P21-T The influence of the navigated TMS mapping protocol on the accuracy of motor representation characteristics

Abstract

Background Navigated transcranial magnetic stimulation enables noninvasive mapping of muscle cortical representations for research and clinical purposes. Several mapping protocols are in use, with no consensus regarding the optimal one and insufficient data about their influence on the mapping accuracy. We aimed to determine this influence for protocols that employ a stimulation point grid. Methods We mapped the abductor pollicis brevis muscle representations in 8 healthy volunteers (3 women, age median 28, quartiles 24, 29) three times on consecutive days. We used a 7 × 7 stimulation grid, with 10 stimuli per cell applied in a pseudorandom order at an intensity of 110% of the resting motor threshold. The grid size was chosen to cover on average 98% of the area of 121 representations previously mapped without grid. The studied parameters were the area of cells with mean motor evoked potential (MEP) amplitude above 50 μV, the area of cells with at least one (or 6) suprathreshold MEP, the area weighted by the mean amplitude (or the probability of suprathreshold MEP), and the location of the center of gravity. Results Parameter accuracy significantly increased with the number of stimuli. The area weighted by the probability of suprathreshold MEP had the highest accuracy and differed significantly between sessions in three subjects. Conclusions The obtained accuracy dependencies can guide the choice of a sufficient number of stimuli for a given problem, e.g. the detection of significant area changes between sessions in our dataset was possible with at least 7 stimuli per cell. Funding RSF17-75-10062.