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Visual tilt effects
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)〔Gibson, J. J. (1937). Adaptation, after-effect, and contrast in the perception of tilted lines. II. Simultaneous contrast and the areal restriction of the after-effect. Journal of Experimental Psychology, 553-569.〕 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)〔Gibson, J. J., & Radner, M. (1937). Adaptation, after-effect and contrast in the perception of tilted lines. I. Quantitative studies. Journal of Experimental Psychology, 453-467.〕 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.〔〔Morant, R. B., & Harris, J. R. (1965). Two different after-effects of exposure to visual tilts. The American Journal of Psychology, 218-226.〕〔O'Toole, B., & Wenderoth, P. (1977). The tilt illusion: repulsion and attraction effects in the oblique meridian. Vision Res., 367-374.〕 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).
==The original experiments showing the TI and TAE==
These effects were first studied by Gibson in 1937. The subject's vision was restricted so that he could see a black line (the test line) bisecting a white circular field, and he could grasp the edges of a disk to rotate the line about its midpoint. An experimenter would sit behind the disk to set the stimuli and to record the subject's adjusted position of the line. During the tilt aftereffect experiment,〔 the subject was required to look at an oriented line for four minutes, and then to adjust another line to a position which appeared to be vertical. In the simultaneous tilt illusion experiment,〔 a tilted grating was introduced into the circular field of the subject, and the subject was supposed to set the adjustable line to vertical before and after the tilted grating had been superposed on it. Both experiments showed that the position which appeared to follow the subject's perceived vertical was slightly off the objective vertical, and the perceived orientation shifts depended on the relative orientation between the test line and the adapted line or the simultaneously induced line.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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