reading the background

When reading any EEG you start with the background, which reflects the overall health of a person's brain and can be affected by many factors including acute illness, medications, degenerative disease and normal state changes.

There are four background components
AP Gradient

organization & ap gradient

The organization of an EEG tracing refers, broadly, to how the waveforms appear across the entirety of the page, and includes continuity, symmetry, and the anterior posterior gradient.

Continuity refers to the waveforms being uninterrupted by periods of flat or very attenuated activity. Healthy children and adults should always have a continuous record, but early neonatal tracings can show periods of discontinuity (this is discussed further in the neonatal and pediatric section). Below are examples of both a continuous and discontinuous record.

The next component of organization is symmetry, in which both the left and right sides appear, largely, the same in terms of both amplitude and frequency. Healthy EEGs should always be symmetric, and intermittent or persistent asymmetries can arise from structural entities such as tumors or bleeds. Changes in symmetry can be subtle, but note how on the asymmetric example below how the left hemisphere has higher amplitude, slower delta activity compared to the right side.

In addition to symmetry and continuity, consider the anterior posterior gradient, in which faster, lower amplitude frequencies are present towards the front of the brain while slower, higher amplitude frequencies are found in the back of the brain. The AP gradient leads into the last component of organization, the posterior dominant rhythm (PDR), discussed in the next section.

posterior dominant rhythm

The posterior dominant rhythm (PDR) is the resting frequency of the occipital region when eyes are closed and the patient is resting quietly. It is a vital part of a normal EEG and among the first things you should look for; the PDR used to be called the alpha rhythm because the normal PDR (8.5-12 Hz ) is in the alpha range (7-13 Hz).

The PDR should be symmetric in both frequency and amplitude; if there is a more than 50% difference in amplitude or a more than 1 Hz difference in frequency between sides, this is abnormal. Of note, it is normal for the PDR on the left to be slightly attenuated compared to the right, thought to reflect a thicker skull on the left side in most people.

To determine the PDR, wait till the eyes are closed and then count the number of waves per second in the occipital region. It is helpful to check at more than one period, as the PDR can fluctuate mildly and you want to give a patient their best possible PDR. Of note, up to 5% of normal people have no PDR at all.

Note that the PDR emerges right after the patient closes their eyes (the eye closure is seen as the large positive wave right before the blue box -- we'll discuss why that is in the Artifact section). In adults, the normal PDR should be between 8.5 and 12 Hz and symmetric, but in children a normal PDR depends on the age, as discussed in the Pediatric EEG section. When the PDR is slower than 8.5 Hz, there may be generalized slowing present, as discussed in the Abnormal EEG section.

Of note, when finding the right PDR, be careful of two things in particular: alpha squeak and drowsiness. Alpha squeak describes a transient quickening of the PDR immediately after eye closure, and is named because, when EEG was still traced out on paper, the pen would squeak from moving so quickly. If you choose a PDR based on an area of alpha squeak you'll think it is faster than it actually is. On the other hand, if you choose the PDR when a patient is very drowsy or entering stage I sleep, even though it may be more apparent than when they're more awake, it might look a little slower than it actually is.

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The PDR is 10

In this normal EEG tracing, note first that the anterior-posterior gradient is intact and normal, with faster, lower amplitude frequencies seen over the frontal regions and slower, higher amplitude frequencies seen over the posterior regions. Furthermore, the PDR, best seen after the eye closure (the large frontal positive wave right before the blue box, due to Bell's Phenomenon), is a crisp and symmetric 10 Hz. Note that the PDR recedes upon eye opening (the large frontal negative wave) several seconds later, as expected.

variability & reactivity

Another facet of a normal awake EEG is the presence of variability and reactivity. Variability refers to the presence of shifts in the waveforms across the span of a tracing. A normal brain should have regular fluctuations in the waveforms from second to second. Reactivity is simply the presence of shifts in frequency according to external stimuli; for example, the tracing of a drowsy patient whose background is mostly theta may become again mixed with faster frequencies if they hear a noise or other stimulus.

In patients who are in altered states of consciousness, such as under sedation for hypothermia protocol or refractory status epilepticus, reactivity and variability may be temporarily absent or reduced; in brain dead patients, the tracing is irreversibly neither reactive nor variable.

See the Answer

No, this tracing has absolutely no variability

This is not a trick question, just an extreme example. This tracing of an unfortunately brain dead patient has no background activity or variability. Note the cardiac tracing does still have activity, but shows multiple abnormalities including profound bradycardia. Compare this flat EEG tracing to the other examples on this page to get a sense of how the EEG tracing should change second to second.

State of consciousness

The last part of reading the background for a tracing is determining the patient's state, or whether they're awake, drowsy, or asleep. An awake adult EEG is marked by a plethora of findings including a symmetric PDR with predominant alpha and beta activity (there should be no delta activity in a healthy adult background), and the presence of many artifact types includine eye blinks, movement artifact (usually seen as very high amplitude, chaotic appearing changes in the background), myogenic artifact (seen as high frequency, low amplitude activity usually maximal over the frontal regions, due to the forehead's movement), and even chewing artifact. These artifacts will be further discussed in the Artifacts section.

