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Two separable stimulus - independent modulations of visual cortex during threshold contrast detection


, : Two separable stimulus - independent modulations of visual cortex during threshold contrast detection. Society for Neuroscience Abstract Viewer & Itinerary Planner : Abstract No 767 5

Whole brain 3 Tesla fMRI was performed while subjects (n=3) detected the presence/absence of a near threshold contrast reversing (2Hz) checkerboard annulus (1.5 to 3 degree radius, duration 750ms, present on 50% of trials). Audible beeps occurred 1 sec prior to stimulus onset, at stimulus onset, at stimulus offset, and to signal the time of response. In separate blocks, subjects were cued to respond either 0.1 or 7.6 sec after stimulus offset. In additional sessions, we examined the effect of: (a) withholding (no-go) or emitting (go) a response; (b) self-paced (sub-vocal counting) vs. externally triggered (beep) response. Retinotopic areas and eccentricity representations were mapped in a separate session.Two stimulus-independent endogenous responses were observed: (1) Retinotopically specific modulation of V1 and adjacent areas, corresponding in time to the stimulus presentation window, occurred on both stimulus present and stimulus absent trials (Ress, Backus & Heeger, 2000). (2) A considerably larger activation was observed corresponding to the time of response. This modulation was clearly seen in V1, yet the activation did not map to the stimulus. It was stronger in the more peripheral (anterior) representations, and extended along the calcarine sulcus into regions anterior to visuotopic cortex. This response also occurred in the no-go condition (a); and in the absence of any external stimuli, including auditory (b). It did not correspond in time to any visual attentional demands. Subjects denied any visual imagery at the time of response.This large modulation of visual cortex thus appears to play no direct role in perception; nor does it appear to be associated with the actual act of response. However, it does represent a reliable reactivation of visual cortex associated with the completion of a visuo-motor task.

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