SPATIAL-FREQUENCY MASKING WITH THE SWEEP-VEP

A suprathreshold sinewave grating can change the amplitude of the stea dy-state visual evoked potential (VEP) in response to a test grating i f 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 variabl e 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 fi xed at one of nine SFs or was swept across 19 SFs (5 octaves). The amp litude of the test response (at 14 Hz) was measured as a function of t he 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 r evealed minima near 1 and 3 c/deg, Functions for 8 c/deg tests reveale d minima at 3 and 8 c/deg, Doubling the contrast of the mask from 20 t o 40% increased masking but in a nonlinear fashion that enlarged the o ff-peak minima. Swept masks caused slightly more masking than fixed ma sks, 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 mu ltiple spatial analyzers, that a discrete set of analyzers may underli e the data, and that the efficient sweep-VEP can measure SF tuning. (C ) 1997 Published by Elsevier Science Ltd.