The different mechanisms of the direct and indirect tilt illusions.

Both the tilt illusion and aftereffect exhibit indirect effects under certain conditions: these are negative (assimilation) effects which occur with large (70-90 deg) angular separations between test and inducing gratings. They are opposite in direction to the positive, and much larger, contrast effects which occur at smaller (10-15 deg) separations. Evidence from six experiments shows that stimulus manipulations which reduce direct effects have little or no effect on indirect effects and vice versa, suggesting that the two effects have different determinants. It is proposed that direct effects arise from lateral inhibitory interactions between populations of neurones in striate cortex and that indirect effects occur at a higher level, possibly in areas concerned with stimulus-specific interactions beyond the classic receptive field. The implications of the data for theories of the tilt illusion are considered.