EEGs and ERPs | AQA A-Level Psychology Revision

For 7182 specification, first teach in September 2025

AQA A-Level Psychology | Free Revision Notes

Estimated study time: 45 minutes

These EEGs and ERPs A-Level Psychology revision notes explain how electrical activity in the brain can be recorded using electrodes placed on the scalp. You will learn how an EEG produces a continuous record of general brain activity and how event-related potentials are extracted from EEG data to identify responses linked to specific stimuli or tasks. The lesson builds on measuring changes in blood oxygenation and prepares you to compare living-brain measurements with examining the brain after death.

Learning Objectives 🎯

By the end of this revision page, you should be able to:

• Define an electroencephalogram.

• Explain how EEGs measure brain activity.

• Identify the type of information produced by an EEG.

• Define an event-related potential.

• Explain how ERPs are extracted from EEG recordings.

• Compare EEGs and ERPs as ways of studying the brain.

• Evaluate the strengths and limitations of both methods.

Revision Notes 📚

EEGs and ERPs A-Level Psychology revision overview

An electroencephalogram, usually abbreviated to EEG, records electrical activity in the brain using electrodes placed on the scalp.

The recording shows patterns of electrical activity over time.

An event-related potential, usually abbreviated to ERP, is a small electrical response associated with a specific stimulus or event. ERPs are extracted from EEG recordings by presenting the same event repeatedly and averaging the recorded activity.

The central distinction is:

• An EEG provides a continuous record of general electrical activity.

• An ERP isolates electrical activity linked to a particular event.

The AQA specification identifies EEGs and ERPs as required ways of studying the brain. It does not require named ERP components, so these notes focus on how the methods work, what information they produce and how they should be evaluated.

Electrical activity in the brain

Neurons communicate using electrical and chemical processes.

Information travels:

• Electrically along neurons.

• Chemically across synapses.

• Electrically through the next neuron.

When large groups of neurons are active, their combined electrical activity can be detected using electrodes placed on the scalp.

The EEG records this combined activity.

It does not record:

• The firing of one individual neuron.

• Neurotransmitters crossing a synapse.

• Changes in blood oxygenation.

• A detailed image of brain structure.

• A person’s thoughts directly.

The electrical and chemical processes connecting neurons are covered in neurotransmitters, excitation and inhibition.

What does EEG stand for?

EEG is usually used to refer to an electroencephalogram, which is the recording produced by the method.

The term can be broken down as follows:

Part	Meaning
Electro	Relating to electrical activity
Encephalo	Relating to the brain
Gram	A record or recording

An EEG is therefore a record of electrical activity in the brain.

How an EEG is recorded

During an EEG recording:

1. Electrodes are attached to the participant’s scalp.

2. The electrodes detect small changes in electrical activity.

3. Signals from several areas of the scalp are recorded.

4. The signals are amplified.

5. A computer displays the activity as a pattern of waves over time.

The participant may be:

• Resting.

• Completing a task.

• Sleeping.

• Responding to stimuli.

• Being monitored for unusual electrical activity.

The method records activity continuously during the selected period.

The role of electrodes

An electrode is a sensor that detects electrical activity.

In an EEG investigation, several electrodes are positioned across the scalp. They may be attached individually or held in place using a specialised cap.

The electrodes detect voltage changes produced by the combined activity of groups of neurons.

Using several electrodes allows researchers to compare activity detected at different positions on the scalp.

However, scalp electrodes do not identify the precise origin of activity within the brain. Electrical activity must pass through brain tissue, the skull and the scalp before it is recorded.

What information does an EEG produce?

An EEG produces a continuous record of electrical activity over time.

The recording may show differences in:

• The frequency of brain waves.

• The amplitude of brain waves.

• The timing of changes in activity.

• Patterns occurring during different states or tasks.

• Unusual electrical activity.

The information is usually represented as a series of waveforms.

Feature	Meaning
Frequency	How often the waves occur within a period of time
Amplitude	The size or height of the electrical change
Timing	When a change in electrical activity occurs
Pattern	The overall form of electrical activity across time

An EEG is particularly useful for identifying when brain activity changes because electrical signals can be recorded extremely quickly.

