Attentive tracking can modulate the illusory misalignment of a flash

Abstract

Purpose: The perceived position of a static flash can be shifted by nearby motion (flash-shift: Whitney & Cavanagh, 2000) but the role of attention in the flash-shift effect has not been explored. To investigate whether attention drawn to a moving, tracked stimulus can modulate the position information of static objects, the perceived misalignment of a flash was measured while the offset between the attentively tracked stimulus and the flash was varied. Methods: Subjects were instructed to attentively track a pair of stimulus bars (dark bars) of the rotating radial grating (a twelve-cycle sinusoidal luminance grating), indicated by a pointer. The pointer rotated with the target pair for 667ms and subjects continued to track the indicated bars with attention after the pointer disappeared. A further 833ms later, two horizontal lines were briefly flashed adjacent to the grating at the 3 and 9 o'clock positions. The perceived misalignment of two flashes was measured as a function of the angular position of the tracked bars at the moment of the flashes. At the moment of the flash, the radial grating was always at the same orientation (light bars aligned with vertical and horizontal) and the location of the tracked bar was an integer number of cycles of the grating away from the flash (−2 to +2). Results: When the test flashes were presented before the tracked bars had reached their location, the two flashes showed a misalignment in the direction of the motion of the grating. However, when the flashes were presented after the tracked bars had passed their location, the illusory displacement was considerably reduced or reversed. Conclusion: Even though the moving stimulus was physically identical for each of the offsets between the tracked bar and the test, the perceived position of the tests depended on the locus of attention. This result shows that attention to a moving stimulus can influence the perceived position of a nearby, briefly presented stationary object.