Transcranial static magnetic field stimulation of the human motor cortex

Abstract

The aim of the present study was to investigate in healthy humans the possibility of a non-invasive modulation of motor cortex excitability by the application of static magnetic fields through the scalp. Static magnetic fields were obtained by using cylindrical NdFeB magnets. We performed four sets of experiments. In Experiment 1, we recorded motor potentials evoked by single-pulse transcranial magnetic stimulation (TMS) of the motor cortex before and after 10 min of transcranial static magnetic field stimulation (tSMS) in conscious subjects. We observed an average reduction of motor cortex excitability of up to 25%, as revealed by TMS, which lasted for several minutes after the end of tSMS, and was dose dependent (intensity of the magnetic field) but not polarity dependent. In Experiment 2, we confirmed the reduction of motor cortex excitability induced by tSMS using a double-blind sham-controlled design. In Experiment 3, we investigated the duration of tSMS that was necessary to modulate motor cortex excitability. We found that 10 min of tSMS (compared to 1 min and 5 min) were necessary to induce significant effects. In Experiment 4, we used transcranial electric stimulation (TES) to establish that the tSMS-induced reduction of motor cortex excitability was not due to corticospinal axon and/or spinal excitability, but specifically involved intracortical networks. These results suggest that tSMS using small static magnets may be a promising tool to modulate cerebral excitability in a non-invasive, painless, and reversible way.

Figures

Figure 1. The magnet and its location

A, the magnet. B, schematic illustration of the magnet located over the cortical representation of the hand (hot spot).

Figure 2. The set-up for Experiments 1–3

A, experimental set-up for real and sham stimulation (Experiment 1). B, experimental set-up for real and sham stimulation (Experiment 2, double blind). C, experimental set-up for real stimulation using different duration of the tSMS (Experiment 3).

Figure 3. Experiment 1. Effects of transcranial static magnetic field stimulation (tSMS) on motor evoked potential (MEP) amplitude evoked by transcranial magnetic stimulation (TMS) pulses in a representative subject

The left column shows the baseline MEPs and the right column shows the MEPs 2–4 min after the application of the tSMS at different intensities (Big Magnet and Small Magnet) and different polarities (south and north). Sham stimulation is reported in the first line.

Figure 4. Experiment 1. Time course of the effects of the different kinds of tSMS and of sham stimulation on normalized MEPs

The grey column represents schematically the time of stimulation. Each time point is the average of 2 min. To reduce the number of multiple comparisons, statistical analyses were limited to the first 10 min after the end of stimulation (baseline plus 5 time points of 2 min each). The dotted circles indicate those data that are statistically significant. Error bars are standard errors of the mean.

Figure 5. Experiment 2. Time course of the effects of tSMS and sham stimulation on the mean amplitudes of the MEPs evoked by TMS

This experiment was conducted using a double blind protocol. Time course of the effects of 10 min of tSMS using a big magnet and south polarity and of sham stimulation on normalized MEPs. The grey column represents schematically the time of stimulation. Each time point is the average of 2 min. To reduce the number of multiple comparisons, statistical analyses were limited to the first 10 min after the end of stimulation (baseline plus 5 time points of 2 min each). The dotted circles indicate those data that are statistically significant. Error bars are standard errors of the mean.

Figure 6. Experiment 2. MEP amplitudes in individual subjects

Time course of the effects of 10 min of tSMS using the Big Magnet and south polarity and of sham stimulation on normalized MEPs. Each time point is the average of 2 min. Here we show the baseline plus 3 time points of 2 min each (6 min). The bold line and circles indicate the mean.

Figure 7. Experiment 3. Time course of the effects of the different duration of tSMS on normalized MEPs

The grey column represents schematically the time of stimulation. Each time point is the average of two minutes. To reduce the number of multiple comparisons, statistical analyses were limited to the first 6 min after the end of stimulation (baseline plus 3 time points of 2 min each). The dotted circles indicate those data that are statistically significant. Error bars are standard errors.

Figure 8. Experiment 4. Effects of tSMS on MEP amplitude evoked by transcranial electrical stimulation (TES)

Upper part of the figure shows active and resting motor thresholds before and 2–4 min after the application of the tSMS. Lower part of the figure shows effects of tSMS on MEP amplitude evoked by TES at rest and during voluntary contraction. Error bars are standard deviations.