Spatial frequency mechanisms in human vision investigated by evoked potential recording

Two visually evoked brain responses were elicited simultaneously by stimulating the eye with two superimposed sinewave grating patterns that were temporally modulated at slightly different rates. The VEP to one grating was comparatively little affected by the presence of the other grating when the two spatial frequencies were very different, but mutual attenuation grew stronger and stronger as the spatial frequencies of the two gratings were progressively brought together. The attenuation rose to a sharp maximum when the two spatial frequencies were equal. This held at each of the five spatial frequencies tested. This finding can be explained if the spatially-selective mechanisms responsible for grating VEPs contain multiple subunits of narrower spatial frequency bandwidth. The two-grating technique was also used to search for evidence of multiple subunits that are most sensitive at the same spatial frequency, but are tuned to different temporal frequencies. Findings were quite different for temporal and for spatial tuning. Attenuation of one grating VEP was greatest when the temporal frequency of the other grating fell within a broad frequency range of about 3-30 reversals sec-1: maximum attenuation could occur when the gratings had quite different temporal frequencies. This finding denies that for every given temporal frequency there is a subunit maximally sensitive to that frequency.