An oblique effect of spatial summation.

Contrast thresholds for sinewave gratings were measured as a function of the number of cycles in the grating for two different orientations and three spatial frequencies. At high spatial frequencies, where an oblique effect of contrast sensitivity was demonstrated, there was a greater increase in contrast sensitivity as a function of the number of cycles for the vertical gratings than for oblique gratings. At a lower spatial frequency, where the contrast sensitivity anisotropy was greatly diminished, both orientations displayed a similar increase in contrast sensitivity when the number of cycles in the gratings was increased.