RETINOTOPIC ORGANIZATION OF THE PRIMARY VISUAL-CORTEX OF FLYING FOXES(PTEROPUS-POLIOCEPHALUS AND PTEROPUS-SCAPULATUS)

The representation of the visual field in the occipital cortex was stu died by multiunit recordings in seven flying foxes (Pteropus spp.), an esthetized with thiopentone/N2O and immobilized with pancuronium bromi de. On the basis of its visuotopic organization and architecture, the primary visual area (V1) was distinguished from neighboring areas. Are a V1 occupies the dorsal surface of the occipital pole, as well as mos t of the tentorial surface of the cortex, the posterior third of the m esial surface of the brain, and the upper bank of the posterior portio n of the splenial sulcus. In each hemisphere, it contains a precise, v isuotopically organized representation of the entire extent of the con tralateral visual hemifield. The representation of the vertical meridi an, together with 8-15-degrees of ipsilateral hemifield, forms the ant erior border of V1 with other visually responsive areas. The represent ation of the horizontal meridian runs anterolateral to posteromedial, dividing V1 so that the lower visual quadrant is represented medially, and the upper quadrant laterally. The total surface area of V1 is abo ut 140 mm2 for P. poliocephalus, and 110 mm2 for P. scapulatus. The re presentation of the central visual field is greatly magnified relative to that of the periphery. The cortical magnification factor decreases with increasing eccentricity, following a negative power function. Co nversely, receptive field sizes increase markedly with increasing ecce ntricity, and therefore the point-image size is approximately constant throughout V1. The emphasis in the representation of the area central is in V1 is much larger than that expected on the basis of ganglion ce ll counts in flat-mounted retinas. Thus, a larger degree of convergenc e occurs at the peripheral representations in the retino-geniculo-cort ical pathway, in comparison with the central representations. The mark ed emphasis in the representation of central vision, the wide extent o f the binocular field of vision, and the relatively large surface area of V1 reflect the importance of vision in megachiropterans. (C) 1993 Wiley-Liss, Inc.