The functional organization of area V2, II: the impact of stripes on visual topography.
We have examined the visuotopic organization of area V2 of macaque monkeys in relation to its modular construction, comprising repetitive cycles of stripes running perpendicular to the border with area V1. Receptive fields were plotted in anesthetised animals, mainly using long penetrations parallel to the V1 border crossing several stripes in dorsal V2 within the representation of paracentral, inferior visual field. We confirm that each set of modules (thick, thin, and interstripes) mounts an unbroken coverage of the visual field, since there is almost invariably some overlap between the aggregate fields recorded in successive stripes of the same class, at intervals of one cycle. Also as expected, penetrations perpendicular to the stripes record changes in eccentricity along an isopolar visual meridian. We measured the size of the point image along such an isopolar meridian in nine cases, and showed that on average it exceeds the length of a typical cycle; again, this implies that no point in space escapes analysis by any of the functional modules. The representation of eccentricity across a cycle of stripes resembles a "ratchet" model, in which the gradient of eccentricity across a single stripe exceeds the gradient across the full cycle, leading to discontinuities ("switchbacks") at the borders between stripes. The shift in eccentricity across the width of a stripe is sufficient to maintain a virtually continuous map across successive stripes of the same class; when coupled to receptive field scatter about the mean trend, this creates the overlap of aggregate fields. The presence of topographic discontinuities, in addition to functional ones, at stripe boundaries, reflects autonomy of function at the local level. At a global level, however, the organization of V2 may promote integration across stripes, since the range of visual overlap, and intrinsic connections, exceeds a single cycle. Furthermore, the reduplication of modules along the isopolar axis has the effect of increasing the isopolar magnification factor (mm/deg); the ratio of isopolar to iso-eccentric magnification is approximately 1.5:1, and this elongation abets the external configuration of V2 that, stretching concentrically around V1, has long been hypothesized to facilitate functional interaction between the two areas.
Shipp
Stewart
S
Wellcome Department of Cognitive Neurology, University College, London, UK. s.shipp@ucl.ac.uk
Zeki
Semir
S
D000818:Animals; D001931:Brain Mapping; D008251:Macaca; D010775:Photic Stimulation / Q000379:methods; D014793:Visual Cortex / Q000502:physiology*; D014794:Visual Fields / Q000502:physiology