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Ansgar Koene, Yu-Chin Huang, Chien-Chung Chen; Percept dependent acitivity in the occipitotemporal cortex for Ebbinghaus illusion. Journal of Vision 2008;8(6):954. doi: 10.1167/8.6.954.
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Early visual areas encode visual information in retinotopic coordinates, signaling retinotopic size and orientation properties. Distance and orientation invariant object recognition however requires stimulus processing based on relative stimulus dimensions (e.g. relative lengths and orientation of edges with respect to each other) independent of absolute retinotopic size or orientation. Here we used event-related fMRI to measure BOLD response to a variant of the Ebbinghaus illusion to test where visual information processing starts to encode relative, opposed to retinotopic, sizes. The stimulus consisted of a white test disk superimposed on a background texture of overlapping disks whose size was smaller, equal or larger than the test disk. Visibility of the individual inducer disks was achieved through the use of different gray levels. We chose this variant of the Ebbinghaus illusion to avoid BOLD response differences due to changes in the retinotopic area stimulated by the inducers. Perceived size was determined using a 2-interval-forced-choice design. A comparison disk of variable size was presented for 400ms in one hemi-field then after a 100ms blank period the Ebbinghaus stimulus was presented for 600ms with the test disk in the other hemi-field. Psychophysical comparison with the traditional Ebbinghaus illusion showed that the strength of the illusion in our variant was comparable. Comparison of BOLD response across conditions (equal sized vs. small inducers; equal sized vs. large inducers; small vs. large inducers) revealed illusion related activity in the intraoccipital sulcus (IOS), the middle temporal gyrus and the temporal pole. The IOS and the posterior middle temporal activation may relate to depth judgment suggesting that the Ebbinghaus illusion is related to textural depth cues. The temporal pole activation may relate to categorical object processing. Our result is consistent with the notion that the Ebbinghaus illusion results from depth judgment.
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