Two experiments were done t o find the effect on simultaneous brightness contrast of placing test patches in, as against outside, the fovea. When the display was magnified SO that the test patches were outside the fovea no statistically significant effect was found. Very short exposures, to eliminate eye movements, did not appear to reduce contrast.
INTRODUCTIONIt has usually been held that the familiar phenomenon of simultaneous contrast is partly central and partly peripheral (i.e. in this context, retinal) in origin. Reviews of the earlier literature are readily available (eg. in Graham, 1934; or Woodworth, 1938). A recent review has been made by Diamond (1960).If we suppose that the retinal component is important, it might be expected that the amount of contrast would be different inside and outside the foveal area, since there are so many well known structural and functional differences between the fovea and the periphery. Recent electro-physiological work on lateral inhibition in the retina (Barlow, FitzHugh and Kuffler, 1957 ;Wiesel, 1960 ;Barlow, 1953 ;Kuffler, 1953) suggests that there are changes in the size of the receptive field (both excitatory and inhibitory parts) and in the amount of inhibition, as we go from the centre towards the periphery. Other experiments (Hubel and Wiesel, 1959, 1960a, 1960b have demonstrated a somewhat similar organization at higher levels in the central nervous system, and suggest a highly complex interaction of retinal and central factors. These experiments do not yet, however, give a clear indication of the kind of change to be expected in simultaneous contrast with variation of retinal area in the human being.I n this paper we are discussing the effect of magnifying the contrast-inducing display, so as to throw the test patches of Figure I wholly within or wholly outside the fovea, as opposed to viewing peripherally a display of constant size. The effect of very short exposures, so as to preclude eye-movements, is also considered.