Functional topography in cat area 18.

Using closely spaced microelectrode penetrations, we have mapped the representation of the visual field and of several functional response properties on the surface of cat area 18. The representation of the visual field was anisotropic, with the magnification factor for vertical visual space being about 2-5 times greater than that for horizontal. The topographic anisotropy was paralleled by an anisotropy in the cortical point-spread function, a measure of the distribution in the cortex of cells with overlapping receptive-field centers. The spread of this distribution (measured as twice the standard deviation) was positively correlated with the magnification factor anisotropy, averaging about 0.6 mm along the mediolateral (M-L) axis and 1.2 mm along the anteroposterior (A-P) axis. Units with similar response properties were clustered across the cortical surface, but different response features were laid out in different ways. For units with orientations within a 90 degrees range, the clusters took the form of branching bands, with a center to center spacing of 1.25 +/- 0.13 mm. These bands ran from postero-medial to anterolateral across the cortical surface, roughly perpendicular to the 17/18 border. Units with similar preferred eye input were laid out in patches without a well-defined direction of elongation, and with a less well-defined periodicity that averaged 1.86 +/- 0.75 mm. Local correlations in direction preference extending over distances of 300 micron were found, together with frequent 180 degrees differences in the direction preferences of units close together. The organization of the map of orientation preference, and the way in which direction selectivity is overlaid on this map, is discussed in more detail in the accompanying paper (Swindale et al., 1987). No obvious structural relationships between the ocular dominance patches and the pattern of iso-orientation domains were observed. There were, however, obvious interrelationships between the topographic map and other functional response properties. The iso-orientation bands ran approximately perpendicular to the direction in which both the point-spread function and magnification factor were elongated. This will have the effect of maximizing the allocation of a full range of orientations by the cortex to each location in visual space.