Action Potentials and Event-Related Potentials: Back to Basics

[Chip (OP)]

Action Potentials and Event-Related Potentials: Back to Basics

see also A brainwave for catching a criminal ?

The Action Potential

Every thought, sensation, and movement you have ever experienced began with an action potential — the fundamental electrical event of the nervous system.

A neuron at rest maintains a charge difference across its membrane: negative inside, positive outside, held in place by ion pumps. When a neuron receives sufficient incoming signals, voltage-gated sodium channels snap open, positive sodium ions flood in, and the membrane potential reverses rapidly — from around -70mV to +40mV in less than a millisecond. This spike propagates down the axon like a wave. Potassium channels then open, the charge reverses back, and the neuron enters a brief refractory period before it can fire again.

This is the action potential. It is all-or-nothing — a neuron either fires fully or not at all. What varies is not the size of the spike but the frequency of firing and which neurons are firing. The entire complexity of human cognition emerges from variations in those two parameters across billions of neurons firing in coordinated patterns.

It was Emil du Bois-Reymond who first characterised this electrical event in 1848, calling it the "negative variation." Without that discovery, none of what follows would exist.

Event-Related Potentials (ERPs)

When the brain processes a specific stimulus — a sound, a word, an image, a surprise — populations of neurons fire in synchrony, producing voltage fluctuations detectable at the scalp via EEG. These are event-related potentials: brief, reproducible waves time-locked to a stimulus, each reflecting a distinct stage of cognitive processing.

They are named by polarity (P = positive, N = negative) and approximate timing in milliseconds after the stimulus.

P300
Occurs approximately 300ms after a stimulus that is unexpected, meaningful, or task-relevant. Reflects attention, recognition, and working memory updating. Larger amplitude = greater significance to the individual. The basis of the guilty knowledge test in forensic applications — a person who recognises a crime-relevant detail produces a larger P300 even if they deny it. First described by Sutton et al. in 1965. Applied to deception detection by Peter Rosenfeld in 1987.

N200 (N2)
Occurs around 200ms, negative deflection. Reflects conflict detection and response inhibition — the brain noticing a mismatch between expectation and reality and preparing to override an automatic response. Active when you stop yourself from doing something habitual.

N400
Occurs around 400ms in response to semantically unexpected words or concepts — for example, reading "he spread butter on his socks." Discovered by Marta Kutas and Steven Hillyard in 1980. Reflects the brain's real-time language comprehension and meaning integration processes. Larger amplitude = greater semantic surprise.

P600
Occurs around 600ms in response to grammatical violations or syntactic reanalysis — when the brain detects that a sentence structure doesn't parse correctly and has to reprocess it. Considered the grammatical counterpart to the N400's semantic role.

MMN — Mismatch Negativity
A negative deflection occurring 100–250ms after an unexpected change in a repetitive sound sequence — for example, a slightly different tone in a series of identical tones. Crucially, it occurs automatically, without attention or conscious awareness, and is present even during sleep. Used clinically to assess auditory processing in coma patients and infants. Reflects the brain's pre-attentive change detection system.

N170
Occurs around 170ms specifically in response to faces — much larger for faces than for any other visual object category. Reflects the brain's dedicated face processing system. Disrupted in prosopagnosia (face blindness).

C1 / P1 / N1 (Early Visual Components)
Occurring within the first 100–200ms of seeing something, these early components reflect low-level sensory processing in the visual cortex before any higher cognitive interpretation has occurred.

Summary Table

Component	Timing	What it reflects
Action Potential	~1ms	Single neuron firing — the base unit of all neural signalling
MMN	100–250ms	Automatic pre-attentive change detection; present during sleep
N170	~170ms	Face recognition; dedicated visual processing pathway
N200	~200ms	Conflict detection; response inhibition
P300	~300ms	Attention, recognition, working memory update; guilty knowledge detection
N400	~400ms	Semantic processing; meaning violation detection
P600	~600ms	Syntactic reanalysis; grammatical error detection