EEG waveforms

An EEG waveform is a visual representation of electrical activity.

The horizontal direction represents time, while the vertical direction represents changes in electrical voltage.

A waveform may show:

• Faster or slower activity.

• Larger or smaller electrical changes.

• Regular or irregular patterns.

• Differences between electrodes.

• Changes between psychological or behavioural states.

Researchers interpret the overall pattern rather than treating every small movement in the line as a separate psychological process.

EEG and states of consciousness

EEGs can be used to compare broad patterns of brain activity during different states.

For example, researchers may investigate activity while a participant is:

• Awake.

• Resting.

• Completing a task.

• Moving through different stages of sleep.

Different states may be associated with different electrical patterns.

This makes EEG useful when the research question concerns general changes in brain activity across time.

However, an EEG usually provides less precise information about the exact brain structure producing the activity.

EEG and clinical use

EEGs can help identify unusual patterns of electrical activity.

For example, the method may contribute to investigations involving:

• Epileptic activity.

• Sleep disorders.

• Disturbances in normal patterns of brain activity.

This is useful because EEG provides a direct record of electrical activity while it is occurring.

Clinical usefulness does not mean that an EEG alone explains the psychological cause of a behaviour or condition. The recording must be considered alongside other evidence.

EEG and cognitive tasks

Psychologists may record an EEG while a participant completes a cognitive task.

For example, a participant might:

• Listen to sounds.

• View pictures.

• Read words.

• Make decisions.

• Respond to a signal.

• Try to remember information.

The EEG records electrical activity throughout the task.

However, the continuous recording includes activity related to many processes, such as:

• General alertness.

• Movement.

• Attention.

• Sensory processing.

• Background brain activity.

• The specific process the researcher wants to investigate.

This can make it difficult to isolate the response to one particular event using the raw EEG alone.

What is an ERP?

An event-related potential is a small electrical response in the brain associated with a specific stimulus, event or response.

An ERP is derived from an EEG recording.

It is not recorded using a completely different type of machine. The same general equipment is used, but the data are collected and analysed in a more specific way.

Examples of events that could be investigated include:

• Seeing a particular image.

• Hearing a sound.

• Recognising a word.

• Detecting an unexpected stimulus.

• Making a decision.

• Pressing a response button.

The event must occur at a clearly identified time so that the electrical activity following it can be examined.

Why ERPs are needed

The electrical response to one event is usually very small compared with the brain’s overall electrical activity.

A single EEG recording contains:

• Activity related to the event.

• Activity related to other mental processes.

• Ongoing background activity.

• Possible interference from movement or blinking.

• Random electrical variation.

The event-related response may therefore be difficult to identify in a single trial.

Researchers overcome this problem by repeating the event and averaging the recordings.

How ERPs are produced

ERPs are produced through a sequence of data collection and analysis.

Stage 1: Electrodes are placed on the scalp

The participant wears electrodes in the same general way as during an EEG recording.

Stage 2: A specific stimulus or event is presented

The event occurs at a precisely recorded time.

For example, a particular sound may be played.

Stage 3: Electrical activity is recorded

The EEG records the electrical activity occurring before, during and after the event.

Stage 4: The event is repeated

The same type of stimulus or event is presented many times.

Each trial creates a new section of EEG data.

Stage 5: The recordings are time-locked

The sections of EEG data are aligned according to the exact time at which the event occurred.

This is known as being time-locked to the event.

Stage 6: The recordings are averaged

Electrical activity from all the trials is averaged.

Background activity that varies randomly across trials is reduced, while activity that occurs consistently after the event remains visible.

Stage 7: The ERP waveform is produced

The resulting waveform shows the average electrical response associated with the event.

ERP production in one sequence

The complete process can be represented as:

Specific event presented repeatedly → EEG recorded for each trial → recordings aligned with the event → trials averaged → consistent event-related response identified

The key idea is that averaging separates the consistent response from less consistent background activity.

Why repeated trials are necessary

Suppose a researcher plays one sound and records the participant’s electrical brain activity.

The recording may include activity related to:

• The sound.

• A passing thought.

• A small movement.

• General alertness.

