Simple solutions: concentrations in the surface region.

data of this kind have not yet been reported. Tyler's statement concerning poor spatial resolution in the chromatic channels does not seem to add much to our understanding of these effects, especially given his comment that "large receptive fields could also occur in the achromatic channels." He is correct in predicting achromatic aftereffects. As yet unpublished data from several laboratories indicate that results commensurate with our color aftereffects can be obtained with achromatic stimuli. Threshold elevations, obtained under conditions of successive adaptation (2), simultaneous contrast (3), and interocular suppression (4), occur whenever the major Fourier components of checkerboards and gratings have the same orientation. Using achromatic contrast thresholds and checkerboards with Fourier frequencies as low as two cycles per degree, the aftereffect occurs on the diagonals (2). Thus, it seems clear that poor spatial resolution occurs with achromatic aftereffects as well as with chromatic aftereffects. Fourier analysis has made a considerable contribution to the study of pattern perception. Without this powerful conceptualization the argument discussed here would not have occurred. The contribution of electrophysiological data to the understanding of perception has also been great; however, as this argument indicates, the exact relationships between single-unit responses and psychophysical judgments remain to be specified. Fourier components are essentially a feature of the stimulus; our experiment asked what features were salient in this situation rather than directly addressing the question of the type of mechanisms involved. JAMES G. MAY Department of Psychology, University of New Orleans, New Orleans, Louisiana 70122 HALSEY H. MATTESON Department of Psychology, Tulane University, New Orleans, Louisiana 70118