P 96. Prismatic lenses as a novel tool to directionally manipulate motor cortex excitability: Evidence from paired-pulse TMS

Abstract

Prismatic adaptation (PA) is a visuo-motor procedure requiring participants to adapt to prismatic lenses shifting the visual scene horizontally. Such an adaptation produces a phenomenon called “after-effect”, opposite to the side of lenses deviation. The after-effect has been frequently associated with a shift of spatial attention in the same direction and with a restoration of hemispatial neglect symptoms. PA has captured the interest of neuroscientists in the last decades, since it affects high-order spatial cognition even thought consisting of low-level visuo-motor processes. Despite a huge literature on this procedure, the basic neural processes related to PA and its effects on cortical plasticity are misunderstood. The aim of the present study was to explore whether PA induces a direct effect on the motor cortices (M1) excitability. This investigation will provide useful implications on how low-level visuo-motor processes can affect more general brain functions. Fourteen healthy participants were submitted to a paired-pulse transcranial magnetic stimulation (TMS) protocol. In detail this technique allows to assess how much a conditioning TMS stimulus (CS) influences a following test TMS stimulus (TS) at different inter-stimulus intervals (ISIs = 1–15 ms), delivered on M1 and measured as the amplitude of the motor evoked potential (MEP) recorded in the first dorsal interosseus (FDI) muscle controlateral to the stimulated M1. The greater the MEP amplitude, the more the controlateral M1 has been excited, the lower the MEP amplitude and the more the controlateral M1 has been inhibited. Short ISIs (1, 2, 3, 5 ms; short-intracortical-inhibition = SICI) usually produce a reduction in the MEP amplitude while long ISIs (7, 10, 15 ms; intracortical facilitation = ICF) usually produce an increase in the MEP amplitude. To investigate whether PA procedure modulated SICI and ICF, paired-pulse TMS was applied on both the left and right M1 before and after a session of PA inducing a leftward or a rightward after-effect. We found an increase of intracortical-facilitation (ICF) in the M1 controlateral to the after-effect direction. Interestingly, the extent of facilitation and after-effect correlated with each other. This finding reveals that PA influences directly M1 cortices raising their excitability. The present investigation represents an innovative step to the understanding of neurophysiological underppinings of PA, suggesting motor areas as a possible neuronal link between low and high levels brain functions.