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Harald Stogbauer, Virginie van Wassenhove, Shinsuke Shimojo; Neural correlates of a saltation illusion. Journal of Vision 2007;7(9):989. doi: 10.1167/7.9.989.
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© ARVO (1962-2015); The Authors (2016-present)
Stationary flashes shown before the presentation of an apparent motion stimulus are mislocalized in the direction of apparent motion (visual rabbit illusion, Geldard 1976). This perceptual displacement depends on the position of the last flash suggesting that a “backward reconstruction” is taking place in perception. Here, participants were tested on a visual rabbit paradigm while being recorded under electroencephalography (EEG). The stimuli consisted of three flashes: the first and third flash were 4 degrees apart (either to the left or right) and 25 degrees away from fixation. The second flash was shown either at the same location as the first flash (RAB) or at a location between the first and third flashes (SAL). Psychophysical results show that participants are unable to discriminate between RAB and SAL conditions. The EEG study investigated the time course of cortical processing by contrasting the SAL and RAB conditions. First, event-related potentials (ERPs) in occipital electrodes showed a significant amplitude difference between stimulus types shortly (∼100 milliseconds) after the presentation of the second flash. At about 180 ms after presentation of the third flash, a significant amplitude difference was found in parietal electrodes. This ERP profile clustered RAB and SAL according to the perceptual outcome for the direction of motion (left or right). Starting at about 240 ms after presentation of the last flash, a significant difference between the RAB and SAL conditions was found independent of the direction of apparent motion. This difference was observed mainly in the frontal electrodes. The ERP results suggest three distinct stages in the processing of the visual rabbit illusion, a low-level feature stage in which the different locations for the second flash can be seen, a discrimination of motion direction stage and an “anti-illusion” stage which shows a categorization of RAB and SAL, despite no perceptual difference.
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