Cross-modal reduction of phosphene threshold and the role of intraparietal sulcus in remmapping of visual-tatile interactions

Nadia Bolognini, Department of Psychology University of Milano Bicocca

Abstract
We explored whether phosphenes threshold (PT) can be modulated by irrelevant tactile input. Blindfolded subjects indicated the presence of phosphenes, after receiving a TMS pulse on the occipital cortex with or without a simultaneous touch. Subjects’ hands were held at the spatial position where the phosphenes appeared, either crossed or uncrossed. PT was significantly reduced by a touch in the same external location as the phosphenes (experiment 1), regardless of the hands posture (experiment 2). Signal detection measures showed that spatially aligned touches significantly enhanced sensitivity, without shifts in response criterion. Finally, we found that after disruption of intraparietal sulcus by low-frequency repetitive TMS (experiment 3), the ipsilateral touch was still effective in reducing PT with uncrossed hands, while the reverse was found with crossed hands (i.e. stronger effect of a controlateral touch).
In conclusion, the present study shows that PT can be cross-modally modulated by touch, likely increasing visual cortex excitability. Such cross-modal “boosting” critically depends on the spatial alignment of the multimodal stimuli in external space. Moreover, we highlight for the first time in humans the causal involvement of parietal cortex in updating the spatial congruence of the sensory maps by taking into account postural information.

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