Basic Neurosciences · Paper A
EEG rhythms
An EEG (electroencephalogramA recording of the brain's electrical activity, usually shortened to EEG.Read more →) is a recording of the brain's electrical activity, shown as waves of different speeds (frequencies). Each frequency band has a typical meaning, and the exam tests these. For example, imagine seeing slow delta waves on the EEG of a fully awake, alert adult. In practice, that specific mismatch, delta activity outside sleep, is itself the abnormal finding worth investigating.
Alpha (8-13 Hz) is the relaxed, eyes-closed background rhythm at the back of the head; it disappears on opening the eyes or concentrating. Beta (13-30 Hz) is fast, low-amplitude activity during alertness, increased by benzodiazepines. Theta (4-7 Hz) is normal in children and in drowsiness, but abnormal if prominent in a fully awake adult. Delta (under 4 Hz) is the big, slow wave of deep sleep. For example, imagine a drowsy toddler whose EEG shows prominent theta activity, that finding is entirely normal at that age and state. In practice, this means the same wave pattern can be normal or abnormal depending on the patient's age and level of alertness.
An EEG (electroencephalogram) records the brain's electrical activity from electrodes placed on the scalp, and the trace it produces is really just a mixture of waves at different frequencies. Every named EEG rhythm in psychiatry is defined by ONE number (how many cycles per second (Hz, or hertz, the unit of frequency) the wave oscillates at) plus where on the scalp it is strongest and what state the patient is in when it appears. Learn the frequency bands first; everything else in this topic hangs off them.
EEG rhythms are named frequency bands into which the brain's background electrical activity is conventionally divided: delta, theta, alpha, and beta, listed here from slowest to fastest. Each band is associated with a different normal state of arousal, and each becomes clinically informative when it appears somewhere, or at some time, it should not.
A common mistake is treating a slow rhythm as automatically abnormal. It is not: delta and theta waves are entirely normal findings in specific circumstances, for example deep sleep or early childhood. What matters clinically is CONTEXT: the same frequency that is normal in one state (delta in deep sleep) is a red flag for pathology in another (delta over one side of the brain in an awake adult, suggesting a focal lesion, meaning a localised area of brain damage rather than a generalised process).
Imagine tuning a radio dial from the slowest, lowest station to the fastest, highest one. DELTA waves are the slowest, at less than 4 Hz, and dominate deep, dreamless sleep (slow-wave sleep) and early childhood; in an awake adult, focal delta activity points toward a structural lesion such as a tumour or an area of brain injury. THETA waves run at 4 to 8 Hz and are seen in drowsiness, light sleep, and, when excessive, in (diffuse brain dysfunction, often from a metabolic or toxic cause). ALPHA waves run at 8 to 13 Hz and are the classic "relaxed but awake" rhythm, strongest over the posterior (back) of the scalp, appearing specifically when the patient is awake with their EYES CLOSED; opening the eyes attenuates (reduces) the alpha rhythm, a bedside trick used to confirm it is genuine. BETA waves are the fastest of the four, above 13 Hz, and dominate active concentration and anxiety: and, importantly for psychiatry, are INCREASED by benzodiazepines (a class of sedative and anxiolytic drug), making beta activity a recognised EEG marker of benzodiazepine effect.
Consider a patient having a routine EEG. If the technician asks them to close their eyes and a strong, symmetrical rhythm appears over the back of the head at roughly 10 Hz, that is a normal posterior alpha rhythm, confirming a relaxed, awake state. If instead the same patient shows persistent delta-frequency slowing localised over the left temporal region while awake, that focal, awake-state delta is abnormal and should prompt investigation for a structural lesion in that area, for example with an MRI brain scan, rather than being dismissed as a normal sleep-related finding. Benzodiazepines characteristically increase fast beta activity across the whole scalp, which is why a patient's EEG can look different once they start a benzodiazepine even with no underlying brain disease.
An EEG rhythm (a background frequency band) is not the same as an epileptiform discharge (a sharp spike or spike-and-wave complex signalling abnormal, hyperexcitable neuronal firing): a patient can have entirely normal background rhythms between seizures and still have epilepsy, which is why a single normal EEG never fully excludes a seizure disorder. Diffuse background slowing (excess theta or delta replacing normal alpha and beta activity across the whole scalp) is a recognised finding in delirium and in some dementias, reflecting generalised cortical dysfunction, and should be told apart from FOCAL slowing, which points toward a localised structural lesion instead. EEG rhythms are also distinct from evoked potentials, which measure the brain's response to a specific sensory stimulus rather than its resting background activity.
An EEG is recorded using electrodes placed across the scalp in a standard layout (the 10-20 system, so called because electrode spacing is set at 10% or 20% of set skull landmarks), usually with the patient resting, and often including activation procedures such as asking the patient to hyperventilate or exposing them to a flashing light, both of which can bring out abnormal discharges that a resting trace would miss. Reading the trace means asking two questions together: what frequency band is present, and is it appropriate for this patient's current state and this part of the scalp. Diffuse slowing supports a delirium diagnosis (an acute confusional state, often from an underlying medical cause) over a primary psychotic illness, which usually shows a normal EEG, while spike-and-wave discharges support a diagnosis of epilepsy.
EEG is used diagnostically to support, not alone confirm, a diagnosis of epilepsy, to characterise , and to help distinguish organic causes of confusion from primary psychiatric presentations. Any suspected organic contributor identified on EEG should prompt appropriate investigation and management of the underlying cause, for example correcting a metabolic derangement or treating an infection driving delirium, rather than treating the EEG finding itself. NICE guidance on the diagnosis and management of epilepsy places EEG as a supportive investigation alongside clinical history, not a stand-alone diagnostic test, and a normal EEG should never be used on its own to rule epilepsy out.
Questions often present a normal finding (delta in a sleeping patient, or alpha attenuating on eye-opening) and ask candidates to wrongly flag it as abnormal, or present benzodiazepine-related beta increase and expect it mistaken for a primary anxiety finding rather than a drug effect. Anchor each band to its number (delta under 4 Hz, theta 4 to 8 Hz, alpha 8 to 13 Hz posterior with eyes closed, beta above 13 Hz raised by benzodiazepines) and always ask what STATE the patient was in when the trace was recorded.
The four EEG background rhythms run, slowest to fastest, delta (under 4 Hz, deep sleep or a focal lesion if awake), theta (4 to 8 Hz, drowsiness or ), alpha (8 to 13 Hz, posterior, relaxed and awake with eyes closed), and beta (above 13 Hz, alert concentration, increased by benzodiazepines): interpreting any of them always depends on the patient's state and where on the scalp the rhythm appears, and EEG supports rather than replaces clinical diagnosis of epilepsy or delirium per NICE guidance.