P146 Sulcus-based linear mapping of sensorimotor integration in the hand motor area

Abstract

Introduction We have recently introduced neuronavigated linear transcranial magnetic stimulation (TMS) mapping as a method to capture motor somatotopy in the hand motor area (M1 HAND ). In contrast to other mapping methods, linear TMS mapping adjusts the TMS coil position and orientation to the individual shape of the central sulcus (CS). Here we used this technique to map the spatial representation of short-latency afferent inhibition (SAI) in M1 HAND . SAI refers to a suppression of the motor evoked potential (MEP) amplitude by preceding peripheral electrical nerve stimulation of the contralateral hand. SAI is somatotopically specific: it’s stronger when electrical stimulation is applied close to the TMS-target muscle (homotopic stimulation) and weaker when not (heterotopic stimulation). Aim We hypothesized a somatotopic expression of SAI in M1 HAND for homotopic as opposed to heterotopic stimulation. Methods Electrical stimulation of the left index finger or little finger was applied 23 ms before TMS of right M1 HAND . MEPs were recorded from left first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles. SAI was applied randomly at seven M1 target sites following the individual shape of the right CS. Results We found a clear somatotopic representation of SAI. Homotopic SAI of the ADM muscle was expressed more medially than homotopic SAI of the FDI muscle along the M1 HAND . We also found somatotopy for heterotopic stimulation. Here SAI was replaced by a “surrounding” facilitation of the heterotopic muscle. Conclusions Linear sulcal TMS mapping revealed a somatotopically defined centre-surround organization of sensorimotor integration in the human M1 HAND .