Alpha rhythms as physiological and abnormal phenomena.

There are three physiological alpha rhythms in mature healthy humans: (a) the classical posterior alpha; (b) the Rolandic mu rhythm and (c) the midtemporal 'third rhythm'. The classical posterior alpha rhythm develops out of a 4/s rhythm appearing at age 4 months and gradually reaches the alpha frequency band around age 3 years. The mature frequency around 10/s is subject to subtle physiological changes and grossly decelerates in the face of pathology. No posterior alpha rhythm may be detectable in a minority of healthy adults with an inherited low voltage fast EEG. One is tempted to speculate that these individuals may have a hidden alpha rhythm in neuronal level and defective mechanisms of synchronization. Alpha blocking with visual stimuli (eye opening) is a classical response; responses to mental stimuli (mental arithmetic) are inconsistent, presumably due to the involvement of higher cognitive functions. The Rolandic my rhythm is found with scalp EEG in a minority of subjects but there is good reason to presume that all healthy adults have this rhythm. A particularly powerful mu rhythm reaches the scalp but this could be also an indicator of a mild CNS dysfunction. There is even a relationship between mu rhythm and the central spike activity in children with benign Rolandic epilepsy. The midtemporal third rhythm is not detectable in the scalp EEG unless there are local bone defects. Its functional significance is debatable; its blocking responses encompass various higher cognitive tasks and are inconsistent; responses to auditory stimuli do occur but appear to be of secondary significance. This rhythm arises from midtemporal structures which by far exceed the borders of the auditory cortex. Abnormal rhythmical alpha activity-above all the alpha coma in life-threatening cerebral anoxia -is discussed in order to deepen our understanding of the physiological alpha rhythms. Severe cortical de-afferentation may give rise to cortical autorhythmicity-either in alpha frequency or in other frequency bands. Physiological alpha rhythms are likely to have closer relationships to 'events' than one might have thought earlier. The demonstration of event-related desynchronization and synchronization (in Pfurtscheller's work) clearly underscores this view.