Effect of spatial waveform on apparent spatial frequency

We examined the effect of spatial waveform on the perceived spatial frequency of a grating target. The luminance profile of 0.5 c/ degrees sinusoidal gratings was modified by either compressive or expansive power functions, and was presented alternately with a true sinusoidal grating. Subjects matched the apparent spatial frequency of the two gratings using a method of adjustment. Both compressive and expansive power functions lowered the perceived spatial frequency of the grating, irrespective of the stimulus contrast. Rectified sine wave gratings were also found to reduce apparent spatial frequency. The magnitude of the spatial frequency shifts with spatial waveform diminished with successive matches, which may represent a change in matching strategy employed by observers. Calculations and a further experiment suggest that judgements of spatial frequency may in part be determined by the separation between edges in a grating.