The relation of apparent brightness to contrast threshold on a photopic background :Dependence on retinal position and target size

Abstract

The luminance necessary for constant apparent brightness was measured as a function of retinal eccentricity at several photopic brightness levels and compared with the corresponding increment threshold functions. Thresholds rose substantially from the fovea to the periphery, but the contrast required for constant brightness showed little change as a function of eccentricity. A change of target size had relatively little effect on the shapes of the equal brightness curves. Two contributing factors to the difference between the threshold and equal brightness curves are systematic changes in (1) the apparent brightness at threshold (2) the exponent of the brightness power function. A neural mechanism is proposed which accounts for the results. A comparison with known retinal ganglion cell properties leads to the conclusion that this mechanism must be located proximally to the retina.