Reading an EEG means learning to recognize a small set of brainwave patterns, then systematically checking for anything that deviates from what’s expected for a person’s age and level of alertness. The recording is a series of squiggly lines, each representing the electrical difference between two points on the scalp, displayed across time. Once you understand how the electrodes are arranged, what normal brain rhythms look like, and how to spot artifacts and abnormal discharges, the tracing becomes surprisingly readable.
How Electrodes Map to Brain Regions
EEG electrodes are placed on the scalp using a standardized layout called the international 10-20 system. The name comes from the fact that electrodes are spaced at 10% or 20% intervals along measured distances between bony landmarks on the skull. Each electrode gets a letter-number label: the letter tells you the brain region (F for frontal, T for temporal, C for central, P for parietal, O for occipital), and the number tells you the side. Odd numbers are on the left, even numbers are on the right, and “z” marks the midline. So “O1” sits over the left occipital area and “F4” sits over the right frontal area.
CT scanning studies have confirmed that these scalp positions reliably correspond to the brain structures beneath them. When you see activity at a particular electrode, you can reasonably infer which part of the cortex is generating it.
Understanding Montages
Each line on an EEG display is called a channel, and each channel shows the voltage difference between two electrodes. A montage is simply the arrangement of these electrode pairs on the page. There are two main types, and switching between them is one of the most useful skills in EEG reading.
In a bipolar montage, each channel compares two neighboring electrodes in a chain running across the scalp. Because adjacent electrodes share much of the same background activity, this setup is excellent for pinpointing where an abnormal signal originates. The signal will appear largest at the electrode where the activity is strongest and will “cancel out” between two electrodes that see it equally. Bipolar montages come in longitudinal chains (front to back) and transverse chains (left to right).
In a referential montage, every electrode is compared to a single common reference point, such as the ear or an average of all electrodes. This gives you a cleaner picture of the actual voltage at each site, making it easier to compare the same brain region on the left and right sides. However, if the reference electrode picks up noise or its own brain signal, that contamination appears in every channel.
Skilled readers flip between bipolar and referential montages to confirm their findings. If something looks abnormal in one arrangement, switching to the other helps verify where it’s truly coming from.
The Four Main Brainwave Types
EEG activity is categorized by frequency, measured in cycles per second (Hz). Each frequency band has a Greek letter name and appears under specific circumstances.
- Beta (13–30 Hz): The fastest common rhythm. These low-voltage waves (typically 10–20 microvolts, rarely exceeding 30) dominate when you’re alert, focused, or anxious. They’re most prominent over the frontal and central regions.
- Alpha (8–12 Hz): The signature rhythm of relaxed wakefulness with eyes closed. Alpha waves are strongest over the back of the head and disappear the moment a person opens their eyes or concentrates. Their amplitude varies between individuals, but the pattern is unmistakable: a smooth, rhythmic waveform that blocks with attention.
- Theta (4–7 Hz): These slower waves appear during drowsiness and light sleep. Seeing theta in someone who’s fully awake can be a sign of underlying brain dysfunction, though brief bursts are normal during transitions into sleep.
- Delta (0.5–4 Hz): The slowest and largest waves. In adults, delta activity is a hallmark of deep sleep and appears most prominently over the frontal and central scalp. Persistent delta waves in someone who is awake suggest an abnormality such as focal brain injury or metabolic disturbance.
When reading an EEG, you first identify the dominant background rhythm and check whether it matches the patient’s state. A relaxed, eyes-closed adult should show a clear alpha rhythm around 8–12 Hz posteriorly. If the background is dominated by theta or delta instead, that’s your first clue something may be off.
Recognizing Sleep Stages on EEG
If a recording captures sleep, you’ll notice distinct markers at each stage. In N1 (the lightest sleep), the alpha rhythm drops away and is replaced by low-voltage, mixed-frequency activity in the 4–7 Hz range. You may also see vertex sharp transients: brief, pointed waves that pop up over the top of the head. These are normal.
N2 sleep introduces two unmistakable features. Sleep spindles are short bursts of rhythmic 12–16 Hz activity lasting about half a second. K-complexes are large, sharp waveforms followed by a slower component. The first appearance of either one marks the official start of N2. In N3 (deep sleep), large, slow delta waves take over at least 20% of each 30-second viewing window. REM sleep looks surprisingly similar to light wakefulness on EEG, with low-voltage mixed-frequency activity, but the key is that sleep spindles and K-complexes disappear, muscle tone drops to its lowest point, and rapid eye movements appear in the eye channels.
Spotting Artifacts
Not everything on an EEG comes from the brain. Artifacts are signals generated by non-cerebral sources, and learning to identify them is half the battle of reading a tracing accurately.
