Abstract
That our world remains stable in the face of continual rapid eye-movements suggests that there exists in the brain a neural representation, or "map", that does not move with the eyes but remains solid in external world coordinates. However, evidence from psychophysical and imaging studies for such as map remains fiercely controversial. In this study we measured spatiotopicity with the classical motion aftereffect (MAE: illusory motion following adaptation to motion) and the positional motion aftereffect (PMAE: illusory shift in apparent position following adaptation to motion). Subjects adapted to small (1°) vertically aligned patches of gratings (1 c/deg), drifting in opposite directions at 3 deg/sec. They then made a 12° rightward saccade, after which test grating stimuli (same size and spatial frequency) appeared for 500 ms, in the same retinal or the same screen position (or both, with no intervening saccade). For the MAE subjects annulled the motion, for the PMAE they annulled the apparent spatial misalignment of the patches. The MAE was strictly retinotopic (confirming previous research), but the PMAE showed a strong spatiotopic component. We also measured the PMAE with test gratings that were apparently stationary (with illusory MAE annulled) and found that under these conditions, the effects were almost entirely spatiotopic, with no statistically significant retinotopic component. Similarly, with brief (50 ms) stationary test-stimuli (producing only weak illusory motion), the effects were largely spatiotopic. The results imply two causes for the PMAE: one an indirect consequence of the illusory motion of the MAE; the other direct adaptation of a spatiotopic neural map. Taken together, the adaptation results provide very clear evidence for the existence of a spatiotopic map, probably within the dorsal stream, that is highly susceptible to influence from motion.
Meeting abstract presented at VSS 2012