Hemifield differences in perceived spatial frequency

Measurements of the perceived spatial frequency of stationary sinewave gratings were made with the gratings presented at the same eccentricity in the left, right, upper, and lower visual hemifields. Ten subjects performed the task binocularly with spatial frequencies of 1, 2, and 4 cycles deg-1. Two of these subjects also performed the task monocularly at 2 cycles deg-1. In the majority of cases, the spatial frequency of stimuli presented in the left and lower visual hemifields was overestimated relative to stimuli presented in the right and upper visual hemifields. The results were similar for all spatial frequencies tested, and the direction of the asymmetry was the same whether viewing was with the left eye, right eye or binocular, suggesting that the differences in perceived spatial frequency are not retinal in origin.