A bias-free measure of retinotopic tilt adaptation

Abstract

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.

Keywords: methods; signal detection theory; visual adaptation.

Figure 1

Schematic representation of the stimuli used for adaptation and test. The subject adapts (left) to four Gabor patches with their carriers moving in the directions shown by the arrows. They are then tested with four patches in approximately the same position as the adapting patches (right). In each trial, a signed spatial displacement is selected. This displacement shifts one of the two bottom patches horizontally. Negative stimuli shift the left-hand patch, and positive stimuli shift the right-hand patch. The direction of the shift (leftward or rightward) depends on the condition. In condition “Out,” the patch is shifted outward in the same direction expected from the aftereffect. In condition “In,” the patch is shifted inward, opposite to the expected direction of adaptation. The case shown is a positive signal in condition “Out” (bold type). Conditions “Out” and “In” were randomly interleaved.

Figure 2

Stimulus configuration used in the retinotopic version of the experiment. The rectangles represent the outline of the display screen, not to scale. The test stimulus jumps horizontally as soon as the adapting period ends. The vertical position of the patches is unchanged. For further explanation see the text.

Figure 3

Psychometric functions for two observers (MM, left; DM, right) in the control condition in which the adapting rating was stationary (top) and in the combined moving-adaptor conditions for conditions RV, RH, SV, and SH in Experiment 1 (bottom). The y-axis shows the probability of choosing the right-hand stimulus pair as being more tilted; the x-axis shows the actual displacement (in degree of tilt) with positive values presented to the right-hand pair of patches and negative values to the right. In the outward condition (blue), the absolute stimulus level was in the same direction as the bias expected from adaptation; in the inward case (magenta), it was in the opposite direction. The continuous lines are a two-parameter (μ, σ) fit to the combined data in the top and bottom panels. For further details see the text.

Figure 4

Data from the retinotopic condition and the predictions of the model asserting that adaption takes place in a spatiotopic reference frame. The coloring of the curves representing the two independent conditions is the same as in Figure 3. Left-hand panel: subject MM. Right-hand panel: subject DM. Note that the curves are not fits to the data, but predictions based on the assumption that adaptation is purely spatiotopic, using the parameter values μ, σ from the spatiotopic condition.