Figure 2. Results of the first experiment.

(A) A representative psychometric function showing a significant shift in the perceived position of the test Gabors as a function of the direction of motion adaptation. The abscissa shows the physical misalignment between the test Gabors: positive values indicate that the Gabors were misaligned in the same direction as the prior motion adaptation, and negative values indicate that they were misaligned in a direction opposite that of the motion adaptation. The ordinate shows the proportion of trials in which the subject perceived the test Gabors to be misaligned opposite the direction of the prior motion adaptation. The point of subjective equality (PSE, the inflexion point) defines the physical misalignment between the Gabors that appeared to be aligned. Because of the motion adaptation, the Gabors had to be presented ~0.6 deg in the same direction as the prior motion adaptation to appear aligned (t₍₇₎ = 6.06, P = 0.001). When physically aligned, the Gabors appeared shifted opposite the direction of the motion adaptation. (B) Results for one representative subject (left panel), as well as all four subjects averaged (right panel), as a function of the spatial frequency (abscissa) and orientation of the test Gabors. The orientation of the test Gabors was either the same as (open symbols) or orthogonal to (solid symbols) the orientation of the adaptation Gabors. There was a significant shift in the perceived position of the Gabors with both test orientations (same orientation: t₍₃₎ = 4.8, P < 0.01; orthogonal orientation: t₍₃₎ = 4.25, P < 0.02). Across all subjects, there was little variation in the illusory position shift as a function of spatial frequency (F_(2,6) = 2.6, P = 0.15). There was a larger perceived shift when the test stimuli contained the same orientation as the adaptation stimuli, although this difference did not reach significance (for the group results in the right panel, t₍₃₎ = 2.75, P = 0.07). There was a significant overall position shift for each individual subject as well (least significant effect was for subject EV, t₍₅₎ = 5.96, P = 0.002; the least significant within-subject condition effect for subject EV was the orthogonally oriented test Gabor with a spatial frequency of 1 cyc/deg: t₍₃₀₎ = 3.3, P < 0.01). Error bars ± s.e.m.