Dynamic modulation of illusory and physical target size on separate and coordinated eye and hand movements

Measuring the double-drift illusion and its resets with hand trajectories

If a Gabor pattern drifts in one direction while its internal texture drifts in the orthogonal direction, its perceived position deviates further and further away from its true path. We first evaluated the illusion using manual tracking. Participants followed the Gabor with a stylus on a drawing tablet that coincided optically with the horizontal monitor surface. Their hand and the stylus were not visible during the tracking. The magnitude of the tracking illusion corresponded closely to previous perceptual and pointing measures indicating that manual tracking is a valid measure for the illusion. This allowed us to use it in a second experiment to capture the behavior of the illusion as it eventually degrades and breaks down in single trials. Specifically, the deviation of the Gabor stops accumulating at some point and either stays at a fixed offset or resets toward the veridical position. To report the perceived trajectory of the Gabor, participants drew it after the Gabor was removed from the monitor. Resets were detected and analyzed and their distribution matches neither a temporal nor a spatial limit, but rather a broad gamma distribution over time. This suggests that resets are triggered randomly, about once per 1.3 seconds, possible by extraneous distractions or eye movements.

Modulation of visually guided action by the image and familiar sizes of real-world objects

In daily life, two aspects of real-world object size perception—the image size of an object and its familiar size in the real world—are highly correlated. Thus, whether these two aspects of object size differently affect goal-directed action (e.g., manual pointing) and how have scarcely been examined. Here, participants reached to touch one of two simultaneously presented objects based on either their image or familiar size, which could be congruent or incongruent (e.g., a rubber duck presented as smaller and larger than a boat, respectively). We observed that when pointing to target objects in the incongruent conditions, participants’ movements were slower and were more curved toward the incorrect object compared with the movements in the congruent conditions. By comparing performance in the congruent and incongruent conditions, we concluded that both image size and familiar size influenced action even when task irrelevant, indicating that both are processed automatically (Konkle & Oliva, 2012a). Image size, however, showed influence earlier in the course of movements and more robustly overall than familiar size. We additionally found that greater relative familiar size differences mitigated the impact of image size processing and increased the impact of familiar size processing on pointing movements. Overall, our data suggest that image size and familiar size perception interact both with each other and with visually guided action, but that the relative contributions of each are unequal and vary based on task demands.