GloNeuro

Electrophysiology

Neurons communicate with electrical signals. Electrophysiology records these signals, from a single ion channel to the rhythms of the whole brain, with a precision of milliseconds.

MeasuresElectrical activity of neurons and populations
Temporal resolutionMilliseconds or better
Applied toCells, brain slices, animals and people, including EEG in clinics

Methods by scale

Microscopy
about 1 nm to 1 cm
Neuroimaging
about 0.5 mm to the whole brain
Brain networks
single neurons to the whole brain
Electrophysiology
about 10 nm to the whole brain
1 nm10 nm100 nm1 µm10 µm100 µm1 mm1 cm10 cm1 m
  1. Ion channelabout 10 nm
  2. Synaptic vesicleabout 40 nm
  3. Neuron cell bodyabout 10–30 µm
  4. Thickness of cortexabout 2–4 mm
  5. Human brainabout 15 cm long
Spatial scale, logarithmic: each step along the axis is ten times larger than the last. Ranges and sizes are approximate.
Where electrical signals are recorded, from the least to the most invasive. Scalp EEG sums the activity of very large populations of neurons; ECoG electrodes rest on the surface of the cortex; depth electrodes record local field potentials inside the tissue; fine microelectrodes pick up the spikes of single neurons; and a patch pipette sealed onto one cell records its membrane potential directly. Traces are illustrative.

When a neuron fires, channels in its membrane open and close in sequence, letting charged ions flow in and out. The result is a brief electrical pulse, the action potential, which lasts only a millisecond or two and travels along the axon to the next cells.

Electrodes placed inside or immediately next to a neuron record its individual action potentials. Arrays of many electrodes record hundreds of neurons at once. Electrodes on the scalp, used in electroencephalography (EEG), detect the combined activity of large populations of neurons as rhythmic waves.

Because electrophysiology follows activity as it happens, it is central to understanding how neurons encode information, and it is used clinically to diagnose epilepsy, assess sleep and guide brain surgery.

Key techniques

Patch clamp

Recording the currents that flow through ion channels.

A glass pipette with a tip about a micrometre wide is pressed against a cell membrane, and gentle suction forms a tight seal. In this configuration the current through a single ion channel can be measured, appearing as small, step-like openings and closings. Breaking through the membrane gives access to the whole cell, so its voltage and currents can be recorded or controlled.

Applications include studying how ion channels open and close, measuring synaptic currents and testing how drugs affect neurons.

The technique was developed by Erwin Neher and Bert Sakmann, who received the 1991 Nobel Prize.

Single-unit recording

Recording individual neurons in the active brain.

A fine microelectrode placed close to a neuron picks up the brief voltage changes produced by its action potentials. Recording while an animal sees, moves or makes decisions relates a neuron’s firing to specific events. Orientation-selective cells in visual cortex and place cells in the hippocampus were both discovered this way.

Applications include linking neural firing to perception and behaviour, mapping receptive fields and guiding electrodes during deep brain stimulation surgery.

Spikes are assigned to individual neurons by the shape of their waveforms, a step called spike sorting.

High-density probes

Recording hundreds of neurons at the same time.

Silicon probes carry hundreds or thousands of recording sites along thin shanks. A single Neuropixels probe can record from hundreds of neurons across several brain regions simultaneously, revealing how populations of neurons, rather than single cells, represent information.

Applications include studying how brain-wide populations coordinate behaviour, comparing regions at the same moment and long-term recording in freely moving animals.

These datasets are very large, and analysing them relies heavily on computation and machine learning.

Electroencephalography (EEG)

Brain rhythms recorded from the scalp.

Electrodes on the scalp detect tiny voltage fluctuations produced when large populations of cortical neurons are active together, mainly through synaptic currents in pyramidal cells. The signals appear as waves whose frequency and amplitude change with sleep, attention and disease. Averaging responses to repeated stimuli gives event-related potentials, which track processing millisecond by millisecond.

