Electroencephalography (EEG) is like eavesdropping on a conversation happening in your cerebral cortex. But instead of words, it uses electrical signals—and the patterns those signals make can tell us a lot about how the brain is functioning… or misfiring.
This post will outline the major important features of the EEG and how it can help us to understand the function, and dysfunction, of the brain.
⚡ What Is EEG, and Why Do We Use It?
How it works:
- EEG measures electrical activity from groups of neurons.
- Electrodes on the scalp pick up voltage changes produced by postsynaptic potentials in cortical neurons.
- Captures rhythmic patterns—alpha, beta, delta, theta and gamma - each associated with states of consciousness or pathology.
Why it matters:
- EEG helps detect unusual brain activity, especially seizures.
- Useful in diagnosing epilepsy, assessing encephalopathy, and guiding sleep studies.
- It’s non-invasive and accessible, often used in sleep studies and neurology clinics.
🔍 Think of EEG like a seismograph for the brain—tracking tiny tremors and tectonic shifts in real time.
🧠 How Are Brain Waves Classified?
Brain waves vary based on what the brain is doing—awake, asleep, thinking, or drowsy.
Here's a breakdown:
🧠 Alpha Waves (8–13 Hz)
What do they look like?
Smooth, regular waves that roll gently across the EEG screen like a steady rhythm.
Why do they appear?
Alpha waves happen when you're awake but relaxed—especially with your eyes closed and not focusing on anything. They reflect synchronized activity in the visual cortex when it's not being stimulated.
Where are they strongest?
In the occipital lobes, which process vision. When your eyes close, visual input decreases and alpha activity picks up.
🧭 Think of alpha waves as your brain’s “idle mode” when you're relaxed but ready to engage.
🧠 Beta Waves (>13 Hz)
What do they look like?
Fast and small—short bursts that look sharper and more frequent than alpha waves.
Why do they appear?
Beta waves reflect active thinking and concentration. When you're solving problems, speaking, or moving, your brain is busy—and the waves show that by being fast and low in amplitude.
Where are they strongest?
In the frontal lobes, where decision-making, speech, and motor planning live.
🧭 Beta is your brain’s “on-task mode”—switched on and firing rapidly during activity.
🧠 Theta Waves (4–7 Hz)
What do they look like?
Slower, more irregular waves that seem looser and less organized.
Why do they appear?
Theta waves show up when you're drowsy or drifting toward sleep. They're linked with the hippocampus and emotional centres, especially as the brain starts turning inward.
Where are they strongest?
Often seen in the temporal lobes, which handle memory and emotion.
🧭 Theta waves are like your brain easing off the accelerator—prepping for sleep or deep thought.
🧠 Delta Waves (<4 Hz)
What do they look like?
Very slow and high in amplitude—big waves that stand out clearly on the EEG.
Why do they appear?
They dominate in deep sleep, when the cortex is largely shut down from external input. But in an awake person, prominent delta waves can signal dysfunction—like brain injury, metabolic problems, or encephalopathy.
Where are they strongest?
Can be seen widely, but when abnormal, they often appear most in the frontal lobes, which are especially sensitive to cortical dysfunction.
🧭 Delta waves are like brain “power saving mode” in sleep—or a warning signal when awake.
🧠 Gamma Waves (≥30 Hz)
What do they look like?
Very fast and very small waves — they zip across the screen at high speed, often buried in background noise or mistaken for muscle activity.
Why do they appear?
Gamma waves show up when the brain is working hard to integrate information. They help link together thoughts, sights, sounds, and memories — so you can experience them as a single, unified event. Tasks involving attention, learning, and complex perception tend to generate gamma activity.
Where are they strongest?
Most active in regions like the frontal lobes, visual cortex, and hippocampus — especially during focused cognitive effort. But because they’re so fast, they’re difficult to see clearly in standard scalp EEG and may require specialised equipment or filtering.
🧭 Gamma waves are like the brain’s rapid-fire synchronisers — bringing everything together so your thoughts, senses, and memories work in harmony.
⚠️ How Do We Use EEG to Detect Seizures?
What happens in a seizure:
- Groups of neurons fire abnormally and synchronously.
- On EEG, this shows up as spikes, sharp waves, or spike-and-wave complexes.
Why this is important:
- Different seizure types have different EEG patterns. For example:
- Absence seizures → 3 Hz spike-and-wave across both hemispheres.
- Temporal lobe epilepsy → focal spikes in the temporal electrodes.
- These patterns help clinicians locate seizure origins—and guide treatment decisions.
🔍 How do you know it's a seizure focus?
- Look for repeated abnormal discharges from the same area.
- Correlate with clinical signs (e.g., automatisms, aura).
🗓️ When to Order an EEG
An EEG isn’t something you order just because someone has a headache or feels a bit off. It’s best used when there's a specific question about brain function — especially around electrical activity.
So… when is it actually useful?
⚡ Suspected Seizures
If a person has had an episode of altered awareness, jerking movements, staring spells, or unusual behaviours that might be seizure-related, an EEG helps capture abnormal electrical discharges — or interictal changes that suggest epilepsy.
😴 Unexplained Altered Consciousness
When someone is drowsy, confused, or fluctuating between alertness and non-responsiveness, EEG can detect slowing that may indicate encephalopathy or delirium.
🧠 Status Epilepticus
If there’s concern about non-convulsive status epilepticus — a prolonged seizure state without visible convulsions — EEG is crucial for diagnosis and monitoring.
💤 Sleep Disorders
EEGs form part of polysomnography in sleep studies, helping to diagnose conditions like narcolepsy or parasomnias. Sleep also unearths epileptiform discharges that might not appear while awake.
👶 Developmental or Behavioural Changes
In children with regression, staring episodes, or sudden loss of learned skills, EEG can help clarify if seizures are contributing to neurological disruption.
🧠 Important context:
EEG shows function, not structure. If you’re looking for a tumour, stroke, or bleeding, that’s MRI or CT territory. EEG complements those by revealing what the brain is doing — or failing to do — in real time.
🔍 What’s next?
- 📝 All posts on the nervous system →
- 📝 Structure and function of the CNS →
- 📝 Pathophysiology of seizures →
- 📝 Understanding seizure classification →
- 📝 Localisation of seizures →
- 📝 Neurotransmitters 101 →
- 📝 Consciousness and how we can lose it →
- 📝 Clinical cases in seizure localisation →
- 📝Principles of seizure management →
- 📝Neurotransmitters on drugs! →
- 📝A beginner's guide to EEG →
- 📝A beginner's guide to neuroimaging →
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