• Background neural activity.

It would be difficult to determine which part of the recording was specifically related to the sound.

If the sound is presented many times, the event-related response should occur at a similar time after each presentation.

Unrelated activity will be less consistent.

When the recordings are averaged:

• Consistent activity remains.

• Random activity is reduced.

This makes the event-related potential easier to identify.

Time-locking

Time-locking means aligning brain-activity recordings according to the moment a specific event occurred.

For example:

• Time 0 milliseconds: the stimulus appears.

• Activity immediately after the stimulus is recorded.

• Each trial is aligned at time 0.

• Activity across all trials is averaged.

This allows researchers to investigate the timing of electrical processes following the event.

Without precise time-locking, activity from different trials would not line up accurately.

Averaging

Averaging involves calculating the mean electrical response across repeated trials.

The purpose is not simply to create a more attractive waveform. Averaging helps separate:

• Activity consistently associated with the event.

• Activity that varies randomly between trials.

Suppose a particular electrical change repeatedly occurs shortly after a stimulus. When the trials are averaged, this consistent change remains visible.

A random change occurring at different times in different trials is more likely to cancel out.

What information does an ERP produce?

An ERP produces information about:

• The electrical response associated with a particular event.

• When that response occurs.

• The sequence of electrical changes following the event.

• The size of the electrical response.

• Differences between stimuli, tasks or groups.

Two important features are:

Feature	Meaning
Latency	The time between the event and an identified electrical response
Amplitude	The size of the electrical response

Latency helps researchers investigate the timing of information processing.

Amplitude may show that the size of the response differs between conditions, although its meaning depends on the design and process being investigated.

ERP waveforms

An ERP waveform shows averaged electrical activity in relation to a specific event.

The event is normally placed at a defined point on the time scale.

Researchers then examine changes occurring after the event.

For example, they may ask:

• How quickly did the electrical response appear?

• Was the response larger in one condition?

• Did the response occur in both groups?

• Did one type of stimulus produce a different waveform?

• Was the sequence of responses different?

ERPs are therefore useful for investigating the timing of cognitive processing.

ERPs and cognitive neuroscience

Cognitive processes cannot be observed directly.

Researchers can, however:

1. Present a controlled stimulus.

2. Record the participant’s behavioural response.

3. Measure electrical activity.

4. Extract the ERP related to the stimulus.

5. Compare the waveform between conditions.

6. Make an inference about the timing of cognitive processing.

This connects biological activity with cognitive tasks.

The wider relationship between biological and cognitive explanations is developed in linking mental processes with brain activity.

An example of an ERP investigation

Imagine that researchers want to investigate responses to familiar and unfamiliar words.

Participants are shown many words while their brain activity is recorded.

The procedure could involve:

1. Presenting a familiar or unfamiliar word.

2. Recording the EEG.

3. Repeating both types of word many times.

4. Aligning each recording with the moment the word appeared.

5. Averaging familiar-word trials separately from unfamiliar-word trials.

6. Comparing the resulting ERP waveforms.

Suppose the waveforms differ in latency or amplitude.

The researcher may infer that the brain processes familiar and unfamiliar words differently.

However, the ERP does not reveal exactly what the participant was thinking.

Observation and inference

ERPs provide objective electrical measurements, but psychological meaning must still be inferred.

Directly recorded	Inferred
A voltage change occurred	A cognitive process may have taken place
The response occurred after a specified delay	A stage of processing may have occurred at that time
One condition produced a larger waveform	The task may have required different processing
Two groups produced different patterns	Their processing may differ

The researcher should not move from an ERP difference to a psychological conclusion without considering:

• The task.

• The comparison condition.

• Behavioural performance.

• Alternative explanations.

• The quality of the recording.

Comparing EEGs and ERPs

EEGs and ERPs are closely related methods.

Both use scalp electrodes to measure electrical activity, but they produce different types of information.

