Apparent fineness of stationary compound gratings.

Patterns consisting of the sum of a sinusoidal grating and its second spatial harmonic have an apparent spatial fineness, or periodicity, that is about halfway between the two component spatial frequencies. There are also phase dependent modulations of the apparent fineness about the mean fineness shift. Covariance between individuals' phase dependent fineness shifts indicates the presence of four spatial phase channels. The apparent fineness effects, and the putative phase channels, may both be a product of a local, linear, analysis of spatial frequency content. Illusory second harmonics, as generated in the spatial frequency doubling illusion, also change apparent fineness.