Objective evidence for phase-independent spatial frequency analysis in the human visual pathway

Abstract

Electrophysiological responses in human index an interaction between responses to two gratings that is relatively independent of the distribution of light in the retinal image. Two 5 cycle/deg sinewave gratings were superimposed, one counterphase-modulated at F 1 Hz and the other at F 2 Hz. Nonlinear interaction terms of frequency ( nF 1± mF 2) were recorded that could not be produced by superimposing the F 1 Hz grating on blank-field 7 Hz flicker. A local luminance origin could be excluded for the (2 F 1+2 F 2) term and for the suppression of 2 F 1 and 4 F 1, but not for the ( F 1+ F 2), ( F 1+3 F 2) and (5 F 2− F 1) terms. The relative spatial phase of the two gratings was varied, thus altering the light distribution in the retinal image without changing its spatial power spectrum. The (2 F 1+2 F 2) Hz contrast-specific grating-grating interaction term was almost unaffected by these substantial changes in retinal image light distribution providing that the spatial frequency power spectrum of the retinal image was held constant. The (2 F 1+2 F 2) term and the suppression of 2 F 1 were both tuned to spatial frequency.