Tuesday, October 15, 2013

Mu Rhythms for BCI

As discussed previously, there are several approaches that are currently being pursued for Brain Computer Interfaces (BCI).  One approach is to perform an EEG and to measure the "Mu Waves" or "Mu Rhythms" from the sensorimotor portion of one's brain.  The Mu Waves are associated with you moving your body -- either by actually moving your body, or by you *thinking* about moving your body.  Sounds like a great way to command a computer, eh?  Let's dig in a little more...

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.  Specifically, they occur in the sensorimotor cortex, which is the portion of the brain associated with coordinating muscle motion and the perception of ones muscle and joint motion.  Looking at the image below, the sesnorimotor cortex as the areas labeled "Primary Motor Cortex" and "Primary Somatosensory Cortex".   It is relatively narrow strip going from one ear, up over the top of the head, to the other ear.  This is where Mu Waves seem to occur.

Illustration of Sections of the Brain (via UIC)

When Do they Appear?  It is my understanding that Mu Waves appear naturally when your body is physically relaxed.  The appearance of the Mu Waves are an indication that the sensorimotor portion of your brain is "idling".  When you move a major body part, those portions of your brain stop "idling", they get down to real work, and the Mu Waves go away (are "suppressed") during the motor activity.  Amazingly, this portion of your brain exhibits the same Mu Wave suppression simply by imagining the motion of a body part.  Even better, the specific portion of your cortex where the Mu Waves are suppressed is linked to the body part that you're imagining moving.  Now that's cool!

From BCI2000.  The different regions of the sensorimotor cortex, *roughly* correspond to different body parts.  Feet and legs are near the top of the head.  Hands are near the middle.  Face and tongue are near the bottom of the cortex, which on your scalp is located just above your ears.
Difference From 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.  The most common trigger for Alpha waves is simply to close your eyes.  In most people, closing your eyes idles the visual cortex (the whole back portion of your brain...the "occipital" region), which causes Alpha waves to appear throughout the rear portion of the brain.  This called the Posterior Dominant Rhythm.  The Mu Waves, by contrast, are associated with the sensorimotor portion of your brain, so they should only appear in the signals from the electrodes over that part of the brain.

How Can We Measure Our Mu Waves?  Theoretically, if you hook up an EEG sensor system to your scalp, and if you put some electrodes exactly over the sensorimotor portion of your brain, you should be able to see Mu Waves when you relax your body.  I have yet to be successful with this, though I will try again.  In preparation, I have been reading the tutorial from BCI2000 to get a better idea of where to put my electrodes and which electrodes to use for reference and bias.

Mu Waves in EEG Traces:  Below is a cool 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 F4-C4 trace.  This link also shows Mu waves in a raw EEG trace...in this montage, they're seen most strongly in the F3-C3 trace.  In my own trials, I have not specifically plotted these two combinations of electrodes.  I will.

What Could We Do with Mu Waves?  In the EEG traces above, it appears that the presence or absence of Mu Waves is pretty easy to see...we can probably get a computer to detect their presence pretty easily.  Once the computer sees that they're present, we can imagine moving our body, which should make them go away.  The computer can see that they went away and can take some action (like moving a robotic limb). It would only be a simple on/off control, but it still would be cool!

Using Mu Wave for Fine Control of a BCI:  Mu waves are compelling for BCI, though, because we don't have to be satisfied with simple on-off control.  Take, for example, the fact that our bodies and brains are sided -- the left side of your brain controls the right side of your body, and vice versa.  So, if by imagining motion with the left side of your body, the Mu waves should only be suppressed on the *right* side of your brain.  The converse is true as well -- imagining motion on the right side of your body should suppress the Mu waves on the left side of your brain.  As a result, you should be able to use a Mu wave reading BCI to control a robot to move in two ways...say, left or right.  Now it's getting useful!

Using Different Body Parts:  But we're not done.  Mu waves are quite local.  If you imagine moving just your feet, the Mu waves are only suppressed in a small portion of your sensorimotor cortex that, for the feet, is near the top of your head.  Imagining moving your hands suppresses the Mu waves in a different part of the cortex (down closer to the ears).  So, with more electrodes -- electrodes that are carefully placed over the different regions of the sensorimotor cortex -- we should be able to distinguish between thoughts of moving your hands versus moving your feet.   The movie below shows an example of a group who built a BCI that achieves this.  Fantastic.

The Future:  In theory, more electrodes on the scalp could maybe yield an even finer distinction between body parts, though I've only seen BCIs that do hands versus feet.  Maybe now is the time for a break-through!

Follow Up:  Check out my Mu waves!


  1. In the shown tutorial, mu waves are most evident in C3-F3. That sounds slightly odd, because to the proximity of the two electrode positions, one might suggest that an electrode at F3 would pick some of the mu frequencies generated at that region of the sensorimotor cortex and therefore hinder the strength of the mu waves when calculating the difference in potential between C3 and F3.
    Do you have a hypothesis for why that's actually not happening? Maybe the two electrode positions are not close enough for any interference to occur?

    1. My understanding is that the mu waves from the motor/sense cortex are quite localized. Therefore, they might not show up at F3 at all. Externally generated interference, however, might be similar at these two sites, so differencing them would result in the greatest interference rejection. This is my conjecture as to why C3-F3 might suggested.