Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 105

4.1.2 EEG/MEG rhythms
The problem of the origins of EEG rhythmical activity has been approached by
electrophysiological studies on brain nerve cells and by the modeling of electrical
activity of the neural populations [Freeman, 1975, Lopes da Silva, 1996, Wright
et al., 2000, David et al., 2004]. It is generally accepted that cooperative properties
of networks consisting of excitatory and inhibitory neurons connected by feedback
loops play a crucial role in establishing EEG rhythms, but there is some evidence
that the intrinsic oscillatory properties of some neurons may contribute to the shap-
ing of the rhythmic behavior of networks to which they belong. The frequency of
oscillation depends on the intrinsic membrane properties, on the membrane potential
of the individual neurons, and on the strength of the synaptic interactions. The fol-
lowing rhythms have been distinguished in EEG (the same rhythms can be observed
in MEG) [Niedermayer and Lopes da Silva, 2004]: delta (0.1-4 Hz), theta (4-8 Hz),
alpha (8-13 Hz), beta (13-30 Hz), and gamma (above 30 Hz) ( Figure 4.2) . Frequen-
cies of gamma rhythms measured by scalp electrodes usually did not exceed 40-60
Hz because of strong damping of high frequencies by head tissues. High frequency
gamma (60-250) Hz were first observed in ECoG. They may be recorded as well by
means of MEG and also in EEG by application of sophisticated modern recording
techniques.
The contribution of different rhythms to the EEG/MEG depends on the age and
behavioral state of the subject, mainly the level of alertness. There are also con-
siderable inter-subject differences in EEG characteristics. EEG pattern is influenced
by neuropathological conditions, metabolic disorders, and drug action [Niedermayer
and Lopes da Silva, 2004].

•

Delta rhythm is a predominant feature in EEGs recorded during deep sleep.
In this stage delta waves usually have large amplitudes (75-200 μ V) and show
strong coherence all over the scalp.

• Theta rhythms occur in the state of drowsiness and in some emotional states,

but they are also involved in cognitive and working memory processes. In
the latter case theta waves are associated with gamma activity. Occurrence of
theta rhythm can also be connected with the slowing of alpha rhythms due to
pathology. Theta waves are predominant in rodents; in this case the frequency
range is broader (4-12 Hz) and waves have a high amplitude and characteristic
sawtooth shape.

• Alpha rhythms are predominant during wakefulness and are most pronounced

in the posterior regions of the head. They are best observed when the eyes are
closed and the subject is in a relaxed state. They are blocked or attenuated by
attention (especially visual) and by mental effort. Mu waves have a frequency
band similar to alpha, but their shape resembles the Greek letter μ .Theyare
prevalent in the central part of the head and are related to the function of motor
cortex, namely they are blocked by motor actions.