Lateral inhibition between spatially adjacent spatial-frequency channels?

Abstract

A new lateral inhibitory phenomenon is suggested between spatially neighboring channels that detect similar spatial frequencies. This mechanism tends to enhance the apparent contrast of gratings at their boundaries, and Mach-band-type effects are demonstrated at the edge of a suprathreshold half-field grating. We studied the dependence of this effect on the grating parameters of spatial frequency and contrast. The grating spatial frequency must be above about 4 cycles per degree (cpd) for any effect to be seen. In the range 4–14 cpd, the type of effect seen depends on the contrast of the grating: At high contrasts, the effect is an enhancement of grating contrast near its border, whereas at near threshold contrasts, an opposite effect, edge contrast diminution, is seen. Finally, above about 14 cpd, enhancement is found down to threshold contrasts. The latter very-high-frequency effect may be due to a high-spatial-frequency limit of the visual channels available, with the smallest receptive-field summation area being in the order of 2.2′. The near-threshold edge contrast diminution may derive from illumination lateral inhibition. In the intermediate range, however, the enhancement appears only at high contrast levels and therefore cannot be due to a linear mechanism. This enhancement seems to derive from lateral inhibition between neighboring spatial-frequency channels. A Craik-O’Brien-Cornsweet type of visual illusion is demonstrated in the spatial-frequency grating contrast domain. The effect of lateral inhibition on the selectivity of spatial frequency channels is discussed. Lateral inhibition may cause a shift in the optimal spatial frequency of the channels by about 40% at high contrasts, making such gratings appear to have lower spatial frequencies. This shift may also be responsible for the phenomenon of contrast constancy.