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Minyoung Ryoo, Charlie Burlingham, Zvi Roth, David Heeger, Elisha Merriam; A widespread task-related hemodynamic response in human V1 is modulated by task difficulty. Journal of Vision 2018;18(10):1261. doi: 10.1167/18.10.1261.
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© ARVO (1962-2015); The Authors (2016-present)
Background: The brain exhibits large-scale endogenous responses in the absence of visual stimuli at the earliest stages of visual cortical processing. There are at least two distinct types of endogenous responses: 'attention-like' responses that are spatially selective and entrained to attention cues, and 'task-related' responses that are spatially global and entrained to task timing. Purpose: Are task-related hemodynamic responses modulated by task difficulty? Method: Observers performed a two-alternative forced-choice (2AFC) orientation discrimination task, reporting whether a grating patch was tilted clockwise or counter-clockwise relative to vertical. The stimulus was a circular grating that flashed briefly in the lower right visual field. The difficulty of the task was manipulated by changing the tilt angle of the grating. In blocks of easy trials, the grating was tilted ±20° away from vertical. In blocks of difficult trials, the grating was tilted by a much smaller amount (typically ±1°), with a staircase ensuring ~70% correct discrimination accuracy. Each block had a predictable trial structure with a fixed inter-trial interval. Activity was measured in visual cortex with fMRI (3T GE scanner, 32-ch coil, multi-echo pulse sequence, 22 slices covering visual cortex, voxel size 3×3×3 mm). Results: Robust, spatially-global bilateral activity was observed during performance of the 2AFC task. Such activity was observed in all retinotopically-defined visual areas, and in non-retinotopic portions of parietal and temporal cortex. Activity in contralateral regions likely reflected a mixture of stimulus-evoked and task-related activity. Activity in early visual areas, ipsilateral to the visual stimulus, was attributed to the task-related response. This task-related activity ipsilateral to the stimulus was significantly larger during blocks of difficult trials. Conclusions: Endogenous, task-related fMRI activity is spatially extensive in the human brain. It is dissociable from activity related to sensory stimuli, and is modulated by task demands, such as the level of difficulty.
Meeting abstract presented at VSS 2018
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