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Sara Agosta, Duje Tadin, Alvaro Pascual-Leone, Lorella Battelli; Dissociating mechanisms of spatial suppression and summation in human MT: a tDCS study. Journal of Vision 2012;12(9):934. doi: 10.1167/12.9.934.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: In visual motion perception, spatial suppression is behaviorally measured as the increase of direction discriminations threshold with the increasing stimulus size. This counterintuitive result likely reflects inhibitory mechanisms within visual area MT (Churan et al., 2008; Tadin et al., 2011). Spatial suppression, however, is observed only for high-contrast stimuli. At low-contrast, stimuli become easier to discriminate as their size increases, a result indicating spatial summation. Here, we aimed to (1) determine whether both spatial summation and spatial suppression are causally linked to MT, and (2) elucidate the role(s) that MT plays in motion summation and suppression. We addressed these questions by interfering with the normal MT functioning using bilateral cathodal transcranial direct current stimulation (tDCS). Methods: Eight subjects performed a motion direction discrimination task in four conditions: small (1.2°) or large (8°) gratings presented at either high-contrast or low-contrast (1cycle/°, 4°/s, foveally presented). Each subject completed three counterbalanced sessions: baseline, cathodal tDCS and sham tDCS. During tDCS sessions participants completed the task three times: pre-tDCS, post-tDCS and 60 minutes post-stimulation. tDCS was delivered bilaterally over each MT for 20 min (2 mA). Results: At baseline, thresholds for large high-contrast gratings were substantially higher than those for small gratings, a marker of spatial suppression. At low-contrast, large moving stimuli were easier to discriminate, indicating spatial summation. Cathodal tDCS affected both spatial suppression and spatial summation, but in different ways. We found a significant improvement in motion discriminations of large, high-contrast stimuli and a significant impairment in motion discriminations of large, low-contrast stimuli. Sham stimulation had no effect. Conclusion: These findings are consistent with the hypothesis that spatial suppression and summation critically and directly depend on mechanisms within visual area MT. Moreover, dissociable effects of tDCS on suppression and summation indicate that MT plays opposite roles in these visual processes.
Meeting abstract presented at VSS 2012
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