Prismatic adaptation as a novel tool to directionally modulate motor cortex excitability: evidence from paired-pulse TMS.

BACKGROUND:The prismatic adaptation (PA) is a visuo-motor procedure that has captured the attention of neuroscientists in the last decades, hence it seems to affect high-order cognition. However, the basic neural processes related to PA and its effects on cortical plasticity are not clear yet. OBJECTIVE/HYPOTHESIS: The aim of the present study is to explore whether PA induces a direct effect on the motor cortices (M1) excitability. METHODS: Fourteen healthy participants were submitted to paired-pulse TMS to measure short-intracortical-inhibition (SICI) and intracortical-facilitation (ICF) on both the left and the right M1, before and after PA, that could induce a leftward or rightward after-effect. RESULTS: An increase of intracortical-facilitation was found in the M1 contralateral to the after-effect direction. Moreover the extent of facilitation and of the after-effect were correlated to each others. CONCLUSION: This finding reveals that PA influences M1 cortices directly, raising their excitability. The present investigation represents an innovative step for the understanding of neurophysiological processes by which PA affects brain functions.