Frequency

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).

Alpha 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)

Delta

Theta

Alpha

Beta

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

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.

morphology

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).

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).