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.
Highlights
Since the discovery of “mirror neurons” in the macaque brain, researchers have investigated the presence of such neurons in humans, and considered what the functional role of the human mirror neuron system (MNS) might be
We have focused here on alpha and beta frequency ranges, but studies vary in terms of the precise frequency ranges used to define these, and some argue for finer subdivision of these frequency bands (e.g. Pfurtscheller, Neuper, & Krausz, 2000)
We examined whether changes in 8-13 Hz power at the central electrodes are distinct from changes in power at this frequency in the occipital regions, and whether high occipital alpha during baseline tasks could be a confounding factor in previous mu suppression designs
Summary
Since the discovery of “mirror neurons” in the macaque brain, researchers have investigated the presence of such neurons in humans, and considered what the functional role of the human mirror neuron system (MNS) might be. Observes or imagines themselves performing an action, the firing of these cells becomes desynchronised This desynchronisation leads to reduced mu power, compared to when the cells were firing together (Pfurtscheller, Neuper, Andrew, & Edlinger, 1997). If there is a reduction in mu power in the experimental condition compared to the baseline condition, the interpretation is that the experimental condition has activated neurons in sensorimotor cortex. Because mu suppression is seen both when an individual performs and observes an action, it has been taken as a proxy for the activity of the human mirror neuron system (Muthukumaraswamy & Johnson, 2004; Muthukumaraswamy, Johnson, & McNair, 2004; Oberman, McCleery, Ramachandran, & Pineda, 2007; Pineda, 2005)
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