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Duje Tadin, Juha Silvanto, Alvaro Pascual-Leone, Lorella Battelli; Paradoxical improvement of motion perception following disruption of Cortical area MT/V5. Journal of Vision 2009;9(8):647. doi: 10.1167/9.8.647.
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© ARVO (1962-2015); The Authors (2016-present)
As stimulus size increases, motion direction of high-contrast patterns becomes increasingly harder to perceive. This counterintuitive result, termed “spatial suppression,” is thought to reflect center-surround antagonism; a neural mechanism ubiquitous in sensory systems. Our aim is to directly test this hypothesis by investigating the causal involvement of MT/V5 in spatial suppression. Given that MT/V5 center-surround antagonism is not inherited from feedforward inputs, we hypothesize that suppressing MT/V5 using offline low-frequency TMS should interfere with center-surround inhibition, and thus weaken spatial suppression. Moreover, as the main behavioral marker of spatial suppression is poor direction discrimination of large stimuli, we expect that disruption of MT/V5 will paradoxically improve motion perception of such stimuli.
To measure spatial suppression, we presented small (1.2°) or large (8°) high-contrast gratings (1cycle/°, 4°/s) in either the right or the left visual field, and measured duration thresholds for direction discrimination. We applied 15min train of 1Hz TMS to either left MT/V5 or the left occipital site that elicited phosphenes overlapping with stimulus location. Results revealed a significant interaction (p[[lt]]0.05) between stimulus location and TMS condition. Pairwise comparisons showed that spatial suppression was reduced in the visual field contralateral to the stimulation site following TMS over MT/V5 (p=0.01) but not after occipital TMS (p=0.53). No changes in performance were observed for ipsilaterally presented stimuli.
These results confirmed our hypothesis. Disruption of MT/V5 processing significantly reduced the strength of behaviorally measured spatial suppression, a finding mimicking abnormally weakened spatial suppression found in elderly, schizophrenics and depression patients. Furthermore, as hypothesized, this result was principally driven by improvements in motion discriminations of large stimuli after TMS over MT/V5 (p=0.02). This improved motion perception indicates that the critical neural constraints limiting motion perception of large stimuli likely involve MT/V5 and it suggests a causal link between MT/V5 and perceptually observed spatial suppression.
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