The functional organization of area V2, I: Specialization across stripes and layers

Abstract

We used qualitative tests to assess the sensitivity of 1043 V2 neurons (predominantly multiunits) in anesthetised macaque monkeys to direction, length, orientation, and color of moving bar stimuli. Spectral sensitivity was additionally tested by noting ON or OFF responses to flashed stimuli of varied size and color. The location of 649 units was identified with respect to cycles of cytochrome oxidase stripes (thick-inter-thin-inter) and cortical layer. We used an initial 8-way stripe classification (4 stripes, and 4 “marginal” zones at interstripes boundaries), and a 9-way layer classification (5 standard layers (2–6), and 4 “marginal” strata at layer boundaries). These classes were collapsed differently for particular analyses of functional distribution; the main stripe-by-layer analysis was performed on 18 compartments (3 stripes × 6 layers). We found direction sensitivity only within thick stripes, orientation sensitivity mainly in thick stripes and interstripes, and spectral sensitivity mainly in thin stripes. Positive length summation was relatively more frequent in thick stripes and interstripes, and negative length/size summation in thin stripes. All these “majority” characteristics of stripes were most prominent in layers 3A and 3B. By contrast, “minority” characteristics (e.g. spectral sensitivity in thick stripes; positive size summation in thin stripes) tended to be most frequent in the outer layers, that is, layers 2 and 6. In consequence, going by the four functions tested, the distinctions between stripes were maximal in layer 3, moderate in layer 2, and minimal in layer 6.