Second and third visual areas of the cat: interindividual variability in retinotopic arrangement and cortical location

Publisher Website
Google Scholar
Abstract
The cortical location and the retinotopic arrangement of the 2nd (V2) and 3rd (V3) visual areas in the anesthetized and immobilized cat were investigated with single and multiple unit recordings. V2 and V3 are arranged side by side anterior and medial to V1 and occupy the lateral gyrus and the postlateral sulcus. V2 spreads to postlateral parts of the lateral sulcus and, occasionally, to the posterior suprasylvian gyrus. The contralateral lower hemifield is represented on the lateral gyrus, the area centralis and the horizontal meridian are found in most animals in the anterior part of the postlateral sulcus, and the representation of the upper hemifield occupies the posterior part of the postlateral sulcus. The detailed retinotopic arrangement of the visual field maps shows 2 characteristic features. The retinotopy at the V2/V3 border differs between lower and upper hemifield; the lower field arrangement resembles that of rodents, and the upper field arrangement is similar to that of primates. The periphery of a part of the visual field is not continuously represented, but forms patches or islands (Donaldson and Whitteridge, 1977). The islands are bounded by visual field representations closer to the vertical meridian. Cortical location and detailed retinotopic arrangement vary considerably from animal to animal, so that a representative map of V2 and V3 cannot be constructed. The entire hemifield is represented in V2 at least in some animals. In V3 the uppermost part of the vertical meridian seems not to be represented. In other animals only a restricted part of the contralateral visual field is represented in V2 or in V3. In these cases the receptive fields cover not > 50 degrees out in the lower hemifield or on the horizontal meridian. In a few cases the periphery of the horizontal meridian and the upper hemifield are not at all represented in V3, or only in an incomplete manner. The magnification factors (Daniel and Whitteridge, 1961) become progressively smaller from V1 to V2 to V3. The cortical volume occupied decreases from V1 to V3. In V1 and in V2 the magnification is highest along the lower vertical meridian. In V2 the magnification along the horizontal meridian is the smallest, whereas in V1 the magnification decreases progressively from the lower vertical, to the horizontal and to the upper vertical meridian. The relationship between retinal ganglion cell densities and cortical magnification factors is discussed.