Due to the effect of a spatial context or temporal context, the perceived orientation of a test line or grating pattern can appear tilted away from its physical orientation. The tilt illusion (TI) is the phenomenon that the perceived orientation of a test line or grating is altered by the presence of surrounding lines or grating with a different orientation (spatial context; see Fig.1). And the tilt aftereffect (TAE) is the phenomenon that the perceived orientation is changed after prolonged inspection of another oriented line or grating (temporal context; see Fig.2).
It has been reported that the magnitude and the direction of the perceived orientation shift depends on the relative orientation between test and contextual stimuli (see Fig.3). Psychophysics experiments have shown that relative orientations between 0 deg and about 50 deg produce repulsion effects (the test line or grating tends to rotate away from the contextual stimulus), which is known as the direct form of the tilt effect; but larger relative orientations up to 90 deg produce attraction effects (the test line or grating tends to rotate towards the contextual stimulus), which is known as the indirect form of the tilt effect.
It has been observed repeatedly that indirect effects are smaller than direct effects. The repulsion peak is about 3 degrees usually when the relative orientation between the test and contextual stimuli is around 20 degrees; and the attraction peak is usually maximally 0.5 degrees when the relative orientation is around 70 degrees (see Fig.3).