The sensorimotor mu rhythm, also known as mu wave, comb or wicket rhythms or arciform rhythms, are synchronized patterns of electrical activity involving large numbers of neurons, probably of the pyramidal type, in the part of the brain that controls voluntary movement. These patterns as measured by electroencephalography (EEG), magnetoencephalography (MEG), or electrocorticography (ECoG), repeat at a frequency of 7.5–12.5 (and primarily 9–11) Hz, and are most prominent when the body is physically at rest. Unlike the alpha wave, which occurs at a similar frequency over the resting visual cortex at the back of the scalp, the mu rhythm is found over the motor cortex, in a band approximately from ear to ear. People suppress mu rhythms when they perform motor actions or, with practice, when they visualize performing motor actions. This suppression is called desynchronization of the wave because EEG wave forms are caused by large numbers of neurons firing in synchrony. The mu rhythm is even suppressed when one observes another person performing a motor action or an abstract motion with biological characteristics. Researchers such as V. S. Ramachandran and colleagues have suggested that this is a sign that the mirror neuron system is involved in mu rhythm suppression, although others disagree.
The mu rhythm is of interest to a variety of scholars. Scientists who study neural development are interested in the details of the development of the mu rhythm in infancy and childhood and its role in learning. Since a group of researchers believe that autism spectrum disorder (ASD) is strongly influenced by an altered mirror neuron system and that mu rhythm suppression is a downstream indication of mirror neuron activity, many of these scientists have kindled a more popular interest in investigating the mu wave in people with ASD. Assorted investigators are also in the process of using mu rhythms to develop a new technology: the brain–computer interface (BCI). With the emergence of BCI systems, clinicians hope to give the severely physically disabled population new methods of communication and a means to manipulate and navigate their environments.
Mirror neurons
The mirror neuron system consists of a class of neurons that was first studied in the 1990s in macaque monkeys. Studies have found sets of neurons that fire when these monkeys perform simple tasks and also when the monkeys view others performing the same simple tasks. This suggests they play a role in mapping others' movements into the brain without actually physically performing the movements. These sets of neurons are called mirror neurons and together make up the mirror neuron system. Mu waves are suppressed when these neurons fire, a phenomenon which allows researchers to study mirror neuron activity in humans. There is evidence that mirror neurons exist in humans as well as in non-human animals. The right fusiform gyrus, left inferior parietal lobule, right anterior parietal cortex, and left inferior frontal gyrus are of particular interest. Some researchers believe that mu wave suppression can be a consequence of mirror neuron activity throughout the brain, and represents a higher-level integrative processing of mirror neuron activity. Tests in both monkeys (using invasive measuring techniques) and humans (using EEG and fMRI) have found that these mirror neurons not only fire during basic motor tasks, but also have components that deal with intention. There is evidence of an important role for mirror neurons in humans, and mu waves may represent a high level coordination of those mirror neurons.
Development
A fruitful conceptualization of mu waves in pediatric use is that mu wave suppression is a representation of activity going on in the world, and is detectable in the frontal and parietal networks. A resting oscillation becomes suppressed during the observation of sensory information such as sounds or sights, usually within the frontoparietal (motor) cortical region. The mu wave is detectable during infancy as early as four to six months, when the peak frequency the wave reaches can be as low as 5.4 Hz. There is a rapid increase in peak frequency in the first year of life, and by age two frequency typically reaches 7.5 Hz. The peak frequency of the mu wave increases with age until maturation into adulthood, when it reaches its final and stable frequency of 8–13 Hz. These varying frequencies are measured as activity around the central sulcus, within the Rolandic cortex.