How many maps are there in visual cortex?

In addition to a topographic map of the retina, mammalian visual cortex con tains superimposed, orderly periodic maps of features such as orientation, eye dominance, direction of motion and spatial frequency. There is evidence that these maps are overlaid so as to ensure that all combinations of the different parameters are represented as uniformly as possible across visual space. However, it is unknown to what extent geometrical factors limit the number of periodic maps which might simultaneously be present, given this constraint. This paper attempts to investigate the question by using a dime nsion reduction model to generate maps of simple, many-dimensional feature spaces onto a model two-dimensional cortex. The feature space included a mo del retina, plus N binary variables, corresponding to parameters such as oc ular dominance or spatial frequency. The results suggest that geometrical f actors do not sharply limit the ability of the cortex to represent combinat ions of parameters in spatially superimposed maps of similar periodicity. C onsiderations of uniform coverage suggest an upper limit of six or seven ma ps. A higher limit, of about nine or ten, may be imposed by the numbers of neurons (or minicolumns) available to represent each of 2(N) features withi n a given small region of cortex.