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 grating suitably vignetted to give a stimulus localized in both space and spatial frequency. While contrast sensitivity along a vertical line through the fixation point falls off steadily from a maximum at the centre, sensitivity along a horizontal line displaced 42 periods of the grating above the fixation point is approximately constant, at least out to 32 periods from the mid-line. The way 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-line 42 periods above it. The relation between sensitivity and number of cycles can in each case be explained exactly assuming probability summation across space as long as the variation in sensitivity across the visual field is taken into account.