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Peter U. Tse, Gideon P. Caplovitz, Po-Jang Hsieh; Voluntary attention modulates the brightness of overlapping transparent surfaces. Journal of Vision 2005;5(8):244. doi: 10.1167/5.8.244.
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A striking new class of brightness illusions (Tse, VR, 2004) is introduced that cannot be entirely accounted for by bottom-up models. In these illusions, brightness can be modulated by voluntary attention in the absence of eye movements. For example, imagine three partially overlapping gray transparent disks on a white background. Fixating any point and attending to just one disk causes it to darken. These effects may arise from top-down or mid-level mechanisms that determine how 3D surfaces and transparent layers are constructed, which in turn influence perceived brightness.
Attention is not the only factor that influences perceived brightness here; Grouping procedures favor the minimal number of transparent layers necessary to account for the geometry of the stimulus, causing surfaces on a common layer to change brightness together. Attentional modulation of brightness places constraints on possible future models of filling-in, transparent surface formation, brightness perception, and attentional processing.
In addition to behavioral data characterizing the effect, fMRI was used to locate areas of the brain that modulate with perceived brightness change. In an event-related design, 15 subjects viewed just two disks, overlapping at fixation. They pressed a button whenever the righthand disk darkened, and carried out a color detection task at fixaton to guarantee fixation, wakefulness, and attentiveness. The BOLD signal was measured using a 1.5T GE scanner. Voxel volume was 3.75×3.75×5mm in 16 horizontal slices collected using single-shot T2* weighted gradient-recalled EPI sequences. Retinotopy was carried out in a separate session.
Neither retinotopic areas (V1, V2d/v,V3/VP, V4v, V3A) nor MT or LGN were found to modulate with perceived darkening. However, a whole brain GLM random effects analysis revealed temporal and parietal areas that modulated with perceived brightness changes. These areas could trigger the percept or serve as the neuronal correlate of this percept.
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