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Stephen L. Macknik, Susana Martinez-Conde, Alexander A. Schlegel, Peter U. Tse; Dichoptic visual masking reveals localized processing of visibility in human extrastriate cortex. Journal of Vision 2004;4(8):43. doi: 10.1167/4.8.43.
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Tse et al., (VSS 2004) found that circuits within the occipital lobe maintain visual awareness of simple targets. However, this does not suggest that all parts of the occipital lobe are equally important to maintaining the visual awareness of simple targets. Single-unit responses in the awake primate LGN and area V1 do not always correlate with perception during dichoptic visual masking (Macknik & Martinez-Conde, JOCN, 2004). Here we set out to verify whether any areas in the human occipital lobe fail to correlate to the visibility of simple targets during dichoptic masking. Experiments: We broke down the visual hierarchy into area ROIs by conducting retinotopy measurements using standard retinotopic mapping techniques (Tong & Engel, Nature, 2001). We then measured the BOLD responses to a new dichoptic version of the full-screen Standing Wave of Invisibility illusion in which we presented the target monocularly to one eye while presenting the mask monocularly to the other. We then compared the amount of target suppression in the dichoptic versus monoptic conditions (monoptic from Tse, et al., 2004, VSS abstracts) for each occipital visual area (V1, V2, & V4), averaged across hemispheres. 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: 15 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: Activity in extrastriate areas correlated with both dichoptic and monoptic visual masking whereas activity in the striate cortex did not. These findings suggest that the circuits responsible for maintaining the awareness of simple targets may be localized to areas downstream of V1, but within the occipital lobe.
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