and ocular dominance have been obtained for large areas in monkey striate cortex by optical imaging [1-3]. These data allow for the first time a "local" as well as "global" description of the spatial patterns and provide strong evidence for correlations between orientation selectivity and ocular dominance. A quantitative analysis reveals that these correlations arise when a fivedimensional feature space (two dimensions for retinotopic space, one each for orientation preference, orientation specificity, and ocular dominance) is mapped into the two available dimensions of cortex while locally preserving topology. These results provide strong evidence for the concept of topology preserving maps which have been suggested as a basic design principle of striate cortex [4-7]. Monkey striate cortex contains a retinotopic map in which are embedded the highly repetitive patterns of orientation selectivity and ocular dominance. The retinotopic projection establishes a "global" order, while maps of variables describing other stimulus features, in particular line orientation and ocularity, dominate cortical organization locally. A large number of pattern models [8-12] as well as models of development [6,7,13-21] have been proposed to describe the spatial structure of these patterns and their development during ontogenesis. However, most models have not been compared with experimental data in detail. There are two reasons for this: (i) many model-studies were not elaborated enough to be experimentally testable and (ii) a sufficient amount of experimental data obtained from large areas of striate cortex was not available. 83 84 Obermayer, Schulten, and Blasdel Figure 1: Spatial pattern of orientation preference and ocular dominance in monkey striate cortex (left) compared with predictions of the SOFM-model (right). Isoorientation lines (gray) are drawn in intervals of 11.25° (left) and IS.00 (right), respectively. Black lines indicate the borders (ws(rj = 0) of ocular dominance bands. The areas enclosed by black rectangles mark corresponding elements of organization in monkey striate cortex and in the simulation result (see text). Left: Data obtained from a 3.1mm x 4.2mm patch of the striate cortex of an adult macaque (macaca nemestrina) by optical imaging [1-3]. The region is located near the border with area IS, close to midline. Right: Model-map generated by the SOFM-algorithm. The figure displays a small section of a network of size N = d = 512. The parameters of the simulation were: € = 0.02, tTh = 5, vr,:x = 20.48, vrax = 15.36, 9 . 101 iterations, with retinotopic initial conditions and periodic boundary conditions. 1 Orientation and ocular dominance columns in monkey