Orientation selectivity in the common marmoset (Callithrix jacchus): the periodicity of orientation columns in V1 and V2

Orientation selectivity is a ubiquitous property of the primary visual cortex of mammals. Within the primate, orientation selectivity is arranged into vertical columns that are organized into a regular patchy pattern. Previous studies, in old world primates, have noted an anisotropy in this arrangement that appears to be due to the presence of ocular dominance columns within the same tissue. In addition, orientation selective responses appear to be arranged into bands of activity within the adjoining extrastriate region V2. Little is known about the precise arrangement of orientation columns within V2. In this study, we examined the layout of orientation columns within both V1 and V2 of a new world primate, the common marmoset, using optical imaging. New world primates have the advantage that, unlike the macaque, V2 exists on the cortical surface, a requirement for this form of optical mapping. We found the arrangement of orientation columns to be isotropic within marmoset V1 with an average repeat distance of around 575 mum, smaller than the repeat distance previously reported for the macaque. We found no evidence of ocular dominance within the animals tested supporting the claim that ocular dominance columns when present distort the mapping of orientation in V1. In V2 we found that orientation columns were larger and as in other primates were represented in discrete bands throughout V2. Orientation columns were spaced on average around 1 mm apart. This suggests that, at least in the marmoset, the visual system maps orientation at a different scale within V1 and V2.