Mu suppression - A good measure of the human mirror neuron system?

Abstract

Mu suppression has been proposed as a signature of the activity of the human mirror neuron system (MNS). However the mu frequency band (8-13 Hz) overlaps with the alpha frequency band, which is sensitive to attentional fluctuation, and thus mu suppression could potentially be confounded by changes in attentional engagement. The specific baseline against which mu suppression is assessed may be crucial, yet there is little consistency in how this is defined. We examined mu suppression in 61 typical adults, the largest mu suppression study so far conducted. We compared different methods of baselining, and examined activity at central and occipital electrodes, to both biological (hands) and non-biological (kaleidoscope) moving stimuli, to investigate the involvement of attention and alpha activity in mu suppression. We also examined changes in beta power, another candidate index of MNS engagement. We observed strong mu suppression restricted to central electrodes when participants performed hand movements, demonstrating that mu is indeed responsive to the activity of the motor cortex. However, when we looked for a similar signature of mu suppression to passively observed stimuli, the baselining method proved to be crucial. Selective suppression for biological versus non-biological stimuli was seen at central electrodes only when we used a within-trial baseline based on a static stimulus: this method greatly reduced trial-by-trial variation in the suppression measure compared with baselines based on blank trials presented in separate blocks. Even in this optimal condition, 16-21% of participants showed no mu suppression. Changes in beta power also did not match our predicted pattern for MNS engagement, and did not seem to offer a better measure than mu. Our conclusions are in contrast to those of a recent meta-analysis, which concluded that mu suppression is a valid means to examine mirror neuron activity. We argue that mu suppression can be used to index the human MNS, but the effect is weak and unreliable and easily confounded with alpha suppression.

Keywords: Alpha; Baseline; Beta; EEG; Frequency; Methods; Mirror neurons; Mu suppression.

Fig. 1

A diagram depicting the analysis using the three different baselining techniques. The period of the video in which the hand/kaleidoscope pattern moves (the orange section) is compared against one of three baselines: 1. The 2 sec early interval period immediately preceding the video when there is a static picture presented (purple); 2. The average power of the late interval period during the short (8 sec) rest trials (red); 3. The average power of the late interval periods in the long rest condition (green). The long rest period is composed of 40 × 2 sec epochs. In each 40 sec own movement trial, there are five triggers every 8 sec used to divide the movement trials up into five epochs. The own movement trials are analysed the same way as the video trials, comparing the power in the late interval the late interval in the short rest trials and the average power in the long rest condition.

Fig. 2

Graph A shows changes in the 8–13 Hz band (alpha/mu). Graph B shows changes in the 13–35 Hz (beta) band. Kal = kaleidoscope, HNO = Hand (no object), HO = Hand (with object), OM = own movement. Error bars are standard error. Planned comparisons between the video conditions that were significant are highlighted and asterisked: * indicates p < .05, ** indicates p < .01. Where one-sample t-tests found that suppression at central sites was significantly below 0, this is marked with a white *.

Fig. 3

Graph A shows changes in the 8–13 Hz band (alpha/mu). Graph B shows changes in the 13–35 Hz (beta) band. Kal = kaleidoscope, HNO = Hand (no object), HO = Hand (with object), OM = own movement. Error bars are standard error. Planned comparisons between the video conditions that were significant are highlighted and asterisked: * indicates p < .05, ** indicates p < .01. Where one-sample t-tests found that suppression at central sites was significantly below 0, this is marked a white *.

Fig. 4

Graph A shows changes in the 8–13 Hz band (alpha/mu). Graph B shows changes in the 13–35 Hz (beta) band. Kal = kaleidoscope, HNO = Hand (no object), HO = Hand (with object). Error bars are standard error. Planned comparisons between the video conditions that were significant are highlighted and asterisked: * indicates p < .05. Where one-sample t-tests found that suppression at central sites was significantly below 0, this is marked with a white *.