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The functional organization of area V2, II: The impact of stripes on visual topography

Published online by Cambridge University Press:  02 July 2002

STEWART SHIPP
Affiliation:
Wellcome Department of Cognitive Neurology, University College, Gower Street, London WC1E 6BT, UK
SEMIR ZEKI
Affiliation:
Wellcome Department of Cognitive Neurology, University College, Gower Street, London WC1E 6BT, UK

Abstract

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.

Type
Research Article
Copyright
2002 Cambridge University Press

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