Flicker adaptation or superimposition raises the apparent spatial frequency of coarse test gratings

Abstract

Independent channels respond to both the spatial and temporal characteristics of visual stimuli. Gratings <3cycles per degree (cpd) are sensed by transient channels that prefer intermittent stimulation, while gratings >3cpd are sensed by sustained channels that prefer steady stimulation. From this we predict that adaptation to a spatially uniform flickering field will selectively adapt the transient channels and raise the apparent spatial frequency of coarse sinusoidal gratings. Observers adapted to a spatially uniform field whose upper or lower half was steady and whose other half was flickering. They then adjusted the spatial frequency of a stationary test (matching) grating on the previously unmodulated half field until it matched the apparent spatial frequency of a grating falling on the previously flickering half field. The adapting field flickered at 8Hz and the spatial frequency of the gratings was varied in octave steps from 0.25 to 16cpd. As predicted, adapting to flicker raised the apparent spatial frequency of the test gratings. The aftereffect reached a peak of 11% between 0.5 and 1cpd and disappeared above 4cpd. We also observed that superimposed 10Hz luminance flicker raised the apparent spatial frequency of 0.5cpd test gratings. The effect was not seen with slower flicker or finer test gratings. Altogether, our study suggests that apparent spatial frequency is determined by the balance between transient and sustained channels and that an imbalance between the channels caused by flicker can alter spatial frequency perception.