terminology & waveforms


eeg is a language of its own

Once you've got the electrodes in place, montage chosen, and the study started, its your job to interpret and communicate the EEG findings. The first step is learning how to differentiate the many waveforms you'll come across, and the vocabulary you need to communicate what you see.


Recognizing the frequency of the waveforms is fundamental to interpreting EEG. Frequency describes how many waves there are per second, and is measured in hertz (Hz). There are four main frequencies of the human brain seen on scalp EEG, in increasing order: delta, theta, alpha and beta.

Delta is the slowest at 0-4 Hz, and generally speaking should not be present in a normal awake brain. It is the de facto finding of slow wave sleep, where it's seen diffusely, but is also commonly found overlying structural abnormalities such as tumors, or over the bifrontal regions in some patients with encephalopathy.

Theta ranges from 4-8 Hz, and tends to be more prominent in childhood than adulthood. However, theta re-emerges often in drowsy periods, and is the hallmark of some normal findings including rhythmic temporal theta of drowsiness (RMTD).

is 8-13 Hz, and is perhaps the frequency you'll come to know and love best. It is the hallmark frequency of the normal awake adult brain, to the point that the posterior dominant rhythm (PDR), a key finding of the normal background, used to be called the alpha rhythm.

Beta is 13-30 Hz, and is most commonly seen in the setting of myogenic (muscle) artifact in the frontal regions. More diffuse beta activity can be found most often with benzodiazepine use. Of note, there is technically an even faster frequency, gamma (30 Hz and above) but this is not seen with physiologic activity on scalp EEG.

Realistically, frequencies on EEG will almost never be as clean as the above examples. Instead, you'll see a mixture of frequencies overlying one another; for example, you can have beta activity on top slower delta activity, which could be termed delta with overriding beta, or delta with admixed beta.

Below are a set of examples of one predominant frequency per page, but note the multiple intermixed frequencies on each. On these tracings, each dark gray line is one second, with the lighter grey lines in between being 1/5 of a second each; try counting how many of each wave are seen per second to confirm the frequency on each example. Note that "how big" each second appears differs from one example to the next; this is because they were collected on different size screens, and software screen calibration affects the appearance similar to how page speed does (although they are two separate entities)

While this isn't a rule, for physiologic waveforms frequency tends to be inversely related to amplitude, such that lower frequency waves (delta, theta) are often higher amplitude while higher frequency waves (alpha, beta) are lower amplitude.


Amplitude is the height of a waveform, essentially a proxy for the voltage, and on the scalp is measured in microvolts. The normal adult brain has amplitudes from 10 to 100 microvolts on the scalp, mostly in the 10-50 microvolts range. As mentioned above, there is generally an inverse relationship between amplitude and frequency. For epileptiform discharges and other activity, clarifying the amplitude of a discharge or pattern (low, moderate, or high) is an important part of the descriptor.

Don't confuse amplitude and sensitivity--amplitude is an intrinsic variable of the waveform itself, while sensitivity is a technical choice that affects how the amplitude is perceived.


The morphology of a waveform describes its overall shape, and is important for both interpreting a tracing and communicating your findings. For an individual waveform, each part of a wave is considered a phase. Its easiest to think of phases by imaging an imaginary baseline; a monophasic wave begins on one side of the baseline and has only two parts, an up slope and a down slope, and crosses the baseline only once. An example of a monophonic wave is a simple spike. A biphasic wave starts on one side of the baseline with an up slope and downslope going through the baseline, but then has a second part that crosses the baseline again; the classic epileptiform spike and slow wave discharge is an example of a biphasic wave. Polyphasic waves cross the baseline multiple times.

Moving past individual waves, you need to consider how a wave fits into the tracing surrounding it, and part of this is deciding if a set of waves are monomorphic or polymorphic. Monomorphic waves all appear the same in frequency and amplitude; an extreme example of monomorphic waves may be found in seizures, in which repetitive spike and wave activity often looks very similar in portions of the seizure. Often, monomorphic patterns are concerning for epileptiform activity, and monomorphic patterns may also be called rhythmic patterns (rhythmic and periodic patterns are further discussed in the epileptiform abnormalities section).

monomorphic rhythmic spike and wave activity

Polymorphic waves
vary in frequency and amplitude. A classic example of polymorphic slowing is seen with slow wave sleep, in which high amplitude polymorphic delta activity predominates the recording. Polymorphic slowing can also be abnormal, though, as is the case with focal polymorphic slowing caused by tumors or bleeds (slowing is discussed in the nonepileptiform abnormalities section).

See the Answer

Polymorphic Theta to Delta Slowing

This tracing shows a persistent slowing over the left hemisphere. Note how the right side has predominantly alpha activity in the posterior regions--this is the posterior dominant rhythm, discussed in the Normal Awake section. On the left side, however, this alpha activity is absent, replaced by a "messier" and slower theta to delta activity that varies in appearance (including both frequency and amplitude), making it polymorphic.

The four main frequencies of the human brain are (in increasing frequency) delta, theta, alpha and beta

Amplitude (aka voltage) is measured in microvolts, and describes the height of a waveform

The typical amplitude of a normal adult brain is 10-100 microvolts

Morphology describes the shape of a waveform or set of waves; single waves can monophasic, biphasic or polyphasic while a series of waves can be monomorphic (rhythmic) or polymorphic

Left temporal polymorphic delta

This tracing shows persistent delta activity in the left temporal chain. The delta activity varies from second to second and each wave looks different than those around it, making it polymorphic. Note that there is admixed beta, theta and alpha frequencies amongst the delta, as a real EEG almost never has just a single frequency (the brain is a very active place!).

Note also that while much of the right hemisphere is obscured by high frequency myogenic artifact (the very fast activity seen in first half of the page in the right temporal and parasagittal chains--see the artifact section for more details), there is not a similar delta slowing in the right temporal chain. This sort of lateralized slowing suggests underlying cerebral dysfunction of the slowed region; when present in the temporal lobe, it can also suggest increased seizure potential.

Polyphasic and monomorphic

The marked waveforms consist of an initial downward phase, then an upward phase (a spike) followed by an aftergoing positive slow wave (this is actually the prototypical epileptiform discharge, but that's discussed more in the epileptiform abnormalities section). For now, note that each waveform consists of those multiple parts, making them polyphasic, and that while some are more well formed than others they are essentially all quite similar in appearance, making them monomorphic.

Monophasic and continuous

These waveforms consist of a simple up phase and down phase morphology, making them monophasic. There is no aftergoing slow wave. Each wave looks essentially just like the other ones too, so they are monomorphic. These waves are actually a type of artifact, or signal that comes from outside the brain (see the artifact section for details), that happens when an electrode is loose; in this case, the P3 electrode is loose, causing all the portions of the tracing that involve P3 to show this artifact.

Occipital intermittent rhythmic delta activity

This page shows periods of fairly monomorphic or rhythmic delta frequency activity in the occipital regions, more dominant on the left as marked below but also present near the middle of the page on the right side. This pattern is called occipital intermittent rhythmic delta activity (OIRDA). It is more commonly seen in children and can sometimes be associated with increased epileptogenic potential. In this case, there are subtle spikes seen prior to some of the waves; two are marked below.

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