AQA Syllabus focus:
'Ways of studying the brain, including electroencephalograms (EEGs) and event-related potentials (ERPs).'
EEGs and ERPs are indirect measures of brain activity that record electrical signals from the scalp. They are especially useful for showing the timing of neural activity during rest, sleep, and cognitive tasks.
Electroencephalograms
An electroencephalogram records the brain’s ongoing electrical activity using electrodes attached to the scalp.
Electroencephalogram (EEG): A recording of the brain’s overall electrical activity, detected by electrodes placed on the scalp.
EEGs do not measure single neurons. Instead, they detect the combined activity of large groups of neurons, mainly in the cerebral cortex, when these cells are active together. The output appears as a series of brain waves that vary depending on the person’s state, such as alertness, drowsiness, or sleep.
How EEGs are recorded
Researchers place small electrodes on the scalp in standardized positions.
International 10–20 system diagram showing standard scalp electrode locations (e.g., F, C, P, O sites and midline ‘z’). This helps you visualize how EEG recordings are made consistently across participants and labs by placing electrodes at predefined positions. Source
These electrodes detect tiny voltage changes produced by brain activity, and the signal is then amplified and displayed as a waveform over time.
Electrodes are attached to the scalp.
Electrical activity is detected from the cortex.
The signal is amplified because the original activity is very small.
The recording is shown as a continuous pattern of waves.
EEGs can be recorded while a person is resting, sleeping, or completing a task. Because the recording is continuous, EEGs are very good at showing rapid changes in activity from one moment to the next.
Uses of EEGs
EEGs are widely used in sleep research because different stages of sleep produce different patterns of brain waves. They are also used in clinical settings to help identify unusual electrical activity, such as patterns linked to epilepsy. In psychological research, EEGs can be used to study changes in general brain state, attention, and arousal.
Strengths and limitations of EEGs
A major strength of EEGs is their very high temporal resolution. This means they can detect changes in brain activity on a millisecond-by-millisecond basis. EEGs are also non-invasive, so they do not require surgery or injections, and they are relatively economical compared with some other brain-scanning methods.
However, EEGs have poor spatial resolution. They can show that brain activity has changed, but they cannot accurately pinpoint the exact location of that activity within the brain. Another limitation is that recordings can be affected by artifacts, such as blinking, eye movements, or muscle tension, which may reduce accuracy.
Event-related potentials
An event-related potential is a small change in the EEG that is linked to a specific stimulus or event.
Event-related potential (ERP): A measured brain response taken from EEG data that is directly related to a particular sensory, cognitive, or motor event.
ERPs come from the same raw electrical recording as an EEG, but they are more specialized. Instead of looking at overall ongoing activity, researchers focus on what happens in the brain immediately after a particular event, such as seeing an image, hearing a sound, or making a decision. The response of interest is usually very small, so it can be hidden by other background activity.
How ERPs are produced
To produce an ERP, a stimulus is usually presented many times. The EEG is recorded on each trial, and the separate recordings are lined up according to the moment the stimulus appeared. Researchers then average the recordings.
Grand-averaged ERP waveforms illustrating the P300 component at midline electrodes (Fz, Cz, Pz), alongside scalp maps showing where the voltage distribution is strongest around the P300 time window. This visual reinforces that ERPs are extracted from EEG by time-locking to an event and averaging trials to reveal a small, stimulus-linked response. Source
A specific stimulus or event is presented repeatedly.
EEG activity is recorded after each presentation.
The recordings are matched to the timing of the event.
The waveforms are averaged to isolate the event-linked response.
Averaging helps remove random background activity, leaving a clearer waveform that reflects the brain’s response to the event. This allows researchers to examine the timing of processes such as attention, recognition, or evaluation.
Uses of ERPs
ERPs are especially useful for studying cognitive processes. Researchers use them to investigate attention, perception, language, memory, and decision making. They are valuable because they can show when a process happens, even if that process is too fast to observe directly in behavior. This makes ERPs particularly useful when researchers want to study the sequence of mental events after a stimulus is presented.
Strengths and limitations of ERPs
Like EEGs, ERPs have excellent temporal resolution and are non-invasive. A key strength is that they are more specific than the raw EEG because they are tied to a particular event. This makes them very useful for investigating the timing of mental processing in great detail.
However, ERPs also have limitations. They usually require carefully controlled conditions and many repeated trials to produce a clear signal. Small differences in procedure can change the waveform, so research design must be precise. ERPs also share the poor spatial resolution of EEGs, meaning they are much better at showing when activity occurs than where it occurs. In addition, noise from movement or blinking can still interfere with the recording.
Key distinctions between EEGs and ERPs
Although the two methods are closely related, they are not the same.
EEG refers to the continuous recording of overall electrical brain activity.
ERP refers to a specific response within EEG data that is linked to a particular event.
EEGs are often used to study broad brain states, such as sleep or general arousal.
ERPs are more often used to study the timing of sensory and cognitive processing during tasks.
This distinction is important: an ERP is not a separate machine from an EEG. It is a way of analyzing EEG recordings to isolate activity linked to a specific stimulus or response.
Practice Questions
Outline what an EEG is. (2 marks)
1 mark for stating that an EEG records electrical activity in the brain.
1 mark for stating that this is done using electrodes placed on the scalp or that it records overall brain-wave activity.
Explain what ERPs are and discuss one strength and one limitation of using ERPs to investigate brain activity. (6 marks)
AO1: 3 marks
1 mark for stating that ERPs are brain responses linked to a specific event or stimulus.
1 mark for stating that ERPs are derived from EEG recordings.
1 mark for explaining that repeated trials are averaged to isolate the event-related response.
AO3: 3 marks
1 mark for identifying a strength, such as excellent temporal resolution or the ability to study fast cognitive processes.
1 mark for elaborating on that strength.
1 mark for identifying and/or elaborating on a limitation, such as poor spatial resolution, sensitivity to noise, or the need for tightly controlled repeated trials.
FAQ
A standardized placement system helps researchers record from similar scalp locations across different participants and studies.
This improves consistency and makes findings easier to compare. It also reduces the chance that differences in results are caused simply by electrodes being placed in different positions.
These labels describe the shape and timing of an ERP component.
P means a positive peak.
N means a negative peak.
The number usually refers to the approximate time, in milliseconds, after the stimulus appears.
For example, P300 is a positive peak around 300 milliseconds after a stimulus. The label describes the waveform, not a single brain area.
These actions create electrical activity that can be picked up by scalp electrodes.
Common sources of interference include:
blinking
eye movements
jaw clenching
forehead tension
body movement
Because EEG and ERP signals are very small, these unwanted signals can distort the recording. Researchers often ask participants to stay still and may remove contaminated sections during analysis.
An oddball task presents frequent standard stimuli mixed with rare target stimuli.
Researchers use it because rare or unexpected events often produce clear ERP components, making timing differences easier to detect. It is especially useful for studying:
attention
stimulus discrimination
updating of information
This type of task helps researchers link specific ERP patterns to how the brain responds to novelty or significance.
Yes. One advantage of these methods is that they do not depend entirely on spoken answers.
Researchers can present sounds, images, or other stimuli and measure the brain’s response directly. This makes EEGs and ERPs useful with:
infants
young children
people with language difficulties
some clinical groups
The main challenge is that movement can create extra noise, so collecting clean data may take more time.
