Abstract

Adapting to composite gratings may produce a smaller threshold change than adapting to the component gratings alone. This finding suggests that closely spaced spatial frequencies may not be processed independently.¹ If the adaptable units are local frequency filters rather than global Fourier analyzers, the presence of beats in the pattern (areas of constructive or destructive interference) can account for the apparent interactions. The portion of the visual field receiving the low-contrast part of the beating stimulus will be relatively unadapted. An experiment in which a test grating of narrow spatial extent was located in the same position as either a contrast peak or trough of the adapting grating confirmed this prediction. A difficulty with this suggestion, however, is the finding of similar results regardless of whether the composite adaptation stimulus is stationary or drifting. The time-averaged contrast for each part of the visual field is constant for a drifting grating. This difficulty for the beat-frequency hypothesis can be eliminated by another experiment which studied the effect of temporal duty cycle. When a single-component adapting grating is presented with a duty cycle of 50% (the approximate effective duty cycle for each part of the visual field that is being adapted by a drifting composite grating) there is very little adaptation. We conclude therefore that the apparent interaction between spatial frequency gratings results from the presence of beats.

© 1985 Optical Society of America