Biomedical Engineering Reference
In-Depth Information
10.5
EEG Monitoring Techniques
The traditional Rechtschaffen and Kales (R&K) guidelines for human sleep staging
were based on central EEG monitoring [1]. However, most sleep recording today
also includes occipital electrodes. Alpha activity is more prominent in occipital trac-
ings. The terminology for the electrodes adheres to the International 10-20 nomen-
clature in which the electrodes are placed at 10% or 20% of the distance between
structural landmarks on the head. Even subscripts refer to electrodes on the right
side of the head and odd to electrodes on the left side. The usual derivations use the
central or occipital electrodes referenced to the opposite mastoid electrode (C4-A1,
O1-A2). The greater distance between electrodes increases the voltage difference. A
minimum of one central EEG derivation must be recorded for sleep staging. In mod-
ern digital recording, typically all of the electrodes (C4, C3, O2, O1, A1, A2) are
recorded. Of note, additional electrodes may be added if one suspects seizure
activity. This is discussed in detail in later chapters.
10.6
Eye Movement Recording
The main purpose of recording eye movements is to identify REM sleep. EOG (eye
movement) electrodes typically are placed at the outer corners of the eyes—at the
right outer canthus (ROC) and the left outer canthus (LOC). In a common
approach, two eye channels are recorded and the eye electrodes are referenced to the
opposite mastoid (ROC-A1 and LOC-A2). To detect vertical as well as horizontal
eye movements, one electrode is placed slightly above and one slightly below the
eyes [4-7].
Recording of eye movements is possible because a potential difference exists
across the eyeball: front positive (
). Eye movements are detected
by EOG recording of voltage changes. When the eyes move toward an electrode, a
positive voltage is recorded.
There are two common patterns of eye movements (Figure 10.3). Slow eye
movements (SEMs), also called
slow-rolling
eye movements, are pendular oscillat-
ing movements that are seen in drowsy (eyes closed) wakefulness and stage 1 sleep.
By stage 2 sleep, SEMs usually have disappeared. REMs are sharper (more narrow
deflections), which are typical of eyes-open wake and REM sleep.
In the two-tracing method of eye movement recording, large-amplitude EEG
activity or artifact reflected in the EOG tracings usually causes
in-phase deflections.
In Figure 10.4, a K complex causes an in-phase deflection in the eye tracings, while
REM result in
out-of-phase
deflections.
+
), back negative (
−
10.7
Electromyographic Recording
Usually, three EMG leads are placed in the mental and submental areas. The voltage
between two of these three is monitored (for example, EMG1-EMG3). If either of
these leads fails, the third lead can be substituted. The gain of the chin EMG is
adjusted so that some activity is noted during wakefulness. The chin EMG is an
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