Bi-directional illusory position shifts toward the end point of apparent motion

In this study, we examined the relation between motion induced position shifts and the position shifts caused by saccades. When a stimulus is flashed briefly around the time of a saccade, its perceived position is mislocalized toward the saccade target: if the flash is in front of the saccade the test flash appears shifted in the direction of eye movement; but a test flashed beyond the saccade target is displaced back toward the saccade target (bi-directional saccadic compression: Ross, J., Morrone, M. C., and Burr, D. C. (1997). Compression of visual space before saccades. Nature, 386, 598-601. Motion induced position shifts (in the absence of eye movements) have been demonstrated for a variety of stimuli but the illusory position shift is always found to be in the same direction as the motion. However, all previous studies presented the tests either along or beside the motion path, never beyond its end point. We now test this region beyond the motion path and find that the apparent location of a test in this region is shifted in the direction opposite to the motion, back toward the motion end point. In contrast, when the flash was presented between the beginning and end of the motion path, it is shifted in the direction of motion, again, toward the motion end point. These shifts together indicate a compression of perceived locations toward the end point of the apparent motion. Control experiments confirmed that this effect was neither due to the Fröhlich effect induced by apparent motion from the test flash to the second disc nor to foveal compression. The correspondence between compression toward the end point of apparent motion and saccadic compression toward the saccade target suggests that attentional shifts or planned eye movement signals may play a role in both.