A position anchor sinks the double-drift illusion

Abstract

When the internal texture of a Gabor patch moves orthogonally to its envelope's motion, the perceived path, viewed in the periphery, shifts dramatically in position, and direction relative to the true path (the double-drift illusion). Here, we examine positional uncertainty as a critical factor underlying this illusory shift. We presented participants with an anchoring line at different distances from the drifting Gabor's physical path. Our results indicate that placing an anchor (a fixed line) close to the Gabor's path halved the magnitude of the illusion. This suppression was symmetrical for anchors placed on either side of the Gabor. In a second experiment, we used crowding to degrade the anchoring line's position information by embedding it in a set of parallel lines. In this case, despite the presence of the same lines that reduced the illusion when presented in isolation, the illusory shift was now largely restored. We suggest that the adjacent lines crowded each other, reducing their positional certainty, and thus their ability to anchor the location of the moving Gabor. These findings indicate that the positional uncertainty of the equiluminant Gabor patch is critical for the illusory position offset.

Figure 1.

A traversing Gabor patch with texture and envelope component motions was shown on each trial. In the baseline condition no anchor was displayed to find the maximum illusory shift perceived by the participant. In experiment 1, a single vertical high-contrast line (at one of the 13 possible locations demonstrated by dashed lines; ranging from 3 dva from the left of the Gabor to 3 dva to its right, in multiples of 0.5) was shown alongside the drifting patch. On the experimental trials of experiment two, a collection of lines was displayed that varied in their distance from each other on each condition. After each trial, a response line appeared in place of the fixation cross. Participants adjusted the orientation of this line to match the perceived path of the viewed stimulus.

Figure 2.

An anchoring line with closer than 1.5 dva distance to the double-drift's actual path (0 dva distance anchor lies along the Gabor's path) dramatically reduced the magnitude of the illusion compared to more distant placements. Error bars indicate standard error of the mean. Asterisks indicate significant difference from baseline (no flanking lines) condition.

Figure 3.

Multiple anchor lines displayed together act as a surface, texture, or grid. The grid's effect on the illusory deviation is much less than that of a single anchor although the same anchoring line was present within the grid. Distance between grid lines was selected to match the distances of anchors that had the highest effect on the illusion. None of the grid conditions resulted in a significant reduction of the illusory angle.