Attention influence response in the human visual area V1 for Ebbinghaus illusion

Early visual areas encode visual information in retinotopic coordinates, signaling retinotopic size and orientation properties. One of the most fundamental properties of human primary visual cortex (V1) is its retinotopic organization, whichmakes it an ideal candidate for encoding spatial properties, such as size, of objects. However, Ebbinghaus contextual information can lead to size illusions that are reflected in the spatial pattern of activity in V1. Here we used 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 on a black background display whose fixation attention, central attention or illusion than the test disk. We chose this same kind of the Ebbinghaus illusion to measure BOLD response differences due to changes in the attention. 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 (lager inducers vs. small 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 attention position that the Ebbinghaus illusion is related to textural attention. The temporal pole activation may relate to categorical object processing. Our result is consistent with the notion that the Ebbinghaus illusion results from attention influence. This effect was significantly reduced when the focus of spatial attention was narrowed with a demanding fixation and central attention task. We reason that focusing attention on the fixation task resulted in reduced activity in. and therefore reduced feedback from higher visual areas that process the Ebbinghaus illusion.