Retinotopic orgarnzation of the primary visual cortex of flying foxes (Pteropus poliocephalus and pteropus scapulatus)

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Abstract
The representation of the visual field in the occipital cortex was studied by multiunit recordings in seven flying foxes (Pteropus spp.), anesthetized with thiopentone/N2O and immobilized with pancuronmm bromide. On the basis of its visuotopic organization and architecture, the primary visual area (VI) was distinguished from neighboring areas. Area VI occupies the dorsal surface ofthe occipital pole, as well as most of the tentorial surface of the cortex, the posterior third ofthe mesial surface of the brain, and the upper bank of the posterior portion ofthe spiemai sulcus. In each hemisphere, it contains a precise, visuotopically organized representation of the entire extent of the contnateral visual hrmifield. The representation of the vertical meridian, together with 8–15° of ipsilateral hemifield, forms the anterior border of VI with other visually responsive areas. The representation of the horizontal meridian runs anterolateral to posteromedial, dividing VI so that the lower visual quadrant is represented medially, and the upper quadrant laterally. The total surface area of VI is about 140 mm2 for P. poliocephalus, and 110 Ill∼ for P. scapulattlS. The representation of the central visual field is greatly magnified relative to that ofthe periphery. The cortical magnification factor decreases with increasing eccentricity, following a negative power function. Conoversely, receptive field sizes increase markedly with increasing eecentricity, and therefore the point-image size is approximately constant throughout VI. The emphasis in the representation of the area centralis m VI is much larger than that expected on the representation of the area flat-mounted retinas. Thus, a larger degree of convergence occurs at the peripheral representations in the retino-geniculo-Gortical pathway, in comparison with the central representations. The marked emphasis in the representation of central vision, the extent of the binocular field of vision, and the relatively large surface area of VI reflect the importance of vision in megachiropterans.