Modular organization in area 21a of the cat revealed by optical imaging: comparison with the primary visual cortex.

Area 21a, located on the cat's lateral suprasylvian cortex, is considered as a higher-order cortical area. Little is known about its specific role in visual processing. In this study, the functional organization of area 21a was investigated by optical imaging of intrinsic signals and was compared to that of primary visual areas. We found a clear modular pattern for orientation selectivity in area 21a, with signal amplitude being four times lower than that in primary visual areas. There were no significant differences between the domains' characteristics, nor the tuning bandwidth, in areas of the primary visual cortex (17 and 18) and 21a. This suggests that the basic cortical structure is independent of the hierarchical level or function of one area. A uniform representation of spatial frequency was found in areas 17 and 18, as well as in area 21a. The mean preferred spatial frequency in area 21a was 0.30 c/deg. In contrast to area 18, no direction maps were observed in area 21a whether drifting gratings or random dot kinematograms were used. This study supports the proposal that area 21a plays a pivotal role along the ventral processing stream and is mainly involved in form processing.