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The projection of the opossum's visual field on the cerebral cortex

1978; Wiley; Volume: 177; Issue: 4 Linguagem: Inglês

10.1002/cne.901770404

1096-9861

Aglai P. B. Sousa, Ricardo Gattass, Ė. Oswaldo‐Cruz,

Neurobiology and Insect Physiology Research

Abstract The visual cortex of the opossum was studied by means of evoked potential and multi‐unit recordings, and a map of the visual field in V I was constructed: Evoked responses were observed in regions extending beyond the boundaries of striate cortex. Different patterns of response were obtained depending on the cortical region undergoing exploration: in a posteromedial region, coincident with striate area, complex waveforms were obtained, contrasting with the simpler forms observed in a region localized anterolaterally. V I , which corresponds to cytoarchitectonic area 17, has a topography of projections similar to that found in other mammals: the upper visual field represented posteriorly, the temporal field represented medially, the lower field represented anteriorly and the nasal part laterally. A large proportion of V I is devoted to the region of binocular representation, which covers approximately100° at the level of the horizontal meridian. The cortical representation of the vertical meridian extends to approximately 60°above and below the center of gaze, suggesting that the cortical representation of the visual field is more restricted than the visual field available to the contralateral hemiretina. Our results suggest that the extreme periphery of the field is not represented in V I . In its lateral border the projection does not end at the vertical meridian, it extends 5° into the ipsilateral hemififield of vision, so that approximately 10° of visual field is represented in both hemispheres. There is a reversal of representation along the lateral border of V I , in a region which coincides with the cytoarchitectonic boundaries of striate and peristriate areas, suggesting the existence of at least a second representation of the visual field in the neocortex of the opossum, corresponding to V II of other animals. The magnification factor in V I varied from 7°/mm observed at the cortical representation of the center of gaze, to 30°/mm at the extreme periphery of the field, along the horizontal meridian. A comparable decrease was also observed along the vertical meridian, towards the upper and lower limits of the visual field.