Drowsiness is seen as a mild and diffuse slowing with decreased frequency of eye blinks and roving eye movements marked by very slow waveforms in the bilateral frontal regions. This arises because the corneas are positively charged, and when the eyes look to the right the F8 electrode sees the right cornea's positive charge while the F7 electrode sees a relatively negative charge. As the eyes move slowly back and forth when drowsy, this leads to the slow, undulating, opposing frontal waveforms that are classic for the drowsy state as seen below.

The transition from drowsiness to stage I sleep is subtle, and marked mostly by the emergence of POSTS (posterior occipital sharp transients of sleep) and vertex waves, but the asleep EEG is a topic all its own, and will be discussed in the next section.

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This is a normal awake tracing with a PDR of 9 Hz

The key to any EEG interpretation is a consistent approach. Look for a good anterior-posterior gradient, which this tracing shows with faster, lower amplitude beta activity in the frontal regions and slower, higher amplitude alpha activity in the occipital regions. Next, find the PDR and ensure it is symmetric in both frequency and amplitude, which this one is. If the PDR is present, the patient is awake, but eye blinks can help to confirm this. Variability and reactivity are almost always present if all of the above factors are present and normal.


As part of many EEG studies, provocation is done to better assess any underlying risk for seizures. The two main types of provocation are photic stimulation and hyperventilation.

In photic stimulation, a light is flashed in trains of increasing frequencies to look for photic driving, in which the background rhythm becomes time locked and in sync with each light flash. It is a normal response but many patients will not have it on EEG, and that is also normal. A driving response should be largely symmetric in terms of amplitude, and any major asymmetry may be suggestive of underlying dysfunction of the occipital / posterior brain regions. There is a form of driving called harmonic driving in which the background becomes time locked to some multiple of the light flashes; for instance, 5 Hz light flashes lead to a 10 Hz PDR or 6 Hz flashes to a 12 Hz PDR.

In those with a history of seizures, driving can very rarely can give rise to a photoparoxysmal response with epileptiform activity, which is further discussed in the epileptiform abnormalities section. Below is an example of a patient with photic driving from 6 Hz all the way up to 30 Hz; on this tracing, each flash of light is marked with a red line at the bottom of the screen.

The other major type of provocation is hyperventilation, which should not be done in patients over 65 years of age, those with chronic respiratory issues, or those with a recent stroke or myocardial infarction. The prototypical response to hyperventilation is diffuse slowing, but this is not always seen. The classic case for use of hyperventilation in epilepsy is with absence seizures, in which blowing on a pinwheel or other mode of causing hyperventilation can often cause brief absence seizures marked by generalized 2.5 Hz spike and wave activity (discussed further in the seizures section).

hyperventilation slowing
The background consists of the AP gradient, PDR, variability and reactivity, and state

A normal AP gradient is faster in the front and slower in the back

A normal PDR is 8.5 - 12 Hz and symmetric in frequency and amplitude

An awake tracing should show evidence of second-to-second variability and reactivity

Drowsiness is seen as diffuse slowing, attenuation and emergence of slow roving lateral eye movements

Provocations include hyperventilation (can cause generalized slowing) and photic stimulation (can cause photic driving)

The PDR is 7.5 - 8 Hz

This tracing has a mildly slow PDR--recall that the normal range for the PDR is 8.5 - 12 Hz even though the alpha range, which the PDR was historically named after, is from 7 - 12 Hz. Other notable findings here include a normal AP gradient and eye blinks suggesting the patient is awake. There is also good variability in waveforms across the page.

Of note, there is a lateral eye movement artifact marked in the red box--because the cornea is positively charged, when you look to a side the frontal leads (F7 or F8) on that side have a positive deflection while the contralateral side has a negative deflection.


This tracing shows significant myogenic artifact with a normal approximately 9 Hz PDR and multiple eye blinks, all consistent with the patient being awake. Of note, this is a less commonly seen montage that focuses on the temporal chains, by placing the lateral temporal leads (T1, T3) together with the usual temporal chains on top. The middle chain is a transverse central chain comparing left to right. The bottom two chains are the usual parasagittal chains.

The PDR is 8-9 Hz

This normal awake tracing does not have the most clear PDR, but there are segments where a lower amplitude, somewhat sloppily formed PDR of approximately 8 Hz is seen, perhaps best shown in the blue box below. Note also on this page several lateral eye movement artifacts, seen as opposing polarities in the frontal electodes F7 and F8, due to the cornea's positive charge. We also see here several eye blinks (the large frontal positive deflections in the 2nd, 7th and 9th seconds of the page) and some overlying myogenic artifact, seen as very fast low amplitude activity more prominent on the left than right side (see artifacts section).

The PDR is 10

This normal tracing has a PDR of 10, best seen in the middle portion the page. The PDR is symmetric in both frequency and amplitude (note, the amplitude is not exactly the same side to side but it is within the normal variation of no more than 50% difference). Noted on this page are several types of artifact, including frontal myogenic artifact (seen as high frequency activity in the frontal leads, often arising from the forehead) and movement artifact, seen as high amplitude, nonrhythmic activity over the right frontal region (it is possible the patient is lying with their right side of the head on a pillow, for example).

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awake and drowsy states