Feature	EEG	ERP
Full term	Electroencephalogram	Event-related potential
Measurement	Electrical brain activity	Electrical activity associated with a particular event
Output	Continuous brainwave recording	Averaged waveform time-locked to an event
Focus	General patterns of activity	Specific responses to a stimulus or task
Repeated trials	Not always necessary	Usually necessary
Data processing	Can be examined as an overall record	Requires extraction and averaging
Main value	Broad activity patterns and changes in state	Timing of event-specific processing

Central similarity

Both methods:

• Record electrical activity.

• Use electrodes placed on the scalp.

• Measure the living brain.

• Are non-invasive.

• Have high temporal resolution.

• Have relatively poor spatial resolution.

• Can be affected by movement and other sources of electrical interference.

An ERP is produced from EEG data, so the methods are not completely independent.

Central difference

An EEG provides a general record of ongoing brain activity.

An ERP identifies a response linked to a specific event by averaging repeated sections of EEG data.

A direct comparison could state:

EEG provides a continuous record of overall electrical activity, whereas an ERP is an averaged electrical response time-locked to a particular stimulus or event.

EEGs and ERPs compared with fMRI

EEGs and ERPs measure electrical activity, whereas fMRI measures changes in blood oxygenation associated with neural activity.

Feature	EEGs and ERPs	fMRI
Main measurement	Electrical activity	Blood oxygenation
Temporal resolution	High	Relatively low
Spatial resolution	Relatively low	High
Output	Electrical waveform	Brain-activity map
Main strength	Shows when activity occurs	Shows where activity occurs
Main limitation	Difficulty locating the precise source	Delay between neural activity and blood-flow response

This creates a useful memory rule:

• EEGs and ERPs are stronger for when.

• fMRI is stronger for where.

The strengths and limitations of the scanning method can be reviewed in blood oxygenation and brain-activity maps.

Strength of EEG: high temporal resolution

Temporal resolution refers to the ability to identify when activity occurs.

EEG has very high temporal resolution because electrical activity can be measured almost immediately as it changes.

This allows researchers to:

• Track rapid changes in brain activity.

• Compare activity before and after an event.

• Identify changes occurring over very short time periods.

• Examine the sequence of electrical processes.

This is a major advantage over methods based on slower biological changes, such as blood flow.

Why temporal resolution matters

Many psychological processes occur rapidly.

For example:

• Detecting a stimulus.

• Recognising information.

• Shifting attention.

• Selecting a response.

• Making a simple decision.

A method with high temporal resolution can help researchers distinguish the order in which electrical changes occur.

However, knowing exactly when activity occurred does not necessarily show exactly where it originated.

Strength of EEG: records real-time activity

EEG records brain activity as it happens.

A participant can be monitored while:

• Awake.

• Asleep.

• Completing a task.

• Responding to a stimulus.

• Experiencing unusual electrical activity.

This provides information about the functioning of the living brain rather than relying only on evidence collected after an event or after death.

Strength of EEG: clinical usefulness

EEGs can identify unusual patterns in general electrical activity.

This makes the technique useful in clinical contexts, including the investigation of:

• Epileptic activity.

• Sleep-related patterns.

• Other disturbances in brain-wave activity.

This practical usefulness supports the value of EEG beyond laboratory research.

However, identifying an unusual pattern does not by itself explain why it developed.

Strength of ERP: event specificity

A major advantage of ERPs is that they provide more specific information than a raw EEG.

By time-locking and averaging responses, researchers can isolate electrical activity associated with a particular event.

This allows them to investigate:

• Responses to particular stimuli.

• Differences between cognitive tasks.

• The timing of information processing.

• Differences between participant groups.

• Changes produced by experimental conditions.

ERPs therefore improve the specificity of information obtained from EEG recordings.

Strength of ERP: precise timing

ERPs retain the high temporal resolution of EEG.

Researchers can identify the latency of an electrical response after a stimulus.

This may help distinguish stages within a rapid cognitive process.

For example, a researcher might investigate whether a difference between two conditions appears:

• Very soon after stimulus presentation.

• Later during interpretation.

• Close to the participant’s response.

The ERP cannot directly label the mental process, but its timing can help researchers test cognitive explanations.

Strength: non-invasive methods

Both EEGs and ERPs are non-invasive.

Electrodes are placed on the scalp rather than inserted into the brain.

This means:

• Surgery is not required.

• Brain tissue is not damaged.

