The nature of the boundary between cortical visual areas II and III in the cat.

The representation of the visual field in the second and third visual cortical areas (V n and V h i) of the cat was examined by microelectrode recording. The position of the field maps and the arrangement of the map within V n were found to vary greatly from one cat to another so that no single composite map can be made. The horizontal meridian of the visual field was found to run laterally and forward from V i across V n to V ni. The reversal of field sequence, which indicates the V ii/V in boundary, was very variable both from cat to cat and in the same cat for points above and below the horizontal meridian. The commonest situation was one in which the reversal point was 40° for some lines of latitude, but for others the reversal point was only 615° out. This means an ‘island’ of representation of points 40° out was bounded by areas of representa­ tion much closer to the vertical meridian. In some cats one ‘island’ was plotted, in one there were two completely plotted and in others there were two ‘islands’, one complete, one incompletely plotted. In one cat no ‘island’ was found, and the boundary between V ii and V m seemed to be formed anteriorly and posteriorly by the vertical (longitudinal) meridian 20° out. The islands contain many units with markedly elongated receptive fields whose particular function is not yet clear. The arrangement of the V i i /V m boundary found in these experiments is compared to that previously suggested and to present knowledge of the mapping in primate visual cortex.