Electroencephalography (EEG) is a neurophysiological test that measures changes in neuronal activity associated with current flows in the extracellular space produced by local field potentials. Such field potentials can be either excitatory, also known as excitatory postsynaptic potentials (EPSP,) or inhibitory, also known as inhibitory postsynaptic potentials (IPSP) in nature. EEG tracing records not only the physiologic cortical activity, but inevitably also captures fields of electrical activity generated by other sources such as cardiac, myogenic, and electromagnetic, among others. It is substantial for the electroencephalographer to be able to distinguish normal physiologic brain activity from those recorded from non-physiologic sources. Such knowledge is critical to avoid EEG misinterpretation.
Basal cortical rhythms are physiologic alpha-range rhythms that present in normal healthy brains. They encompass 3 rhythms: (1) Alpha rhythm, (2) Mu rhythm, and (3) Third rhythm. Not a lot is known about the Third rhythm. Wickets rhythm is considered a fragmented Third rhythm and will be discussed in depth.
Terminology and Definitions
Rhythm is a term used in electrophysiology to describe continuous, repetitive, EEG activity that occurs without interruption. Alpha rhythm is a descriptive term where alpha refers to the mature normal frequency that falls in alpha range 8 to 13 per second. The terms posterior basic rhythm (PBR) and posterior dominant rhythm (PDR) are used interchangeably to infer the same. The term alpha rhythm should be differentiated from “alpha frequency” that describes an EEG activity falls in 8 to 13 per second. It takes the normal brain until 8 to 10 years of age to achieve the normal occipital alpha frequency. Therefore, the term Alpha rhythm shall be reserved for adult use, and avoided in pediatrics and perhaps among inexperienced practitioners. EEG lab reports use the terms PBR or PDR interchangeably when referring to Alpha rhythm. The term PBR will be used in this discussion. The International Federation of Societies for Electroencephalography and Clinical Neurophysiology (IFSECN) has proposed the following criteria that define the PBR in adults as “a rhythm that has the frequency of 8 to 13 per second (also known as Hertz [Hz]), and an amplitude of 15 to 45 microvolts. It is seen maximally during relaxed wakefulness distributed over the occipital head regions. It is enhanced by eye closure under physical relaxation and mental inactivity, and blocked by (i.e., disappears) or attenuated (i.e., decreases in amplitude) by visual and mental effort”. Such a holistic definition encompasses 3 main criteria to identify such rhythm: morphology, reactivity, and location. All three criteria have to be present in order to identify this essential rhythm correctly. The absence of any of these features shall question the EEG activity of interest.
Mu is the 12th letter of the Greek letters, transliterated as "m," and it stands for “rolandic” rhythm. It was called “precentral alpha rhythm,” “rolandic alpha,”, “central alpha,” and “somatosensory alpha rhythm” referring to its cortical origin. Other names include “alphoid activity” and “arcade rhythm” and “comb rhythm” derived from its morphology.
Wickets spikes were first described by Reiher and Lebel. Wickets arguably may represent normal fragmented Third rhythm . The latter has been separately called “temporal alphoid rhythm” or “Third rhythm,” and distinguished from the posterior predominant rhythm and mu rhythms being the first and second cortical alpha rhythms, respectively. Niedenmeyer first introduced the nomenclature. The third rhythm is often seen in the presence of a skull defect or through intracranial recording.
Alpha rhythm usually archiform (sinusoidal) or rounded with a negative-sharp component and a positive-round component. It may override a delta wave in youth, termed posterior slow wave of youth. It is best seen using ipsilateral ear referential montage with higher amplitude over the non-dominant hemisphere. Amplitude difference should not exceed 50% when higher on the right, and 35% when higher on the left. The normal PBR frequency is 8 to 13 Hz; that is the alpha frequency range. PBR evolves after birth until it reaches adulthood maturity by age 10. Arguably, the earliest sign for a reminiscent of PBR appears by 4 months; at frequency 4 per second. Experts in the field, however, allow the absence of PBR up to 1 year of age. The PBR progressively increases in frequency with age until it reaches about 5 to 6 Hz by 1 year, and a minimum of 8 Hz by the age of 8 years. A PBR of 8 Hz is usually achieved by 3 years, and 10 Hz by 10 years. Alpha harmonics can be seen and are normal. Fast alpha variant denotes a PBR that is twice the frequency, while slow alpha variant infers to half the frequency (appear as a theta or delta waves) of the PBR in a particular subject. Such harmonics continue to exhibit reactivity and localize similarly over the occipital regions as the usual PBR does.