• Participants may be studied repeatedly.

• The same participant can complete different conditions.

• Ethical and physical risks are relatively low.

Participants must still provide informed consent and be protected from discomfort or distress.

Strength: relatively economical and accessible

EEG equipment is generally less expensive and more accessible than an fMRI scanner.

It does not require:

• A large magnetic scanner.

• A purpose-built scanning room of the same kind.

• The same level of operating cost.

• Participants to lie inside a confined machine.

This may make:

• Larger samples possible.

• Repeated testing more practical.

• Replication easier.

• Research with a wider range of participants more manageable.

Cost alone does not guarantee that a study will produce valid data, but it can affect the feasibility of research.

Strength: participants can perform a wider range of tasks

Participants undergoing EEG recording are not required to lie inside a scanner.

Depending on the investigation, they may be able to:

• Sit upright.

• View material on a screen.

• Listen to sounds.

• Give responses.

• Complete tasks in a less restrictive position.

This may make some tasks more natural than those completed inside an fMRI scanner.

Movement must still be limited because muscular activity can interfere with the recording.

Limitation: poor spatial resolution

A major limitation of EEGs and ERPs is relatively poor spatial resolution.

Spatial resolution refers to the ability to identify where activity originates.

Scalp electrodes detect combined electrical activity after it has passed through:

• Brain tissue.

• The skull.

• The scalp.

This makes it difficult to identify the precise brain structure producing the signal.

Researchers may be able to estimate a general area, but the location is less precise than with fMRI.

Why poor spatial resolution matters

Suppose an EEG shows a change in electrical activity during a language task.

The recording may demonstrate when the activity changed, but it may not identify precisely which language area produced it.

This limits the method’s usefulness when the main research question concerns exact localisation.

Research into the functions of particular brain regions may therefore benefit from combining EEG or ERP evidence with methods offering better spatial resolution.

Limitation: activity is summed across many neurons

EEGs record the combined activity of large groups of neurons.

They do not isolate:

• One individual neuron.

• One synapse.

• One specific neurotransmitter.

• One exact brain structure.

The waveform represents the combined electrical signal detected at the scalp.

This means a similar EEG pattern could potentially result from different underlying neural processes.

Limitation: electrical interference and artefacts

EEG recordings can be affected by electrical activity that does not come from the target brain process.

Possible sources include:

• Eye movements.

• Blinking.

• Facial muscles.

• Head movement.

• Other muscular activity.

• Electrical equipment.

• Poor contact between an electrode and the scalp.

These unwanted influences are called artefacts.

Artefacts may distort the recording or create a pattern that could be mistaken for brain activity.

Researchers must identify, control or remove affected sections of data.

Movement artefacts

A participant who moves during an EEG recording may produce electrical changes associated with muscle activity.

This creates a practical problem:

• Natural behaviour may involve movement.

• Accurate recording often requires the participant to remain still.

• Restricting movement may make the task less realistic.

Researchers must balance accurate measurement with the ecological validity of the task.

Limitation of ERPs: small signal

The electrical response to one event is usually very small.

Researchers need many repeated trials to make the ERP visible.

This can lead to:

• Long testing sessions.

• Participant fatigue.

• Reduced concentration.

• Boredom.

• Changes in attention across the task.

• Habituation to repeated stimuli.

These factors may affect the response being measured.

Limitation of ERPs: averaging may hide variation

Averaging is necessary to identify the consistent event-related signal, but it can also remove information about differences between individual trials.

For example:

• A participant may use different strategies on different trials.

• Attention may vary.

• Some responses may be unusually strong or weak.

• The same stimulus may not be processed identically each time.

The average waveform provides a summary rather than a complete record of every response.

Limitation of ERPs: methodological variation

ERP results may depend on decisions made by researchers.

These include:

• Where electrodes are placed.

• How many trials are presented.

• Which trials are rejected.

• How artefacts are removed.

• Which time period is analysed.

• Which comparison condition is selected.

• How the waveform is interpreted.

Differences in procedure can make findings more difficult to compare across studies.

Clear and standardised methods improve consistency in psychological measurement.

