Adaptation to a spatial impulse: Implications for Fourier transform models of visual processing

Spatial frequency adaptation was analyzed in terms of three representative models of the Fourier transform theory of visual processing. Each model predicted that subjects who exhibited normal sine-wave grating adaptation should show substantial adaptation over a wide range of spatial frequencies following exposure to a narrow bar of high luminance (one-dimensional spatial impulse). In the experiments, two highly practiced subjects who showed normal sine-wave adaptation, showed little adaptation to a 140 cd/m² bar subtending 0.6', superimposed on a background of 4.63 cd/m² subtending 1.75°. Two control experiments with a third subject indicated that neither a photoreceptor nonlinearity nor a power function transformation of the luminance distribution could account for the discrepancy. The lack of adaptation to the spatial impulse suggests that present Fourier transform models which postulate phase-independent frequency channels in the visual system are inadequate for the description of the visual response to suprathreshold aperiodic stimuli. A receptive field model of spatial frequency processing and phase encoding is suggested to account for the results.