Probability summation and regional variation in contrast sensitivity across the visual field.

Contrast sensitivity at different positions in the visual field has been measured at various spatial frequencies using a patch of gratin, 0 suitabij tignetted to give a st!mulus localized in both space and spatial frequent\. While contrast sensitivitl along a vertical line through the fixation point f4is ofl’ steadily from a maximum at the crntre. sensitivity along a horizontal line displaced 12 periods of the grating above the fixation point is approximately constant, at least out to 32 periods from the mid-line. The wak in which detectability increases with increasing number of cycles (2 up to 64) has been measured for gratings with short horizontal bars centred on the fixation point and for gratings with short vertical bars centred on the mid-iine 41 periods above it. The relation between sensitivity and number of cycles can in each case be explained exactly assuming probability summation across space ;15 long as the xariation in sensitivity across the visual field is taken into account.