Interhemispheric symmetry of µ-rhythm phase-dependency of corticospinal excitability

Abstract

Oscillatory activity in the µ-frequency band (8-13 Hz) determines excitability in sensorimotor cortex. In humans, the primary motor cortex (M1) in the two hemispheres shows significant anatomical, connectional, and electrophysiological differences associated with motor dominance. It is currently unclear whether the µ-oscillation phase effects on corticospinal excitability demonstrated previously for the motor-dominant M1 are also different between motor-dominant and motor-non-dominant M1 or, alternatively, are similar to reflect a ubiquitous physiological trait of the motor system at rest. Here, we applied single-pulse transcranial magnetic stimulation to the hand representations of the motor-dominant and the motor-non-dominant M1 of 51 healthy right-handed volunteers when electroencephalography indicated a certain µ-oscillation phase (positive peak, negative peak, or random). We determined resting motor threshold (RMT) as a marker of corticospinal excitability in the three µ-phase conditions. RMT differed significantly depending on the pre-stimulus phase of the µ-oscillation in both M1, with highest RMT in the positive-peak condition, and lowest RMT in the negative-peak condition. µ-phase-dependency of RMT correlated directly between the two M1, and interhemispheric differences in µ-phase-dependency were absent. In conclusion, µ-phase-dependency of corticospinal excitability appears to be a ubiquitous physiological trait of the motor system at rest, without hemispheric dominance.

Figure 1

RMT and PHASE for the different stimulation conditions. (a) RMT as a percentage of the maximum stimulator output (%MSO). The bars represent the median over subjects and the whiskers indicate the interquartile ranges. (b) PHASE = (RMT_pos − RMT_neg)/RMT_rand for the motor-dominant and motor-non-dominant hemisphere. The dots show the data of the individual subjects. The white bars indicate the mean and the mean ± standard deviation of the data.

Figure 2

RMT differences for the motor-dominant and motor-non-dominant M1. (a) Difference in RMT for each subject between the µ-phase positive-peak and negative-peak conditions. The gray line shows a linear fit to the data. For 40/51 of the subjects (78%), either RMT_neg < RMT_pos (37/51 subjects, 73%) or RMT_pos < RMT_neg (3/51 subjects, 6%) was observed in both hemispheres, i.e., their data point is either in the upper right or lower left quadrant of the plot, respectively. (b) Number of subjects having a certain probability for RMT_neg < RMT_pos for the motor-dominant and motor-non-dominant M1. For each subject, the probabilities are obtained from their RMT probability density functions (see Methods, Fig. 3). The number of subjects is reported for adjacent 0.05 × 0.05 squares. The white region marks the area with no subjects.

Figure 3

Resting motor threshold. MEP data of a representative single subject for µ-phase positive-peak and negative-peak conditions and the corresponding probability density functions for the RMT. The dashed line indicates the 50 µV MEP amplitude threshold. The arrows indicate the RMT that corresponds to the location of the maximum of the probability density function.