2 Neuroscience2.3 Basic techniques in neuroscience
2.3.1.2 EEG, frequency bands, normal findings, evoked response techniques and applications
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Written by Resident Doctor Committee · Published
Recording the signal
Electroencephalography (EEG) records scalp voltage differences arising mainly from summed postsynaptic activity in similarly oriented cortical pyramidal cells firing in synchrony.
Volume conduction blurs where a signal comes from and can hide small or deep sources.
The inverse problem limits localisation: different sources can produce similar scalp patterns, though EEG still gives millisecond timing at the bedside.
The international 10-20 system places electrodes by measured head distances: letters mark the region, odd numbers the left, even numbers the right and z the midline.
Frequency bands
Band boundaries vary slightly between conventions.
Band | Frequency | Association |
|---|---|---|
Delta | Below 4 Hz, the slowest band | Deep slow-wave sleep; waking infants |
Theta | 4 to under 8 Hz | Drowsiness, light sleep; waking young children |
Alpha | 8 to 13 Hz (8 to 12 Hz in some sources) | Relaxed, awake adult with eyes closed; largest posteriorly |
Beta | About 12 or 13 to 30 Hz | Low-voltage frontal background of the alert adult |
Gamma | Above about 30 Hz | Thought to bind activity across cortical areas |
Sustained delta in the waking adult is pathological, because delta is normal only in deep slow-wave sleep; continuous focal irregular delta points to a structural lesion such as a tumour.
Theta (4 to 7 Hz) rises during meditation, especially over the frontal midline, while the person stays awake.
Alpha blocking is the bilateral, symmetrical attenuation of the posterior alpha rhythm when the eyes open and visual attention is engaged; a one-sided change implies focal pathology.
Mu rhythm runs at about 7 to 11 Hz over the central sensorimotor electrodes with an arch or comb shape, and is blocked by moving or planning to move the opposite limb.
Mu is told from alpha by site and blocking: mu is central and ignores eye opening, while alpha is posterior and blocks on eye opening.
Gamma power and synchrony are reduced in schizophrenia, for example in the 40 Hz auditory steady-state response.
Development and ageing
The waking infant EEG is dominated by slow, irregular delta (about 3 Hz) and theta, which speeds up and becomes rhythmic through childhood until it looks adult by mid-adolescence.
Mild, intermittent, left-predominant temporal slowing with a preserved posterior rhythm is a benign finding of normal ageing.
Sleep
Stage | EEG features |
|---|---|
N1 | Wake-to-sleep transition: theta after alpha drops out, vertex transients, hypnic jerks; no spindles or K complexes |
N2 | Sleep spindles and K complexes; slow waves fill less than a fifth of the epoch |
N3 (slow-wave) | High-amplitude delta at 0.5 to 2 Hz filling a fifth or more of the epoch |
REM (rapid eye movement) | Wake-like mixed activity, rapid eye movements, muscle atonia |
Sleep spindles are brief waxing and waning bursts at about 11 to 15 Hz, a normal thalamocortical rhythm.
Fatal familial insomnia, a prion disease that destroys the thalamus, abolishes sleep spindles.
Sleep deprivation brings REM sleep forward into daytime sleep as sleep-onset REM periods.
Activation procedures
Procedure | What it provokes |
|---|---|
Hyperventilation | Generalised 3 Hz spike-and-wave of childhood absence epilepsy |
Photic stimulation | Generalised spike-and-wave (photoparoxysmal response), pointing to a genetic generalised epilepsy |
Sleep deprivation | Raised cortical excitability, drowsiness and sleep, which bring out discharges a routine EEG can miss |
Epilepsy
A single routine interictal EEG is normal in about half of people with epilepsy, so a normal trace does not exclude the diagnosis.
Absence is the only generalised-onset seizure type that the International League Against Epilepsy classes as non-motor, because impaired awareness defines it.
