Cortical magnification factor predicts the photopic contrast sensitivity of peripheral vision

Abstract

THE area of visual cortex devoted to the analysis of a con-slant-size region in the visual field diminishes progressively for more peripheral locations. The change is described quantitatively by the cortical magnification factor, which indicates the linear extent of cortex in mm corresponding to one degree of visual angle at various eccentricities (angular distances from the middle fovea). The human cortical magnification factor has been estimated by Cowey and Rolls¹ from the data of Brindley and Lewin², who mapped the phosphenes (sensations of light) caused in the lower nasal visual field by electrical stimulation of the human visual cortex. Building on these results, we have studied the spatial contrast sensitivity functions in man at various eccentricities. We used two methods: in one the retinal image sizes of the test gratings were kept similar at different eccentricities and in the other, the calculated sizes of cortical projections of the test gratings were made similar at different locations. Our results indicate that peripheral contrast sensitivity and acuity are inferior to foveal performance, because of the diminished cortical projection area.