Retinotopic organization of areas 20 and 21 in the cat

The retinotopic organization of cat cortex, in the vicinity of areas 20 and 21 as defined by Heath and Jones ('71), was determined using electrophysiological mapping techniques. Although the topography of the visual field representations in this region of cortex is more complex than that found in areas 17, 18, and 19, this region appears to contain four representations of the visual hemifield. The four cortical areas these visual field representations occupy have been labeled 20a, 20b, 21a, and 21b. These representations are not as precise as those found in areas 17, 18, and 19; the receptive fields in areas 20a, 20b, 21a, and 21b are larger and there is more scatter in receptive field location among adjacent cells. The upper visual hemifield is emphasized in all four of these areas, but the extent of the representation ranges from just the central 20° found in area 21a to nearly the entire upper visual quadrant found in area 20b. The peak areal magnification factors found in these areas are all at least one order of magnitude less than those found in area 17. The visual field transformations from retina onto cortex in areas 20a, 20b, 21a, and 21b are similar to the types we found in other cortical visual areas, and the significance of these transformations for visual processing is discussed.