Cortical Visual Pathways

In addition to the existence of distinct transmission channels in the retinocortical pathway, research that started nearly three decades ago suggests that extrastriate cortical areas are specialized for analyzing different aspects of the visual input. In one of the earliest observations of anatomically dissociable functional properties in the primate visual cortex, Zeki ( 1971 ) reported that cells in the middle temporal area (MT) of the macaque monkey (located in the posterior bank of the superior temporal sulcus; see Figure 21 ) responded selectively to visual motion, but not to stimulus color or form. On the other hand, neurons in area V4 (located rostral to the lunate and the inferior occipital sulcus) were found to be sensitive to the color and orientation of visual stimuli, and responded indiscriminately to a large range of directions of visual motion. The idea that higher level visual processes are served by two distinct pathways was further elaborated by Ungerleider & Mishkin ( 1982 ).