Complex-unoriented cells in a subregion of primate area 18.

In primates, both the primary and secondary visual cortical areas can be subdivided histologically by staining for the mitochondrial enzyme cytochrome oxidase. In the primary visual cortex (area 17, the first cortical receiving area for visual information) these histological differences correspond to functional subdivisions, cytochrome-dark regions being concerned with information about colour and cytochrome-light regions concerned with form. Here we report that the second visual area, area 18, which receives its main cortical input from area 17 (refs 7,8), similarly has functional subdivisions that correspond to the cytochrome oxidase staining pattern. In area 18 the segregation between form and colour is maintained, reinforcing our notion that form and colour information follow parallel pathways. The specific differences between cells in areas 17 and 18 suggest that a possible step in hierarchical information processing is spatial generalization, analogous to the difference between simple and complex cells.