Introduction to Mu Waves

Brain-Computer Interfaces (BCI) are currently being pursued in several approaches by different researchers and people around the World. One approach is to measure the “Mu Rhythms”, or also known as “Mu Waves” from the sensorimotor area of one’s brain by registering an EEG signal. The Mu Waves are associated with the movement of the body – either by actually moving any part of your body or by “thinking” about moving that particular part of your body. Given that they only appear when the body is completely relaxed, Mu Rhythms have a specific range of high activity at a frequency of approximately 8 – 12 Hz. Moreover, this activity is thought to be produced by interactions between the thalamus and the cortex.

What are Mu Waves?

The first paragraph on Mu waves in Wikipedia seems decent enough. As it says, Mu Waves are a type of oscillating electrical rhythm within the brain that can be seen in an EEG recording. In particular, they occur in the sensorimotor cortex, which is the area of the brain associated with coordinating muscle motion and the perception of one’s muscle and joint muscle. Looking at the image below, the sensorimotor area involves Primary motor cortex and Primary somatosensory cortex. It is a narrow strip that goes from one ear to the other one by passing up the top of the head. Here it is where Mu Waves seem to occur.

Figure 1. The main functional divisions of the cortex as seen from a lateral view. Note that most of the primary visual cortex is on the medial surface of each cerebral hemisphere, and so only the small lateral portion is visible in this figure. (From Carlson, 1998, p. 69)

When do they appear?

Mu Waves appear naturally when your body is physically relaxed. The appearance of them is an indicator that the sensorimotor area of your brain is “idling”. When you move a major part of your body, this area of your brain stop idling, they get down to real work; there is a decrease in Mu activity over the sensorimotor cortex commonly labeled as Event-Related Desynchronization (ERD) by Pfurtscheller during the motor activity. Its opposite, that appears when the rhythm increase or Event-Related Synchronization (ERS) occurs after one has moved a part of his body or when one’s physically relaxed.

Surprisingly,  this area of the brain exhibits the same Mu activity suppression simply by imagining the motion of a body part. Moreover, the specific portion of your cortex where the Mu Waves are suppressed is linked to the body part that you are imaging moving.

Figure 2. The motor homunculus on the precentral gyrus. The motor cortex can be seen in the lateral view of the cerebral cortex. (From Netter, 1974, p. 68)

The difference between similar rhythms

The Mu Rhythm occurs in the frequency range commonly referred to as Alpha Waves (8-12 Hz). There are several sources of activity in the Alpha range. When you close your eyes, there is an immediate increase in Alpha activity. In general, closing your eyes idles the visual cortex (the whole back area of your brain as it is shown in Figure 1), which causes Alpha Waves to appear throughout the rear portion of the brain. This particular kind of activity is known as the Posterior Dominant Rhythm (PDR). In contrast, Mu Waves are associated with the sensorimotor area of your brain, so they should only appear in the EEG signals from the electrodes located over that part of the brain.

How can we measure Mu Waves?

Theoretically, if you hook up an EEG sensor system to your scalp, and you select the very specific electrodes located in the narrow sensorimotor of your brain (Figure 2), you should be able to see Mu Waves on the screen when your body is physically relaxed. In practice, there is a tutorial from BCI2000’s wiki that helps to get a better idea of which are the most accurate electrodes to register Mu activity, where to locate both the reference and the ground electrodes.

How do Mu Waves look like?

There is a video that shows what Mu Waves look like in raw EEG traces. Being localized to just the sensorimotor cortex, they appear most strongly in the C3 and C4. Moreover, I wanted to try by myself to acquire Mu Waves, so I used my EEG Headset created by OpenBCI and, as a result, I got the Mu Waves presented in Figure 3 by only closing my eyes and being as much relaxed as I could.

Figure 3. Mu Waves detected with OpenBCI EEG Headset by closing only my eyes.

Just to be sure of my results, I sought to the FFT plot of my EEG signal, and I found a high peak at 9 Hz, as it is shown in the figure above.

Figure 4. There is a high peak around 9 Hz.

What could we do with Mu Waves?

It appears in the video above that the presence or absence of Mu Waves is pretty easy to notice the difference. Once you detect them, and you can distinguish between the two kind of activities, a forward step is to imagine moving your body. Accordingly, to different studies, you should be able to identify them again.

Using Mu Waves to control a BCI

As we have seen before, Mu Waves appear in the sensorimotor cortex. Thus, when you move a body part either from the right or the left side, surprisingly it is reflected in the opposite part of your brain. In other words, the right side of your brain controls the left side of your body and vice versa. For example, if you move your right hand, you will notice an increase of activity in electrodes located in C1, C3, and C5. Likewise, imagining motion on the right side of your body should suppress the Mu waves on the left side of your brain.

Using different parts of your body

Mu waves are entirely local. If you imagine moving just your feet, the Mu Waves are only suppressed in a small portion of your sensorimotor cortex that, for your feet, is located near the top of your head (Figure 2). Imagining moving your hands suppresses the Mu activity in a different part of the cortex (electrodes C3 and C4). So, using electrodes placed over different regions of the sensorimotor cortex should let us be able to distinguish between thoughts of moving your hands versus moving your feet.



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