Applications include diagnosing epilepsy, staging sleep and monitoring anaesthesia and brain function in intensive care.

EEG is inexpensive, portable and safe, which makes it one of the most widely used methods in clinics and research.

Magnetoencephalography (MEG)

Measuring the brain’s magnetic fields.

The same neural currents that produce EEG also generate extremely weak magnetic fields. MEG detects them with very sensitive sensors, traditionally superconducting devices cooled with liquid helium inside a magnetically shielded room. Magnetic fields are less distorted by the skull than electrical signals, so their sources can often be located more precisely than with EEG.

Applications include locating epileptic activity and functional areas before surgery, studying language and sensory processing and measuring the timing of brain rhythms.

Newer optically pumped magnetometers work at room temperature and can be worn on the head.

Intracranial recording

Recording directly from the human brain.

In some people with epilepsy that does not respond to medication, electrodes are placed on the surface of the brain (electrocorticography) or within it (stereo-EEG) to find where seizures begin. These recordings, made over days in hospital, are very precise, and with consent they also allow research into human memory, language and movement.

Applications include locating where seizures begin before surgery, mapping essential functions so they can be protected and research on human cognition and brain–computer interfaces.

Such research is carried out alongside clinical care and only with the patient’s consent.

Brain rhythms

RhythmFrequencyOften associated with
DeltaAbout 0.5–4 HzDeep sleep
ThetaAbout 4–8 HzDrowsiness; memory processing in the hippocampus
AlphaAbout 8–12 HzRelaxed wakefulness with the eyes closed
BetaAbout 13–30 HzActive thinking, alertness and movement control
GammaAbove about 30 HzLocal processing and binding of information

Band limits are approximate and vary between laboratories.

The bands in the table, drawn at typical frequencies over one second. Slower rhythms usually have larger amplitudes.

History of the field

  • Discovery
  • Technique
  • Application

  1. 1791Discovery

    Animal electricity

    Luigi Galvani shows that electrical stimulation makes frog muscles contract.

  2. 1924Technique

    The first human EEG

    Hans Berger records the electrical rhythms of the human brain from the scalp.

  3. 1939Technique

    Inside the axon

    Hodgkin and Huxley record an action potential from inside the squid giant axon.

  4. 1952Discovery

    The Hodgkin–Huxley model

    A mathematical description of how sodium and potassium currents generate the nerve impulse.

  5. 1959Discovery

    Feature detectors in the cortex

    Hubel and Wiesel find neurons in visual cortex that respond to edges of particular orientations.

  6. 1971Discovery

    Place cells

    John O’Keefe discovers hippocampal neurons that fire when an animal is in a particular location.

  7. 1976Technique

    Patch clamp

    Neher and Sakmann record the current through a single ion channel.

  8. 1987Application

    Deep brain stimulation

    High-frequency stimulation of the thalamus is shown to suppress tremor, leading to a treatment now used in Parkinson’s disease.

  9. 2006Application

    Brain–computer interfaces

    A person with paralysis controls a computer cursor using signals recorded from an electrode array in the motor cortex.

  10. 2017Technique

    Neuropixels

    High-density silicon probes record hundreds of neurons across several brain regions at once.

  11. 2023Application

    Restoring speech

    Neural recordings are decoded into text and speech for people who have lost the ability to speak.

  12. 2026Discovery

    Optogenetics recognised

    The Nobel Prize in Physiology or Medicine honours the discoveries that let neurons be switched on and off with light, a tool now combined with electrophysiology across neuroscience.

Strengths and limitations

StrengthsLimitations
Follows neural activity with millisecond precisionInvasive recordings are limited to animals or clinical settings
Records single neurons as well as whole-brain rhythmsScalp EEG cannot pinpoint deep sources precisely
EEG is safe, affordable and portableEach electrode samples only a small fraction of neurons

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