2.1 Neuroanatomy & Neurophysiology
Key Takeaways
- The scalp EEG is generated almost entirely by summated excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) of cortical pyramidal cells, not by action potentials.
- A single scalp electrode requires roughly 6 cm2 to 10 cm2 of synchronously active cortex before a potential becomes visible at the scalp.
- The posterior dominant rhythm (alpha) is normally 8 to 13 Hz, best seen over the occipital region with eyes closed, and attenuates ('blocks') with eye opening or mental effort.
- Frequency bands in adults: delta is under 4 Hz, theta is 4 to under 8 Hz, alpha is 8 to 13 Hz, and beta is over 13 Hz; each band maps to characteristic states of consciousness.
- Slow waves (delta) seen in an awake adult are abnormal, while the same frequencies are normal and expected in deep (N3) sleep.
Every decision a registered EEG technologist makes during the Performing the Study domain rests on one question: is what I am seeing physiologic, and is it normal for this patient's age and state? You cannot answer that without understanding where the EEG comes from. This section builds the neuroanatomy and neurophysiology foundation that supports electrode placement, montage selection, artifact rejection, and waveform identification later in the exam.
The Neuron and the Synapse
The functional unit of the nervous system is the neuron. Information arrives on the dendrites, is integrated at the soma (cell body), and travels as an action potential down the axon to the synapse, where chemical neurotransmitters cross the synaptic cleft to the next neuron.
At the receiving (postsynaptic) membrane, neurotransmitters open ion channels and produce a postsynaptic potential (PSP):
- An excitatory postsynaptic potential (EPSP) depolarizes the membrane and makes the neuron more likely to fire.
- An inhibitory postsynaptic potential (IPSP) hyperpolarizes the membrane and makes firing less likely.
This distinction matters for the exam because the scalp EEG is built from PSPs, not action potentials.
What Actually Generates the Scalp EEG
A common exam trap is to attribute the EEG to action potentials. It is not. Action potentials are too brief (about 1 ms) and too desynchronized to summate into a scalp signal. Postsynaptic potentials last 15 to 200 ms, so they overlap in time and add together.
The dominant generators are the cortical pyramidal cells of layers III, V, and VI. These neurons are arranged in parallel, perpendicular (radial) to the cortical surface, so their extracellular currents line up like many small batteries in series rather than canceling out. This vertical alignment creates an open field that a scalp electrode can detect.
The Summation Requirement
One neuron produces a field far too small to record from the scalp. Detectable scalp activity requires synchronous postsynaptic activity across roughly 6 to 10 square centimeters of cortex. Three factors weaken the signal before it reaches the electrode:
| Factor | Effect on Scalp EEG |
|---|---|
| Distance from generator to electrode | Amplitude falls sharply with distance |
| Volume conduction through CSF, skull, scalp | Smears and attenuates the signal (skull is a strong resistor) |
| Generator orientation | Radial (open field) generators are seen well; tangential generators in sulci are seen poorly |
Because the skull attenuates and blurs the signal, scalp EEG amplitudes are tiny (roughly 10 to 100 microvolts) and represent a low-pass, spatially averaged view of underlying cortex.
Brain Anatomy and Lobar Localization
The cerebral cortex is divided into four lobes per hemisphere. Knowing each lobe's function helps you predict where a clinical abnormality should appear and which electrodes will show it.
| Lobe | 10-20 Region (approx.) | Primary Function | EEG Relevance |
|---|---|---|---|
| Frontal | Fp, F | Executive function, voluntary motor, eye movements | Frontal eye-blink artifact; frontal intermittent rhythmic delta in encephalopathy |
| Temporal | T (T7/T8, T3/T4) | Memory, auditory processing, language | Most common site of focal epileptiform discharges |
| Parietal | P | Somatosensory integration, spatial awareness | Mu rhythm and sensory responses |
| Occipital | O | Primary visual processing | Source of the posterior dominant (alpha) rhythm |
Deeper structures also matter conceptually. The thalamus acts as a pacemaker that helps synchronize cortical rhythms (notably sleep spindles and the alpha rhythm), and the brainstem reticular activating system controls arousal level, which is why the EEG changes so dramatically between wakefulness and sleep.
The two hemispheres are connected by the corpus callosum. Because homologous regions are normally well connected, the awake adult EEG should be roughly symmetric between the left and right sides; persistent asymmetry is a flag for the technologist to verify it is not an electrode or reference problem before assuming it is real.
Normal Rhythms and States of Consciousness
EEG frequency is reported in hertz (Hz) — cycles per second. The four classic adult bands and the states they index are the single most testable concept in this section.
| Band | Frequency | Typical Normal State |
|---|---|---|
| Delta | < 4 Hz | Deep (N3) slow-wave sleep; normal in infants; abnormal if awake adult |
| Theta | 4 to < 8 Hz | Drowsiness and light sleep; normal in children; some appears in awake adults |
| Alpha | 8 to 13 Hz | Relaxed wakefulness with eyes closed (posterior dominant rhythm) |
| Beta | > 13 Hz | Active alertness, concentration; prominent with sedative/benzodiazepine medication |
The Posterior Dominant (Alpha) Rhythm
The alpha rhythm, also called the posterior dominant rhythm (PDR), is the technologist's anchor for a normal adult recording:
- 8 to 13 Hz, maximal over the occipital and posterior head regions.
- Present with eyes closed during relaxed wakefulness.
- Attenuates (blocks) with eye opening, mental effort, or anxiety — a normal response called alpha attenuation or blocking.
- Should be reactive and symmetric; a posterior rhythm that fails to attenuate with eye opening, or is persistently asymmetric, is abnormal.
Beta, Theta, and Delta in Context
Beta is the fast, low-amplitude background of an alert brain and is markedly increased by barbiturates and benzodiazepines (a key medication-effect link). Diffuse theta is normal in children and during drowsiness but suggests mild diffuse dysfunction if dominant in an alert adult. Delta is the hallmark of deep sleep and is normal in N3 and in neonates and young infants — but focal or diffuse delta in an awake adult points to pathology. The recurring exam principle: the same frequency can be normal or abnormal depending on the patient's age and state of consciousness.
Adult EEG Frequency Bands (Hz)
| Item | Value |
|---|---|
| Delta | 3 |
| Theta | 6 |
| Alpha | 10 |
| Beta | 20 |
The electrical activity recorded by scalp EEG is primarily generated by which neuronal event?
A relaxed adult has eyes closed. An 8-13 Hz rhythm is maximal over the occipital region and disappears when the patient opens their eyes. What is this finding?
Approximately how much synchronously active cortex is generally required before a postsynaptic potential becomes visible at a single scalp electrode?
Diffuse delta activity (<4 Hz) is recorded. In which scenario is this finding NORMAL rather than abnormal?