Position shifts following crowded second-order motion adaptation reveal processing of local and global motion without awareness

Abstract

Adaptation to first-order (luminance defined) motion produces not only a motion aftereffect but also a position aftereffect, in which a target pattern's perceived location is shifted opposite the direction of adaptation. These aftereffects can occur passively (when the direction of motion adaptation cannot be detected) and remotely (when the target is not at the site of adaptation). Although second-order (contrast defined) motion produces these aftereffects, it is unclear whether they can occur passively or remotely. To address these questions, we conducted two experiments. In the first, we used crowding to remove a local adapter's second-order motion from awareness and still found a significant position aftereffect. In the second experiment, we found that the direction of motion in one region of a crowded array could produce a position aftereffect in an unadapted, spatially separated region of the crowded array. The results suggest that second-order motion influences perceived position over a large spatial range even without awareness.

Figure 1

A second-order Gabor, used in each of the experiments. Each second-order Gabor was a dynamic random-dot background (only one frame is shown here) with a contrast-modulated sine wave and a Gaussian contrast envelope. The contrast modulation depth (the contrast between the dark and light random dots in the background) is exaggerated here; contrast was lower in the experiments.

Figure 2

The stimulus used in the first experiment. (A) An array of 25 second-order Gabor patches was presented during adaptation. Subjects fixated on the bull’s-eye to the right of the Gabors throughout each session. The motion of each crowder Gabor was randomly determined in the initial and top–up adaptation periods, whereas the motion of the adapting Gabors (circled) remained fixed throughout an experimental session. (B) The test period consisted of Gabors containing orthogonal drift-balanced motion. These Gabors were superimposed on the location of the adapting Gabors (circled). (C) After adapting to the stimuli, test Gabors appear to be shifted in the direction opposite that of the prior motion adaptation. The circles were not visible in the actual display. (D) The procedure in each session.

Figure 3

Stimuli used in the second experiment. (A) An array of 30 Gabor patches was presented during adaptation. The two adapted locations (circled) contained drift-balanced motion (no net directional motion). The eight Gabors immediately surrounding the adapted locations also contained drifting sine wave contrast modulations in both directions simultaneously, but the contrast of one direction was increased (to 0.79 Michelson contrast), whereas the contrast of the oppositely drifting component was reduced (to 0.37 Michelson contrast). The size of the arrows in the Gabors surrounding the adapted locations therefore indicate an imbalanced motion signal. The Gabors in the two columns closest to the fixation point contained randomized directions on each trial to increase the effectiveness of crowding and to prevent discrimination of global motion direction in the top or bottom halves of the display. (B) The test Gabors were drift-balanced orthogonal versions of the adapted Gabors in Panel A. (C) Perceived misalignment between the test Gabors following motion adaptation in the surrounding region. The circles were not visible in the actual experiment.

Figure 4

Representative psychometric functions in the first experiment for one subject. (A) Representative results for the crowded condition, in which the adaptation Gabors had a contrast modulation depth of 0.31. Positive values along the abscissa indicate that the Gabors were misaligned in the direction of adaptation (positive values therefore indicate the presence of an aftereffect). The subject’s PSE for this condition was 0.15°, χ²(1) = 48.6, p < .001. (B) Representative results for the noncrowded condition, in which the adaptation Gabors are presented, but no other crowders were present. The format is the same as in Panel A. The contrast modulation depth of the adapting Gabors was 0.58. The PSE for this condition was 0.123°, χ²(1) = 8.02, p < .01.

Figure 5

Results of the first experiment. (A) The graph shows the second-order motion-induced position shift at three contrast modulation depths when crowders were present. Subject T.H. did not show a significant PSE at the midcontrast (0.58), but all other PSEs were significantly above zero; the least of the significant PSEs was for D.B., at the 0.58 contrast condition, χ²(1) = 8.81, p < .01. (B) The second-order motion-induced position shift without crowding. Each PSE was significant; the least significant of which was for subject T.H. in the 0.31 contrast condition, χ²(1) = 6.87, p < .01. There was no significant difference in the PSEs with and without crowding, F(1,2) = 0.37, p = .61. Averaged across both conditions, the perceived misalignment was ~0.10°, which is just above threshold vernier discrimination at the tested eccentricity and separation (Klein & Levi, 1987; Levi & Klein, 1990; Levi, Klein, & Aitsebaomo, 1985). Error bars denote ±SE of nonlinear regression.

Figure 6

Effectiveness of crowding in the first experiment. (A) In the first experiment, during crowded adaptation, subjects were instructed to report the direction of motion in the adapting Gabor (top, circled Gabor in Figure 2). Response accuracy is plotted as a function of adapting Gabor contrast. All three subjects were at chance in all conditions. (B) Response accuracy when there was no crowding (only the adapting Gabors were present). All subjects responded with more than 85% accuracy.

Figure 7

Results of the second experiment. (A) Following adaptation to the stimulus in Figure 3, in which the crowded adapting Gabors had no directional motion, but the surrounding Gabors had a net directional motion signal, the test Gabors appeared shifted in position. The PSE was 0.067° for D.B., χ²(1) = 7.17, p < .01, 0.11° for T.H., χ²(1) = 8.0, p < .01, and 0.092 for S.A., χ²(1) = 8.9, p < .01. The results indicate that there was a significant, directionally specific aftereffect at a location that was not adapted to directional motion; the motion in the surrounding region therefore generated a remote aftereffect. (B) The position aftereffect for missed trials. The same analysis was conducted as in Panel A but only for those trials in which the subject incorrectly reported the direction of motion adaptation. There was no significant difference between correct and incorrect trials, t(2) = 1.1, p > .05. Error bars denote ±SE of nonlinear regression.