Normal Variants

Wickets are 6-11 Hz monophasic, arciform (or mu-like), sharp, surface-negative waveforms that are not followed by a slow wave. They are typically located in the bilateral, often independent, temporal regions and can occur in both the awake and light sleep states.

Rhythmic mid temporal theta of drowsiness is characterized by 4-7 Hz (theta rhythm) notched, flat-topped, or sharply contoured waveforms seen most prominently in the unilateral or bilateral mid-temporal regions. These typically last from a few seconds to several minutes and are of medium to high amplitude. RMTD is more common in adults and is usually seen during drowsiness or light sleep. RMTD can be confused with a temporal lobe seizure because of its sharp waveforms and rhythmicity; however, RMTD doesn't evolve. As such, it was previously known as the "psychomotor variant."

Small sharp spikes (SSS), also known as benign epileptiform transient of sleep (BETS), are mono or diphasic spikes with a rapid ascending and steep descending slopes. They are usually sporadic, not occuring in runs, and primarily seen in adults. As the name implies, they are usually short in duration (<50 ms) and low in amplitude (<50 mV). They do not typically disrupt the background activity, and are typically not followed by an aftergoing slow wave; when present, however, the aftergoing slow wave is shorter in amplitude than the spike component. Further, they generally have a broad field but are maximal in the anterior and midtemporal regions. They occur in drowsiness and light sleep and disappear during wakefulness and deep sleep.

This normal variant is characterized by bilateral, diffuse, high-amplitude, synchronous, 2-4 Hz (delta) activity that occurs after the start of hyperventilation, an EEG activation procedure. It is often seen in children (maximally between the ages of 8 and 12 years). It is hypothesized that this phenomenon occurs as a result of hypocapnia leading to either activation of thalamocortical projecting systems or decreased activity in the mesencephalic reticular formation. Hyperventilation-induced slowing should not be considered abnormal unless it is focal/unilateral, significantly asymmetric or admixed with epileptiform discharges. Lastly, studies have shown that low blood glucose levels (<80 mg per dL) may favor the appearance of this response, even in adults.

Photic driving consists of evoked cerebral responses that are synchronous and time-locked to the intermittent photic stimulation (IPS) typically at 5 to 30 Hz. The driving response is a frequency that is identical to or a harmonic of the IPS frequency. In practice, the frequency seems to be maximal around the patients’ posterior dominant rhythm (in the occipital region).

Breach rhythm is the EEG representation of an underlying skull defect. There is a change in transmission of amplitude because the skull acts as an attenuator. Notably, breach-related effects mainly impact amplitude with little or no effect on the frequency; however, low voltage fast activity is better seen due to the enhancement of amplitude. As such, the resulting activity appears to have a sharp or spiky morphology.

Hypnagogic hypersynchrony consists of medium- to high-voltage rhythmic sinusoidal, symmetrical, slow (delta or theta) activity seen upon falling asleep. This activity is diffuse but more prominent in the frontocentral region, and may have a notched appearance, especially in younger patients (most prominent between 1-10 years of age, generally disappearing by 13 years of age).