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Ichiro Kuriki, Hiroshi Ashida, Ikuya Murakami, Akiyoshi Kitaoka; Functional brain imaging of the ‘Rotating Snakes’ illusion. Journal of Vision 2008;8(6):64. doi: 10.1167/8.6.64.
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© ARVO (1962-2015); The Authors (2016-present)
The ‘Rotating Snakes’ (abbreviated as ‘snake’) figure (Kitaoka, 2003) is one of the static figures that induce perception of smooth illusory motion. Previous studies have suggested that cortical motion sensors are actually involved, but there has been no direct evidence for humans. Here we investigated whether this illusion activates motion sensitive areas in the human visual cortex by using functional magnetic- resonance-imaging (fMRI) technique. We compared the blood- oxygenation- level- dependent (BOLD) signals for ‘snake’ and control stimuli. The ‘snake’ stimulus consisted of an array of micropatterns, each having four colored blobs (black - blue - white - yellow; perceived direction is in this order). Circular repetition of this micropattern in the same color order yields illusory motion of slow rotation. The control stimulus was made by reversing the color order of adjacent micropattern in this circular array. No motion was perceived in this control stimulus. The stimulus sequence was designed as the repetition of 15 s of either ‘snake’ or ‘control’ stimulus and 15 s of uniform gray screen. We used a 1.5 T MRI scanner to obtain functional images (TR=3000 ms, voxel size of 3 mm × 3 mm × 3 mm, 120 scans/run). Separate fMRI runs were conducted for spontaneous- (SEM), guided- (GEM), and no- eye- movement (NEM) conditions. A stimulus for attentional control was constantly presented at the fixation point in GEM and NEM conditions. Significant differences in BOLD signals between ‘snake’ and ‘control’ were found under SEM and GEM conditions in human MT/MST complex (hMT+), but not under NEM condition. SEM condition exhibited the largest difference. Under no conditions did we find a significant difference in the primary visual cortex. Our results show activation of hMT+ underlying the ‘Rotating Snakes’ illusion and also support a significant role of eye movements in this illusion.
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