Limitation: interpretation is still required

EEGs and ERPs produce objective electrical measurements, but the psychological meaning of the waveform must be interpreted.

For example, an ERP difference between two conditions might reflect:

• Attention.

• Recognition.

• Task difficulty.

• Decision-making.

• Preparation for a response.

• Another process not controlled by the researcher.

The researcher must use:

• An appropriate task.

• A suitable control condition.

• Behavioural data.

• Existing theory.

• Careful operationalisation.

An electrical difference does not automatically identify one specific cognitive process.

Limitation: artificial tasks

ERP investigations often require:

• Repeated presentation of the same type of stimulus.

• Precise timing.

• Restricted movement.

• Simple button-press responses.

• Long series of trials.

These procedures improve experimental control but may differ from everyday cognitive activity.

For example, repeatedly viewing isolated words on a screen is not identical to understanding language during a natural conversation.

This may reduce ecological validity.

Limitation: participant fatigue

ERP procedures may involve hundreds of trials.

As testing continues, participants may:

• Become tired.

• Lose concentration.

• Respond less carefully.

• Anticipate the stimulus.

• Become used to repeated material.

Changes in attention may alter the electrical response.

Researchers may need to use:

• Breaks.

• Shorter testing blocks.

• Counterbalancing.

• Clear instructions.

• Checks of behavioural performance.

Evaluating EEG validity

Validity concerns whether a method measures what it claims to measure.

EEG clearly records electrical activity, but several questions remain:

• Does the task isolate the intended psychological process?

• Is the recorded pattern from the brain rather than muscular movement?

• Does the activity represent the person’s ordinary behaviour?

• Does the researcher’s interpretation match the evidence?

• Is a broad electrical pattern being treated as more specific than it really is?

The main measurement is objective, but the psychological conclusion may still have limited validity.

The wider issue is developed in whether psychological research measures what it intends to measure.

Evaluating ERP validity

ERP validity depends heavily on the task and comparison condition.

For example, if researchers compare familiar and unfamiliar images, the conditions may differ in more than familiarity.

They might also differ in:

• Complexity.

• Emotional meaning.

• Visual features.

• Attention.

• Previous experience.

An ERP difference could therefore reflect another variable.

Good experimental control is needed before the difference can be attributed to the process named by the researcher.

Evaluating reliability

EEGs and ERPs may be reliable when researchers use:

• Standardised electrode placement.

• Consistent equipment.

• Clear task instructions.

• The same stimulus timing.

• Objective data-processing procedures.

• Appropriate artefact removal.

However, electrical recordings may vary because of:

• Participant movement.

• Changes in attention.

• Individual differences.

• Poor electrode contact.

• Different analytical decisions.

Repeated measurements and replication help establish whether a pattern is consistent.

EEGs, ERPs and localisation

EEGs and ERPs can suggest that electrical activity occurred during a task, but they are less precise at identifying the exact source.

This means they are particularly useful for studying the timing of brain processes.

They are less useful when the main aim is to locate activity precisely within a small brain area.

Researchers may combine methods:

• EEG or ERPs provide information about timing.

• fMRI provides more precise information about location.

• Post-mortem examinations provide detailed evidence about structure.

Different methods can therefore provide complementary information.

Applying EEG to an unfamiliar scenario

Consider this study:

Researchers record participants’ brain activity throughout a night of sleep. They identify changing patterns of electrical waves at different times.

This describes an EEG because:

• Electrical activity is recorded continuously.

• The focus is on broad patterns over time.

• The recording is not limited to one repeated stimulus.

• The method can identify changes between states.

The researchers could describe when brain-wave patterns changed, but the EEG would provide limited information about their precise source within the brain.

Applying ERPs to an unfamiliar scenario

Consider another study:

Participants hear a frequent tone and an occasional unexpected tone. Each type of tone is presented many times while electrical activity is recorded.

An ERP investigation would involve:

1. Recording the EEG for every tone.

2. Marking the exact moment each tone occurred.

3. Separating frequent-tone and unexpected-tone trials.

4. Aligning the recordings with tone onset.

5. Averaging each set of trials.

6. Comparing the resulting ERP waveforms.

A difference in latency or amplitude may suggest that the two tones were processed differently.

