A bias-free measure of retinotopic tilt adaptation.

The traditional method of single stimuli for measuring perceptual illusions and context effects confounds perceptual effects with changes in the observer's decision criterion. By deciding consciously or unconsciously to select one of the two response alternatives more than the other when unsure of the correct response, the observer can shift his or her psychometric function in a manner indistinguishable from a genuine perceptual shift. Here, a spatial two-alternative forced-choice method is described to measure a perceptual aftereffect by its influence on the shape of the psychometric function rather than the mean. The method was tested by measuring the effect of motion adaptation on the apparent Vernier offset of stationary Gabor patterns. The shift due to adaptation was found to be comparable in size to the internal noise, estimated from the slope of the psychometric function. By moving the eyes between adaptation and test, it was determined that adaptation is retinotopic rather than spatiotopic.