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Peter U. Tse, Susana Martinez-Conde, Alexander A. Schlegel, Stephen L. Macknik; Visibility and visual masking of simple targets is confined to occipital cortex. Journal of Vision 2004;4(8):42. doi: 10.1167/4.8.42.
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© ARVO (1962-2015); The Authors (2016-present)
A visible target object can be “masked”, or rendered perceptually invisible, by modifying its spatiotemporal context. Visual masking illusions can thus be used to distinguish neuronal circuits that correlate with the physical stimulus from circuits that correlate with perception (Macknik & Martinez-Conde, 2004). Experiments: Here we conduct whole-brain fMRI in humans while they view targets that are either visible or invisible, to localize brain regions that show visibility-correlated modulation of activity. The visual masking illusion we used was a new version of the “Standing Wave of Invisibility” (SWI), modified to tile an entire monitor screen (so as to increase brain area activation). The subjects fixated the convergence point of a field of radiating wedges — 16 target wedges subtended in width 7.5°, interlaced with 16 mask wedges of 15° width. In order to guarantee that observers maintained fixation, they viewed all stimuli while conducting a simple task at the fixation point not directly related to issues of visibility (press button when fixation point blinks). Subjects: 23 subjects were run in a standard fMRI block-design, GE 1.5T, one-shot EPI, FA 90 degrees, epochs 20s, TR = 2.5secs, 25 axial slices. Results: We generated a whole-brain ROI of the voxels that responded better to targets than to masks and determined the voxels within that ROI that had a higher response to the target-visible as opposed to the target-invisible condition. Surprisingly, only voxels within the occipital lobe showed significant target suppression, suggesting that neurons important to the maintenance of visibility reside in the occipital lobe. These results provide the first evidence of an upper boundary in the visual hierarchy for the processing of visual masking and visual awareness of simple targets, thus constraining the location of circuits that maintain the visibility of simple targets to occipital cortex.
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