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Javier Garcia, Ramesh Srinivasan, Emily Grossman; TMS-induced oscillations in orientation discriminations. Journal of Vision 2008;8(6):482. doi: 10.1167/8.6.482.
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Dynamic theories of visual information processing emphasize the importance of recurrent or feedback processing, or continuous updating of visual information, in conjunction with the feedforward cascade of processing through the visual hierarchy (Lamme & Roelfsema, 2000; Bullier, 2001). Transcranial magnetic stimulation (TMS) has proven a powerful tool to study this interactive theory of visual processing with several studies suggesting that primary visual cortex (V1) is required for visual awareness (e.g., Pascual-Leone & Walsh, 2001). TMS has also been proposed to disrupt oscillatory behavior emerging from dynamic interactions between highly-connected regions of visual cortex, possibly disturbing reentry of visual information into V1 from higher level visual areas (Lamme & Roelfsema, 2000). The following experiments explore functional characteristics of these recurrent networks by measuring the impact of TMS over V1 and the motion-sensitive human middle temporal complex (hMT+). Subjects made a 2AFC orientation discrimination on limited lifetime cinematograms moving coherently in a single direction, or on oriented gabors. The 4 deg stimuli were viewed 3 deg in the periphery within the region of visual space affected by the TMS pulse (assessed via reported spatial location of phosphenes as measured prior to the experiment). TMS was applied over V1 or hMT+ at various latencies between 100 msec pre-stimulus onset to 100 msec post-stimulus onset. We find multiple latencies of stimulation affect performance on these tasks, both pre- and post-stimulus onset. Both before and after stimulus onset, the TMS pulses alternately facilitate and impair performance at regular intervals, in an oscillatory manner. The TMS-induced facilitation/impairment alternate at a higher frequency for the motion discrimination. These experiments suggest that a virtual lesion analogy may be inadequate for TM stimulation; instead, the TMS pulse interacts with oscillatory activity in networks processing the visual stimulus.
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