Seizure type or syndrome | EEG |
|---|---|
Typical (childhood) absence | Regular, symmetrical, generalised 3 Hz spike-and-wave; abrupt start and stop; normal background |
Atypical absence | Slow generalised spike-and-wave below about 2.5 Hz (1.5 to 2.5 Hz); gradual onset; abnormal, slowed background |
Lennox-Gastaut syndrome | Interictal generalised spike-and-wave slower than 2.5 Hz; bursts of fast activity in sleep |
Juvenile myoclonic and other myoclonic epilepsies | Faster, often asymmetrical generalised polyspike-and-wave at about 3 to 6 Hz, activated on waking |
Infantile spasms (West syndrome) | Hypsarrhythmia: chaotic, high-amplitude, multifocal spikes with no organised background |
Video-EEG telemetry confirms functional (dissociative, non-epileptic) seizures by recording a habitual attack on video with no epileptiform change on the simultaneous EEG.
Slowing, periodic patterns and encephalopathy
Diffuse, symmetrical theta and delta slowing in an awake patient indicates diffuse cortical dysfunction, the typical picture of metabolic encephalopathy.
Most delirium slows the EEG: alpha fades and theta and delta take over.
Alcohol or sedative withdrawal delirium (delirium tremens) is the exception, showing low-voltage fast beta or a normal record, from cortical hyperarousal.
Condition | Characteristic EEG |
|---|---|
Sporadic Creutzfeldt-Jakob disease (CJD) | Generalised periodic sharp-wave complexes about once a second on a flattened, slow background, in roughly two thirds; usually absent in variant CJD |
Herpes simplex encephalitis | Periodic lateralised epileptiform discharges every 1 to 3 seconds over one temporal region, with local slowing |
Subacute sclerosing panencephalitis (late complication of measles) | Identical high-voltage periodic complexes at long fixed intervals of 4 to 15 seconds, often time-locked to myoclonic jerks |
Hepatic encephalopathy | Triphasic waves: blunt, frontal-maximum complexes with a front-to-back lag on a slow background |
Triphasic waves are not specific, because uraemia, sepsis and lithium toxicity also produce them.
Dementia
Condition | EEG |
|---|---|
Alzheimer's disease | Alpha slows and diffuse slow activity builds as the illness progresses; amplitude is preserved |
Huntington's disease | Diffusely low-voltage, flattened record with poorly formed or lost alpha, at times near flat, from cortical neuronal loss |
Early frontotemporal dementia | Often normal |
Drug effects
Drug | EEG effect |
|---|---|
Benzodiazepines, barbiturates (enhance gamma-aminobutyric acid type A function) | More diffuse fast beta, most obvious frontally; less alpha |
Acute alcohol intoxication | More fast beta, through the same GABA-A action |
Alcohol | More alpha and theta |
Antipsychotics | Slowing: less beta, more theta and delta; low-potency phenothiazines such as chlorpromazine slow more than high-potency drugs |
Clozapine | Less alpha, more theta and delta |
Lithium at therapeutic levels | Slower alpha, diffuse theta and delta, paroxysmal sharp or spike activity |
Lithium toxicity (usually above 1.5 mmol/L) | Diffuse slowing with triphasic waves |
Anticonvulsants such as lamotrigine | Waking background largely unchanged at therapeutic levels |
Opioids such as heroin | Less alpha, more high-voltage theta and delta; slow waves dominate in overdose |
Cannabis, cocaine | More alpha alone, most markedly frontally with cannabis |
Liability to EEG abnormality among antipsychotics runs from clozapine (highest) through olanzapine, risperidone and typical agents to quetiapine (lowest).
Sedating drugs lower alpha, while recreational substances raise it.
Evoked and event-related potentials
Evoked potentials are responses to repeated visual, auditory or somatosensory stimuli; averaging stimulus-aligned segments strengthens time-locked activity while unrelated background cancels.
Latency reflects conduction and processing time, and amplitude reflects the summed response; delayed or absent components suggest pathway dysfunction.
The P300 is a positive response at about 300 ms or later to a relevant, infrequent stimulus.
Mismatch negativity is an automatic response to a change in an auditory stimulus.
Group differences in psychiatric research do not make these individual diagnostic tests.
Magnetoencephalography
Magnetoencephalography (MEG) records magnetic fields arising mainly from summed postsynaptic currents along the parallel apical dendrites of cortical pyramidal cells.
MEG is selectively sensitive to tangential currents in sulcal walls, because radially oriented currents produce magnetic fields that do not escape the head.