Small-angle attraction in the tilt illusion

Abstract

The tilt illusion (TI) describes the phenomenon in which a surround inducer grating of a particular orientation influences the perceived orientation of a central test grating. Typically, inducer-test orientation differences of 5-40 deg cause the test orientation to appear shifted away from the inducer orientation, i.e. showing repulsion. For orientation differences of 60-90 deg, the inducer typically causes the test grating orientation to appear shifted towards the inducer orientation, termed here “large-angle” attraction. Both repulsion and large-angle attraction effects have been observed in contrast-modulated as well as luminance-modulated grating patterns. Here we show that a secondary, “small-angle” 0-10 deg attraction effect is observed in contrast-modulated and orientation-modulated gratings, as well as in luminance-modulated gratings that are relatively low in spatial frequency, low in contrast or contain added texture. The observed small-angle attraction, which can exceed in magnitude that of the repulsion and large-angle attraction effects, is dependent on the spatial phase relationship between the inducer and test, being maximal when in-phase. Both small-angle attraction and repulsion effects are reduced when a gap is introduced between test and inducer. Our findings suggest that small-angle attraction in the TI is a result of assimilation of the inducer pattern into the receptive fields of the neurons sensitive to the test.