Eye blinks produce large, slow deflections that are most prominent in the frontal electrodes, because the eyeball acts as a small battery (positive at the front, negative at the back). Lateral eye movements create a similar pattern but with opposite polarity between the left and right frontal electrodes. Muscle artifact from jaw clenching or scalp tension shows up as high-frequency, spiky activity that can obscure the underlying brain signal entirely. It’s common over the temporal regions, where chewing muscles sit close to the electrodes.
Cardiac artifact creates a small, rhythmic blip that repeats in sync with the heartbeat. You can confirm it by checking if the frequency matches the pulse. Pulse artifact is a variant where an electrode sits over a scalp artery, producing rhythmic slow waves at the cardiac rate. Glossokinetic artifact comes from tongue movements and can be reproduced by having the patient say “la, la, la” repeatedly. Movement artifact is the trickiest, because a loose electrode jostled by patient movement can mimic a seizure discharge. Checking whether the pattern respects known brain anatomy, or whether it appears in a single electrode with no logical field, helps sort real signals from false ones.
Identifying Abnormal Discharges
The most clinically important abnormalities are epileptiform discharges: spikes and sharp waves that stand out from the background and suggest a tendency toward seizures.
A spike is a brief, pointed waveform lasting 20–70 milliseconds, typically exceeding 50 microvolts in amplitude. A sharp wave has the same pointed morphology but lasts longer, between 70 and 200 milliseconds, with amplitudes commonly reaching 100–200 microvolts. Both are usually negative in polarity (pointing upward on a standard display) and are clearly distinguishable from the surrounding background. They often occur with a following slow wave, forming a spike-and-wave complex.
To confirm that a waveform is truly epileptiform rather than an artifact or a normal variant, look for several features: it should have an electrical field that makes anatomical sense (appearing in a cluster of neighboring electrodes rather than just one), it should disrupt the background rhythm, and it should have a characteristic sharp morphology that’s asymmetric in shape, rising faster than it falls or vice versa.
Activation Procedures and What They Reveal
During a standard EEG, the technologist will ask the patient to do two things designed to provoke abnormal activity that might not appear at rest.
Hyperventilation involves three minutes of deep, rapid breathing. This lowers carbon dioxide in the blood and can trigger generalized slowing or, in someone with epilepsy, bring out spike-and-wave discharges. In a normal recording, you’ll see a gradual buildup of slow waves that resolves within a minute or two after breathing returns to normal. In children and young adults, the response tends to be more dramatic, which is why population data shows a higher yield of epileptiform findings in people under 20 (about 10%) compared to adults (about 5%).
Photic stimulation uses a strobe light flashing at varying frequencies. A normal response called photic driving produces rhythmic activity in the occipital region that matches the flash rate. An abnormal response, called a photoparoxysmal response, produces epileptiform discharges triggered by the flashing light. The overall yield is modest (around 3–6% of recordings), but when it appears it’s highly informative. Sleep, whether natural or induced by sleep deprivation before the test, consistently produces the highest yield of epileptiform abnormalities across all activation methods.
Age Matters: Pediatric EEG Differences
Children’s EEGs look fundamentally different from adults’, and what’s normal at age 3 would be abnormal at age 20. The dominant background frequency increases with age: preschoolers (ages 3–6) still have prominent theta activity in the background, and the alpha rhythm doesn’t reach the adult minimum of about 8.5 Hz until around age 16.
Two normal pediatric patterns commonly trip up inexperienced readers. Posterior slow waves of youth are intermittent delta waves mixed into the alpha rhythm over the back of the head. They’re common from childhood through adolescence and can persist into the late 20s. Hypnagogic hypersynchrony is a dramatic buildup of high-voltage delta activity during drowsiness, often with sharp-looking components over the frontal, central, and parietal regions. It’s especially prominent between ages 2 and 4 and is easily mistaken for epileptiform activity. Both of these patterns are completely normal.
A Practical Approach to Reading
When you sit down with an EEG, a systematic approach prevents you from missing important findings. Start by noting the patient’s age and clinical state (awake, drowsy, asleep) because these determine what “normal” looks like. Next, identify the dominant background rhythm. In an awake adult, confirm a posterior alpha rhythm in the 8–12 Hz range and check that it’s roughly symmetric between the two hemispheres. Asymmetry in frequency or amplitude can indicate a problem on the slower or lower-amplitude side.
Then scan for focal abnormalities: regions of persistent slowing, loss of normal fast activity, or epileptiform discharges that appear in one area. Check both bipolar and referential montages to localize anything suspicious. Review the activation procedures and note whether hyperventilation, photic stimulation, or sleep provoked any new findings. Finally, look carefully at the recording for artifacts and make sure nothing you’ve flagged as abnormal is actually coming from the heart, the eyes, or a loose electrode.
The most common mistake beginners make is over-reading: calling normal variants or artifacts abnormal. When in doubt, look at the morphology, the distribution across electrodes, and whether the pattern fits a known normal variant for the patient’s age. A true abnormality will repeat, have a sensible electrical field, and stand out from the background in a way that artifacts and benign patterns generally don’t.