Evoked potential evidence of adaptation to spatial Fourier components in human vision.

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INVESTIGATIONS of the nature of human spatial vision have yielded considerable evidence consistent with the hypothesis that the human visual system contains channels selectively sensitive to narrow ranges of spatial frequencies1. Unfortunately, much of this evidence, which comes from studies of the detection of gratings, can also be explained in terms of the detection of single edges or lines2. However, two studies3,4 of orientation-contingent colour aftereffects have involved attempts to discriminate between these explanations. Through the use of checkerboard stimuli, which have major Fourier components at 45° to the check edges5, these studies showed that chromatic adaptation occurs at orientations determined by the Fourier components of the adapting pattern and not by the edges present. After subjects had adapted to upright red and oblique green checkerboards, achromatic test gratings appeared pink when vertical or horizontal and green when oblique. Comparable aftereffects were observed when achromatic checkerboards were viewed after adaptation to coloured gratings3. The strongest aftereffects were obtained4 for test gratings with a fundamental spatial frequency a factor of 1.5 above the periodicity of the adapting checkerboard; that is, close to the fundamental Fourier component of the checkerboard. We present here evidence, obtained from evoked potentials, that attenuation of the activity of orientation-specific cells in one region of the human visual cortex occurs largely in cells which are tuned to the orientations of the fundamental Fourier components of an adapting pattern rather than to the orientations of the edges contained in the pattern.