A method for identifying EEG or ERP

Ask the following questions.

Is electrical activity being recorded continuously?

This suggests an EEG.

Is the researcher examining general brain-wave patterns?

This suggests an EEG.

Is the activity linked to one specific event or stimulus?

This suggests an ERP.

Is the event repeated many times?

This strongly suggests an ERP.

Are the recordings time-locked and averaged?

This identifies an ERP.

A method for answering comparison questions

Use direct comparison points.

Measurement

Both methods measure electrical activity through scalp electrodes.

Focus

EEG records general ongoing activity, whereas ERP isolates a response associated with a particular event.

Production

An EEG is recorded continuously. An ERP is extracted by averaging repeated, time-locked EEG sections.

Temporal resolution

Both have high temporal resolution.

Spatial resolution

Both have relatively poor spatial resolution.

Practical demands

ERP research usually requires repeated trials and more complex data analysis.

Writing an effective comparison

A strong comparison might state:

Both EEGs and ERPs use scalp electrodes to measure electrical brain activity and both have high temporal resolution. However, an EEG produces a continuous record of general brain-wave activity, whereas an ERP is an averaged response time-locked to a specific stimulus. Producing an ERP requires repeated trials so that random background activity can be reduced. ERPs therefore provide more event-specific information, although both methods have relatively poor spatial resolution.

This answer:

• Gives a similarity.

• Gives several direct differences.

• Explains how ERPs are produced.

• Uses methodological terminology.

• Evaluates the information obtained.

Writing an effective evaluation paragraph

A developed evaluation paragraph should connect a feature with its effect on conclusions.

For example:

One strength of ERPs is their high temporal resolution. Researchers can identify electrical changes occurring very shortly after a stimulus, which helps them investigate the timing and sequence of cognitive processing. However, ERPs have relatively poor spatial resolution because scalp electrodes detect combined activity after it has passed through the skull. Researchers may therefore know when activity occurred without identifying its precise origin.

This paragraph balances the method’s main strength against its central limitation.

Key Words 🔑

Key word	Student-friendly definition	How it may be used in an exam
Electroencephalogram	A recording of electrical activity in the brain detected using scalp electrodes.	Explain how an EEG measures brain activity.
EEG	The abbreviation for electroencephalogram.	Use accurate terminology in short and extended answers.
Electrode	A sensor placed on the scalp to detect electrical activity.	Describe how EEG data are collected.
Electrical activity	Voltage changes produced by the combined activity of groups of neurons.	Identify what EEGs and ERPs measure.
Brain wave	A pattern of electrical activity shown in an EEG recording.	Describe the type of information produced by an EEG.
Frequency	How often waves occur within a period of time.	Describe a feature of an EEG waveform.
Amplitude	The size of an electrical change in a waveform.	Compare electrical responses between conditions.
Event-related potential	An averaged electrical response associated with a specific event or stimulus.	Explain how ERPs differ from general EEG recordings.
ERP	The abbreviation for event-related potential.	Use accurate terminology in examination answers.
Time-locking	Aligning recordings according to the exact time at which an event occurred.	Explain how ERPs are produced.
Averaging	Combining activity across repeated trials to identify a consistent response.	Explain how background activity is reduced.
Latency	The time between an event and an identified electrical response.	Explain the temporal information produced by an ERP.
Temporal resolution	The ability to identify when activity occurs.	Explain a major strength of EEGs and ERPs.
Spatial resolution	The ability to identify where activity occurs.	Explain a major limitation of EEGs and ERPs.
Artefact	Unwanted activity in a recording, such as interference caused by blinking or movement.	Evaluate the accuracy of EEG or ERP data.
Non-invasive	Not requiring surgery or instruments to be inserted into the brain.	Explain a practical or ethical strength.
Continuous recording	A record of activity collected throughout a period of time.	Distinguish EEG data from an event-specific ERP.
Cognitive inference	A conclusion about mental processing drawn from observable biological or behavioural evidence.	Explain why an ERP does not directly reveal thoughts.

Common Mistakes ⚠️

Mistake: Saying that an EEG directly records individual neurons firing.

