No abstract

Recent studies have shown that low-frequency repetitive transcranial magnetic stimulation (rTMS) to the left dorsal premotor cortex has a lasting influence on the excitability of specific neuronal subpopulations in the ipsilateral primary motor hand area (M1 HAND ). Here we asked how these premotor to motor interactions are shaped by the intensity and frequency of rTMS and the orientation of the stimulating coil. We confirmed that premotor rTMS at 1 Hz and an intensity of 90% active motor threshold (AMT) produced a lasting decrease in corticospinal excitability probed with single-pulse TMS over the left M1 HAND . Reducing the intensity to 80% AMT increased paired-pulse excitability at an interstimulus interval (ISI) of 7 ms. Opposite effects occurred if rTMS was given at 5 Hz: at 90% AMT, corticospinal excitability increased; at 80% AMT, paired-pulse excitability at ISI = = 7 ms decreased. No effects were seen if rTMS was applied at the same intensities to prefrontal or primary motor cortices. These findings indicate that the intensity of premotor rTMS determines the net effect of conditioning on distinct populations of neurones in the ipsilateral M1 HAND , but it is the frequency of rTMS that determines the direction of the induced change. By selecting the appropriate intensity and frequency, premotor rTMS allows to induce a predictable up-or down-regulation of the excitability in distinct neuronal circuits of human M1 HAND .

Through electrodes implanted for deep brain stimulation in three patients (5 sides) with Parkinson's disease, we recorded the electrical activity from the human basal ganglia before, during and after voluntary contralateral finger movements, before and after L-DOPA. We analysed the movement-related spectral changes in the electroencephalographic signal from the subthalamic nucleus (STN) and from the internal globus pallidus (GPi). Before, during and after voluntary movements, signals arising from the human basal ganglia contained two main frequencies: a high beta (around 26 Hz), and a low beta (around 18 Hz). The high beta (around 26 Hz) power decreased in the STN and GPi, whereas the low beta (around 18 Hz) power decrease was consistently found only in the GPi. Both frequencies changed their power with a specific temporal modulation related to the different movement phases. L-DOPA specifically and selectively influenced the spectral power changes in these two signal bands.