Periodicity of visual cortical modules in normal and strabismic monkeys

Purpose. Some models of visual cortical development are based on the assumption that the tangential organization of V1 is unspecified, and correlation between the two eyes' inputs drives the formation of cortical modules. In these models, correlated activity is a key element in the formation of visual cortical modularity. When this correlation is reduced, the constraint is weakened and these models predict an increase in the spacing of visual cortical modules. To examine this prediction we measured the periodicity of cytochrome-oxidase blob defined modules in the visual cortex of monkeys reared with either normal binocular vision or strabismus. Methods. The distribution of blobs was examined in 7 normal and 5 strabismic monkeys. Tangential sections through the upper layers of the visual cortex were stained to reveal the 2-dimensional pattern of blobs. An observer, blind to the experiment, located the center of the blobs and digitized the 2D array of blob-centers. The center-to-center spacing of the blobs was calculated using a 2D nearest-neighbor spatial analysis (Delaunay Triangles). Results. The mean center-to-center spacing of the blobs is 590μm for the normal and 598μm for the strabismic monkeys. The spacing of the blobs in monkeys reared with a strabismus is not significantly different from the spacing in monkeys reared with normal binocular vision. Conclusions. The periodicity of blobs is not changed by a reduction in the correlation between the two eyes' inputs. While neural activity must play a key role in the elaboration and refinement of visual cortical modules it does not set the periodicity. This suggests that the neural circuitry underlying cortical periodicity is established prenatally and is not modifiable by experience.