Abstract
It is known that temporal modulation of a sinusoidal grating of low spatial frequency causes an increase in the apparent spatial frequency of the grating. A possible explanation for the illusion is proposed. Temporal modulation would favour channels which respond only transiently to prolonged presentation of a grating. These channels may be the human analogues of Y-cells found in the cat retina. Y-cells respond nonlinearly to gratings, and the nonlinearity may be the root of the apparent spatial-frequency illusions.