Mu rhythm (rhythm en arceau) – pertains to an archiform waveforms with sharp positive and rounded positive components. The amplitude is comparable to PBR; <50 microV. Frequency 7-12 Hz. It is bilateral, usually asymmetric and asynchronous with shifting side.
Wickets rhythm has a monomorphic archiform waves with surface negative polarity reaching 60-210 microV in amplitude. It usually occurs in runs of 6-11 Hz.
The Alpha rhythm can be temporarily blocked, or attenuated by sustained eye opening and/or mental activity (also called Berger’s effect)  and enhanced with sensory stimulation. PBR has been reported to vary throughout the menstrual period. The frequency is highest during preovulatory (days 5-14) and premenstrual (days 23-28) phases, and lowest in both menstrual (days 1-5) and luteal (days 15-23) phases. An asymmetric reactivity reflects an abnormality on the non-reactive (or less reactive) side. Hyperthermia (>41 C) results in slower PBR. Cardiac pacemaker increases Alpha frequency by 1-2 Hz, which is thought to be related to increased cerebral perfusion.
Mu rhythm is present in waking and disappears in drowsiness similar to PBR. It has been described during REM sleep as well; this is in contrast to Alpha rhythm. It is blocked by active, passive or a mere thought of movement in the contralateral limb. Niedermeyer reported that such reactivity was demonstrated in amputees attempting to ambulate phantom limb. Such attenuation is recognized as EEG desynchronization due to hypothesized cortical inhibition induced by motor planning. Mild tactile stimulation has shown to abolish contralateral mu rhythm as well. It is enhanced by intermittent photic stimulation and pattern vision.
Wickets rhythm can be seen during wakefulness and sleep. It is best seen during drowsiness and light sleep. It located over the temporal regions and can be seen unilaterally or bilaterally with shifting variable predominance. When related, the Third Rhythm is thought to be blocked by auditory stimulation.
Alpha rhythm (PBR) is a cortical rhythm with genetically determined morphology. The exact generator remains uncertain but multiple brain regions seem to be involved including occipital lobes and thalamic pacemakers.
Mu rhythm is perceived as a cortical idling rhythm of the somatosensory cortex via thalamocortical projections. It was recorded from the ventroposterolateral (VPL) nucleus of the thalamus. It has two components: an alpha component that represents the post-rolandic sensory cortex, and a supraharmonic beta originates from the pre-rolandic motor region. The early somatosensory stimulation in utero could explain the earlier appearance of Mu; in contrast to the posterior basic rhythm (aka. alpha rhythm) present in neonates following visual stimulation.
Wickets rhythm is seen over the temporal region; however, the exact generator is uncertain. Wickets are suspected to represent normal fragmented Third rhythm.
EEG, similar to other tests, has its technical limitations. It relies heavily on skilled lab technicians for acquiring high-quality study with prompt artifact resolution. It also requires a well trained and experienced interpreter to minimize any chance of overcalling a normal phenomenon. Following are some abnormalities that pertain to those cortical rhythms. It is never acceptable for the PBR to drop below the normal range (i.e., less than 8 Hz), and would be an abnormal finding. An abnormal decrease in the PBR frequency can be seen in the elderly is also abnormal and may suggest an underlying vascular or degenerative etiology. A persistent nonreactive PBR has been described in comatose patients with pontine lesions. A unilateral non-reactive PBR referred to as Bancaud’s phenomenon and indicates an abnormality in the posterior head regions on the same side.A unilateral mu rhythm could result from an ipsilateral skull defect. The presence of a nonreactive unilateral mu rhythm along with focal slow activity is indicative of a focal abnormality.Wickets rhythm can be seen in singlets that frequently gets misinterpreted. It is usually helpful to compare the morphology of spikes to those seen in trains on other EEG pages whenever in doubt. The absence of focal slowing or after-going slow wave supports fragmented physiologic rhythm.