Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 123

eye movements). Recently new criteria for sleep scoring were proposed by the Amer-
ican Academy of Sleep Medicine [Ibert et al., 2007], however R&K criteria are still
the golden rule for sleep staging. The differentiation of the sleep stages involves
measurement of several signals: not only EEG, but also electrooculogram (EOG),
muscular activity—electromyogram (EMG), and respiration, sometimes also mea-
surement of blood flow, electrocardiogram (ECG), and oxygen level in blood. This
kind of recording is called a polysomnogram. EOG is measured by means of elec-
trodes placed at the canthi of the eyes. The EOG and EMG help to distinguish Rapid
Eye Movement (REM) state, which is connected with dreaming. During the night
there are usually four or five cycles of sleep consisting of a period of NREM (non-
REM) sleep followed by REM. The sequence of sleep stages is usually illustrated in
the form of the hypnogram (hypnogram is shown in Figure 4.13). The recognition
of stages is based on the contribution of the different rhythms and the occurrence of
characteristic signal transients absent in wake EEG, namely: sleep spindles, vertex
waves, and K-complexes.
Sleep spindles are rhythmic waves of frequency 11-15 Hz and duration 0.5-2 s;
characteristic increase and then gradual decrease of amplitude is not always ob-
served. Vertex wave is a compound potential: a small spike discharge of positive
polarity preceding a large spike and followed by a negative wave of latency around
100 ms and often another small positive spike. The K-complex consists of an initial
sharp component, followed by a slow component that fuses with a superimposed fast
component. The sharp component may be biphasic or multiphasic. There is hardly
universal consent among encephalographers concerning definition of K-complexes,
which makes their automatic detection difficult. The most precise definition of K-
complexes may be found in [Paiva and Rosa, 1991]. K-complexes and vertex waves
are presumably a kind of evoked responses. Sleep stages may be briefly characterized
as follows:
Stage 1 (drowsiness) is associated with decrease of alpha rhythm, rhythms in 2-7 Hz
frequency band, and low amplitude rhythms of 15-25 Hz band. Sometimes
occurrence of vertex waves.
Stage 2 is characterized by appearance of K-complexes and sleep spindles; the slow
wave activity (SWA) should be less than 20% of the epoch.
Stage 3 is scored when 20-50% of the epoch contains delta waves (SWA) of peak-
to-peak amplitude greater than 75 μ V.
Stage 4 is dominated by slow wave activity of high amplitude. Stage 4 is scored
when more than 50% of the epoch contains delta activity.
REM is characterized by decrease of EEG amplitude, occurrence of faster rhythms,
rapid eye movements and a loss of muscular activity. Spectral characteristics in
REM is polyrhythmic and on the basis of EEG only, it is difficult to distinguish
REM from stage1.