Why this is incorrect:Scalp electrodes detect the combined electrical activity of groups of neurons.

How to improve:Describe EEG as a broad recording of electrical brain activity.

Mistake: Saying that EEG measures changes in blood oxygenation.

Why this is incorrect:Changes in blood oxygenation are measured by fMRI. EEG measures electrical activity.

How to improve:Remember: EEG measures electricity, fMRI measures the BOLD response.

Mistake: Describing an ERP as a completely separate scanning machine.

Why this is incorrect:ERPs are extracted from EEG recordings.

How to improve:Explain that EEG data are recorded, time-locked to an event and averaged.

Mistake: Saying one presentation of a stimulus is enough to produce a reliable ERP.

Why this is incorrect:The event-related response is small and may be hidden by background activity.

How to improve:State that the event is repeated and recordings are averaged.

Mistake: Explaining averaging as simply making the waveform larger.

Why this is incorrect:Averaging reduces inconsistent background activity while preserving activity that occurs consistently after the event.

How to improve:Link averaging to the separation of event-related and random activity.

Mistake: Saying that EEGs and ERPs have high spatial resolution.

Why this is incorrect:Scalp recordings cannot locate the exact origin of activity with the precision of fMRI.

How to improve:Remember: high temporal resolution, relatively poor spatial resolution.

Mistake: Confusing temporal and spatial resolution.

Why this is incorrect:Temporal resolution concerns timing, while spatial resolution concerns location.

How to improve:Use the questions: “When?” for temporal and “Where?” for spatial.

Mistake: Claiming that an ERP directly reveals a participant’s thoughts.

Why this is incorrect:An ERP records an electrical response. The cognitive process must be inferred from the task and pattern.

How to improve:Use cautious language such as “may indicate” or “is associated with”.

Mistake: Ignoring artefacts in EEG recordings.

Why this is incorrect:Eye movements, blinking and muscular activity can distort the electrical signal.

How to improve:Explain how artefacts may reduce the validity or reliability of the recording.

Mistake: Treating a larger ERP amplitude as automatically meaning better processing.

Why this is incorrect:The meaning of amplitude depends on the component, task and comparison being investigated.

How to improve:Describe the measured difference before making a carefully supported inference.

Mistake: Writing two separate descriptions when asked to compare EEGs and ERPs.

Why this is incorrect:A comparison requires direct similarities or differences.

How to improve:Use connecting language such as “both”, “whereas”, “unlike” and “in contrast”.

Exam-Style Questions ✍️

Question 1

Which one of the following best describes an EEG?

A. A measure of blood oxygenation in the brain

B. A continuous recording of electrical brain activity

C. An examination of the brain after death

D. A record of hormones released into the bloodstream

[1 mark]

Question 2

Define an event-related potential.

[2 marks]

Question 3

Explain how electrodes are used to produce an EEG recording.

[3 marks]

Question 4

Explain how an ERP is produced from EEG data.

[4 marks]

Question 5

Explain why repeated trials are needed when researchers produce an ERP.

[4 marks]

Question 6

State one similarity and one difference between EEGs and ERPs.

[4 marks]

Question 7

A researcher presents participants with the same sound many times. Electrical activity is recorded after every presentation and the recordings are then averaged.

Explain why this procedure is likely to produce an ERP rather than a general EEG measure.

[4 marks]

Question 8

A psychologist claims:

Because EEGs measure brain activity immediately, they can identify the precise brain structure responsible for a behaviour.

Explain why this conclusion is incorrect.

[4 marks]

Question 9

Researchers compare electrical responses to familiar and unfamiliar images.

Condition	Mean response latency	Mean response amplitude
Familiar images	280 milliseconds	8.4 microvolts
Unfamiliar images	340 milliseconds	6.1 microvolts

a) Identify which condition produced the shorter mean latency.

[1 mark]

b) Calculate the difference in mean response amplitude between the conditions.

[2 marks]

c) Explain one conclusion the researchers might draw from these results.

[2 marks]

Question 10

Compare EEGs and ERPs as ways of studying the brain.

Refer to the information they produce and their strengths and limitations in your answer.

[8 marks]