Normal Electroencephalogram
Normal patterns seen on the EEG vary somewhat among individuals
but are consistent enough to allow for accurate recognition of normal and pathologic
patterns. The usual base frequency in the awake patient is the β range (>13
Hz). This high-frequency and usually low-amplitude signal is common from the alert
attentive brain and may be recorded from all regions. With eye closure, higher amplitude
signals in the α frequency range (8 to 13 Hz) appear and are seen best in the
occipital region ( Fig. 38-2
).
This
Figure 38-2
The loss and return of α activity can be seen as
the eyes open and close. The large spikes are muscle artifact. (Courtesy
of the Mayo Foundation, Rochester, MN.)
eyes-closed resting pattern is the baseline awake pattern used when anesthetic effects
on the EEG are described. When events that lead the brain to produce higher frequencies
and larger amplitudes occur, the EEG is described as activated,
and when slower frequencies are produced (theta = 4 to 7 Hz and δ < 4
Hz), the EEG is said to be depressed. The EEG for
the sleeping patient may contain all of these frequencies at various times. The
slower frequencies occur during deep natural sleep with sleep spindles ( Fig.
38-3
), but during light sleep or rapid eye movement (REM) sleep, the EEG
becomes activated, and the eye muscle EMG appears on the EEG.
On normal EEGs for awake and asleep patients, patterns recorded
from corresponding electrodes on each hemisphere are symmetric in terms of frequency
and amplitude, the patterns are predictable if clinical states are known, and spike
(epileptic) waveforms are absent. In most cases, normal electroencephalographic
patterns are associated with normal underlying brain function.
