Walsh & Hoyt: Segregation of Visual Inputs

The functional segregation of visual inputs in the primate visual system is now well documented. Retinal information is communicated to cortical neurons through a set of pathways that appear specialized to convey a particular class of visual information. For example, the parvocellular or P-pathway, named for its connections to simian striate cortex (area V1) via parvocellular layers 36 of the lateral geniculate nucleus (LGN), is characterized by color opponency and slow-conducting axons that convey sustained signals. This pathway, which corresponds to a psychophysical sustained channel, has stronger projections to secondary areas such as V4 and inferior temporal (IT) cortex, located in the inferior occipital lobe and adjacent temporooccipital regions. These regions, along the ventral or temporal cortical pathway (the ""what"" pathway), are presumed to play a role in the perception of color, luminance, stereopsis, and pattern recognition. In contrast, the magnocellular or M-pathway is characterized by large, fast-conducting axons that convey information about more transient visual signals. This pathway connects to areas in visual association cortex, including the middle temporal (MT) and medial superior temporal (MST) areas. These regions, located along the dorsal or parietal cortical pathway (the ""where"" pathway), are thought to analyze the spatial location and movement of objects in the panorama. Recent research has identified a koniocellular or K-channel of retinal information that is distinct from the M- and P-channels; less is known about its central inputs, but it might be important for color.