Normal Waking EEG Background Organization
Normal Waking EEG Background Organization[edit | edit source]
The normal waking EEG background is characterized by anterior-posterior voltage and frequency gradients. The EEG activity is generally lower in amplitude and faster in frequency anteriorly and higher in amplitude and slower in frequency posteriorly.
Another feature of a normal waking EEG background is the posterior dominant rhythm, sometimes called the alpha rhythm, which is best seen at O1 and O2 electrodes and is sinusoidal in morphology. This particular rhythm has other physiological characteristics that define it. It is most apparent in the relaxed wakeful state when the eyes are closed. It reacts to eye opening and mental concentration by attenuating or disappearing as the neuronal population responsible for its generation desynchronizes. This rhythm is present beginning at about 6 months of age and gradually increases in frequency with maturation (see table). Most adults have a posterior dominant rhythm in the range of 8.5 to 13 Hz. Slowing of the posterior dominant rhythm below what is considered appropriate for a given age is a potential indication of dysfunction. During childhood into adolescence, this rhythm can be intermixed with delta waves on which the posterior rhythm rides. These underlying delta waves are called posterior slow waves of youth and should not be considered abnormal.
In these examples we can appreciate a rudimentary ~5 Hertz posterior dominant rhythm in a 7-month-old (a) contrasted with the well-developed 7 Hz posterior rhythm of a 2-year-old (b) that emerges with eye closure and is intermixed with slower 2-3 Hz posterior slow waves of youth.
In both examples, there are well-developed anterior-posterior voltage and frequency gradients, and despite their overall difference in appearance, both show normal organization for the patient’s age.
8 year old EEG with less delta more theta compared to 2 year old EEG
95 year old EEG not much different