Spatial frequency masking with the sweep-VEP.

A suprathreshold sinewave grating can change the amplitude of the steady-state visual evoked potential (VEP) in response to a test grating if the two are close in spatial frequency (SF). The change in amplitude provides clues to underlying pattern analyzers. Masking was measured in 12 observers using the steady-state VEP. As a test grating reversed at 7 Hz, a masker of similar temporal frequency (9 Hz) but of variable SF was superimposed on it. Test gratings were 1, 3 and 8 c/deg (20% contrast). Within a 10 sec trial, the mask (20 or 40% contrast) was fixed at one of nine SFs or was swept across 19 SFs (5 octaves). The amplitude of the test response (at 14 Hz) was measured as a function of the SF of the masker. Group masking functions were broad (2-3 octaves) and revealed multiple minima. Functions for 1 and 3 c/deg tests each revealed minima near 1 and 3 c/deg. Functions for 8 c/deg tests revealed minima at 3 and 8 c/deg. Doubling the contrast of the mask from 20 to 40% increased masking but in a nonlinear fashion that enlarged the off-peak minima. Swept masks caused slightly more masking than fixed masks, and caused masked amplitudes to exceed unmasked amplitudes (i.e., enhancement) in one condition (3 c/deg.test, 20% contrast mask). The data suggest that each VEP masking function reflects the outputs of multiple spatial analyzers, that a discrete set of analyzers may underlie the data, and that the efficient sweep-VEP can measure SF tuning.