Down and up! Does the mu rhythm index a gating mechanism in the developing motor system?

Abstract

Developmental research on action processing in the motor cortex relies on a key neural marker - a decrease in 6-12 Hz activity (coined mu suppression). However, recent evidence points towards an increase in mu power, specific for the observation of others' actions. Complementing the findings on mu suppression, this raises the critical question for the functional role of the mu rhythm in the developing motor system. We here discuss a potential solution to this seeming controversy by suggesting a gating function of the mu rhythm: A decrease in mu power may index the facilitation, while an increase may index the inhibition of motor processes, which are critical during action observation. This account may advance our conception of action understanding in early brain development and points towards critical directions for future research.

Keywords: Action understanding, execution, and observation; Brain development; Common coding; Motor system development; Mu suppression.

Fig. 1

Two studies reporting mu power increase during action observation. (A) Time-frequency plots show the spectral power at central channels (C3, C4). The dotted rectangle marks infants’ 7–10 Hz activity during the presentation of an action (2–6 s), relative to the pre-stimulus baseline. (B) Topographies display the spectral activity for the time-frequency window indicated by the dotted rectangle at C3 and C4, both p < .001 (from: Köster et al., 2020; under the creative commons license CC BY-NC-ND 4.0). (C) Normalized power averaged over channels C3 and C4 as a function of frequency shows increase in mu frequency range for 4-year-old children during action observation (top). (D) Topographic distribution of the normalized power in the 7–12 Hz frequency range shows increase in power over motor areas. (from: Meyer et al., 2020; under creative commons license CC by 4.0; see also Endedijk et al., 2017).