Quantitative Sleep Monitoring - Quantitative EEG Analysis Methods and Clinical Applications - page 274

and 4 of NREM sleep, and finally theta waves are an indication of REM sleep [42].
Examples of EEG signals in each of the stages have been adapted from Geyer et al.
[43] (Figures 10.5 to 10.9). Figure 10.5 shows a 30-second segment of EEG when
the patient is awake. Alpha rhythms become more accentuated after the patient
closes her eyes, indicated by the dashed line. Figure 10.6 is an example of what an
EEG signal may look like when the patient is in stage 1 of NREM sleep. The alpha
activity that was present in the wake state eventually turns into theta waves. Stage 2
of NREM sleep is shown in Figure 10.7. Stage 2 is denoted by the K complexes (solid
line) and sleep spindles (dotted lines). The final stages of NREM sleep, stages 3 and
4, are depicted in Figure 10.8 and are revealed by the delta activity. Finally, the
sawtooth theta waves, shown by the solid line in Figure 10.9, are a clear indication
that the patient is in REM sleep.
Although a visual inspection of the EEG signal can be an indicator of the fre-
quencies that make up the signal, a clearer representation of the frequencies can be
achieved by transforming the signal into the frequency domain by using the Fourier
transform. The Fourier transform is a mathematical technique that can transform
any time series into a spectrum of the frequencies that produce it. It is a generaliza-
tion of the Fourier series that breaks up any time-varying signal into the frequency
components of varying magnitude that make it up. The Fourier transform is defined
in (10.1), where f ( t ) is any complex function and k is the k th harmonic frequency.
C3-A2
C4-A1
C1-A2
C2-A1
Figure 10.5 A 30-second segment of EEG when the patient is awake. Alpha rhythms become more
accentuated after the patient closes her eyes, indicated by the dashed line. (Courtesy of James Geyer
and Paul Carney.)
C3-AVG
C4-AVG
O1-AVG
O2-AVG
Figure 10.6 An example of what an EEG signal may look like when the patient is in stage 1 of NREM
sleep. The alpha activity that was present in the wake state eventually turns into theta waves. (Cour-
tesy of James Geyer and Paul Carney.)
C3-A2
C4-A1
O1-A2
O2-A1
Figure 10.7 Stage 2 of NREM sleep, denoted by the K complexes (solid line) and sleep spindles
(dotted lines). (Courtesy of James Geyer and Paul Carney.)