User Tutorial:Introduction to the Mu Rhythm
In awake people, primary sensory or motor cortical areas typically display rhythmic EEG activity with a base frequency of 8-12 Hz when they are not engaged in processing sensory input or producing motor output. This idling activity, called mu rhythm when recorded over sensorimotor cortex and visual alpha rhythm when recorded over visual cortex, is thought to be produced by thalamocortical circuits. Unlike the visual alpha rhythm, which is obvious in a large majority of normal people, the mu rhythm was until quite recently thought to occur in only a minority of people. However, computer-based analyses have shown that the mu rhythm is present in a large majority of adults. Such analyses have also shown that mu rhythm activity comprises a variety of different 8-12 Hz rhythms, distinguished from each other by precise location, precise frequency, and/or typical relationship to concurrent sensory input or motor output.
Behavioral Properties
Several factors suggest that mu rhythm activity could be a good carrier for BCI-based communication. These rhythms are associated with those cortical areas that are most directly connected to the brain's normal motor output channels. Movement or preparation for movement is typically accompanied by a decrease in mu activity over sensorimotor cortex, particularly contralateral to the movement. This decrease has been labeled "event-related desynchronization" or ERD by Pfurtscheller (Pfurtscheller, G.: EEG event-related desynchronization (ERD) and event-related synchronization (ERS). In: E. Niedermeyer, F.H. Lopes da Silva (eds.) Electroencephalography: basic principles, clinical applications and related fields, 4th edition, pp. 958–967. Williams and Wilkins, Baltimore, MD (1999)). Its opposite, rhythm increase, or "event-related synchronization" (ERS) occurs in the post-movement period and with relaxation. Furthermore, and most relevant for BCI applications, ERD and ERS occur also with motor imagery (i.e., imagined movement); they do not require actual movement. Thus, they can occur independent of activity in the brain's normal output channels of peripheral nerves and muscles, and could serve as the basis for a BCI.
The figure displays examples of modulated mu rhythm signals (modified from [1]).
- A,B: Topographical distribution on the scalp of the difference (measured as (the proportion of the single-trial variance that is due to the task)), calculated for actual (A) and imagined (B) right-hand movements and rest for a 3 Hz band centered at 12 Hz.
- C: Example voltage spectra for a different subject and a location over left sensorimotor cortex (i.e., C3) for comparing rest (dashed line) and imagery (solid line).
- D: Corresponding spectrum for rest vs. imagery. Signal modulations are focused over sensorimotor cortex and in the mu- and beta-rhythm frequency bands.
Physical Properties
Geometry
Spatially, the origin of the mu rhythm is the hand resp. foot area of the motor cortex (displayed on the left side, in red).
The rhythm's source character is that of a dipole, with the dipole moment pointing perpendicular to the cortical surface. With regard to the scalp, a location in a gyrus will thus have a radial orientiation (1), while a location in a sulcus will result in a tangential orientation on the scalp (2). In the latter case, the dipole moment will be perpendicular to the central sulcus as well as the scalp.
For intermediate locations, the dipole orientation will be a linear combination of (1) and (2), resulting in a linear combination of the associated scalp potential distributions.
The figure displays typical mu rhythm scalp potential distributions (from Blankertz et al., 2007, reproduced with permission of the authors). The distribution on the left is due to a radially oriented source dipole located on the motor gyrus, while the distribution to the right is due to a tangentially oriented source dipole supposedly located in the sulcus that separates motor and sensory cortices.
Temporal Properties
The mu rhythm has an arc-shaped, periodic wave form (top left). In the frequency domain, such a waveform corresponds to a line spectrum with a strong first harmonic (bottom left). This means, that there will be a second peak in the beta band, located at exactly twice the frequency of the first peak. Most often, relative modulation (i.e., change in amplitude relative to mean amplitude) is identical for both peaks (spectrum of actually measured signals to the right).
BCI Construction
As discussed above, a human subject can wilfully influence the amplitude of her/his mu rhythm by imagination of hand or feet movement. Continuous feedback of mu rhythm amplitude can help improve this natural ability by selective reinforcement of successful strategies.
Much like a historic AM radio receiver, a mu rhythm BCI treats the mu rhythm as a carrier signal with information impressed on it by amplitude modulation. Consequently, its signal processing chain is analogous to that of an AM receiver:
Spatial Selection
Using a linear combination of simultaneous input samples, the spatial filtering step favors signals originating from the hand/feet area. In the AM receiver analogy, this step corresponds to a directional antenna that favors radio signals originating from the spatial direction corresponding to the broadcasting station's position.
Frequency Selection
All mu rhythm BCIs employ some type of frequency selection, favoring signals in a narrow band around a single, or multiple, peaks of the mu rhythm's spectrum. There is a number of possibilities to implement frequency selection; most common are
- IIR bandpass filtering,
- windowed spectral estimation methods such as
While IIR bandpass filtering is the direct computational analog of an AM receiver's tuning circuit, spectral estimation methods provide amplitudes for all frequency bands, and require actual frequency selection in a separate classification step.
Carrier Demodulation
After extracting the carrier signal, its amplitude time series (envelope) must be computed to obtain the original signal impressed onto the carrier. In a simple AM receiver, this is done using a rectifier diode in conjunction with a low pass circuit. If a BCI employs bandpass filtering for frequency selection, calculating the RMS over a short interval usually performs the equivalent function.
For BCIs using spectral estimation methods, demodulation is often implemented inside the spectral estimation step, with its output being a distribution of absolute amplitude vallues rather than a complex amplitude distribution.
Practical Aspects
- How to localize the motor cortex with the help of an EEG cap
- Suggested movement imagination
Next Step
As a next step, learn how to set up an EEG measurement.