Ways of Studying the Brain

Psychologists want to link behaviour and mental processes to activity in specific regions of the brain. To do that they need methods that reveal what the brain is doing, either while a person is alive and completing a task or by examining its physical structure afterwards.

The AQA specification names four techniques you must be able to describe and evaluate:

Technique	Studies a...	Living or dead brain?
fMRI	Living brain	Living
EEG	Living brain	Living
ERPs	Living brain	Living
Post-mortem examination	Structure of the brain	After death

Two ideas run through the whole topic and decide most of the marks: spatial resolution (how precisely a technique locates where activity happens) and temporal resolution (how precisely it captures when activity happens). Keep these two words separate in your mind, because the strengths and limitations of every technique come back to them.

Spatial resolution answers where; temporal resolution answers when. A technique can be excellent at one and poor at the other.

fMRI measures brain activity indirectly, by detecting changes in blood flow and oxygenation rather than measuring neural firing itself. When a brain region becomes more active, its neurons consume more oxygen, so oxygenated blood flows towards that area. This increase in blood flow to active regions is called the haemodynamic response.

The scanner detects the difference between oxygenated and deoxygenated blood and builds a dynamic 3D map showing which regions are active while a person performs a task, such as reading or recognising faces.

Strengths:

• Non-invasive — it does not involve inserting anything into the body and uses no radiation (unlike PET scans).
• High spatial resolution — it can localise activity to within a few millimetres, making it strong at showing where activity occurs.

Limitations:

• Poor temporal resolution — there is roughly a 5-second lag between neural activity and the blood-flow change it detects, so it cannot capture the exact moment of activity.
• It measures blood flow, which is only an indirect proxy for neural activity — it does not record the firing of neurons directly.
• It is expensive to run and requires the person to stay completely still, or the image is distorted.

fMRI does not measure electrical activity. It infers activity from the haemodynamic response — changes in blood flow and oxygen use.

EEG measures the brain's electrical activity directly, using a cap of electrodes attached to the scalp. Each electrode picks up the combined electrical signals from the thousands of neurons beneath it, so the output is a record of general brainwave patterns rather than the activity of any single region.

EEG is widely used to study stages of sleep and to diagnose conditions such as epilepsy, which produces a distinctive pattern of bursts in the recording.

Strengths:

• Very high temporal resolution — it records activity in real time, accurate to the millisecond, so it captures exactly when activity changes.
• It is inexpensive compared with fMRI, making it practical for research and clinical use.

Limitations:

• Poor spatial resolution — because each electrode reads a broad area, EEG cannot pinpoint the exact source of the activity within the brain.
• It detects only general or surface-level activity, so it is not suited to isolating the response of a single deep structure.

EEG is the mirror image of fMRI: excellent temporal resolution, poor spatial resolution. It tells you precisely when but not exactly where.

Event-related potentials (ERPs) are specific electrical responses to a particular stimulus — for example, the brain's response each time a target word appears on a screen. The problem is that this specific response is buried within all the other electrical activity an EEG records.

To isolate it, researchers present the same stimulus many times and average the recordings across all the trials. Random background activity is different on every trial, so averaging cancels it out, while the response that is genuinely tied to the stimulus appears the same each time and therefore survives the averaging.

Strengths:

• High temporal resolution — like EEG, ERPs are accurate to the millisecond.
• They are more specific than raw EEG, allowing researchers to measure the brain's response to a precise cognitive event.

Limitations:

• They require a large number of trials to produce a clear signal, which is time-consuming.
• Background noise is difficult to eliminate completely, so the extraction is never perfect.
• There is a lack of standardisation in ERP methods across studies, which makes it hard to compare or replicate findings.

ERPs are derived from EEG data. You cannot record an ERP without first recording the EEG and then averaging over many trials of the same stimulus.

A post-mortem examination studies a person's brain after they have died. Historically these examinations are carried out on people who showed an unusual behaviour or a rare deficit while alive, so that researchers can look for abnormalities or damage in the brain that might explain it.

Post-mortems are foundational to biopsychology. Broca and Wernicke identified the brain's language areas by examining the brains of patients who had lost specific language abilities, and the amnesiac patient HM was studied after death to link his memory loss to damage in the hippocampus.

Strengths:

• They allow detailed examination of deep brain structures — such as the hippocampus — that scanning techniques struggle to isolate.
• They were historically foundational, providing the early evidence for localisation of function.

Limitations:

• Causation is a serious problem — an observed deficit may have had another cause, and damage may have occurred during or after death rather than being the reason for the behaviour.
• There is no possibility of follow-up, since the person has died, so findings cannot be checked against later behaviour.
• Ethical issues around consent arise, because patients such as HM could not give informed consent for the later study of their brain.

A post-mortem can show a correlation between brain damage and a behaviour observed in life, but it cannot on its own prove the damage caused the behaviour.

The examiner often asks you to compare techniques, so learn this table as a single unit. Read it across each row: notice how the two scanning approaches trade spatial resolution against temporal resolution.

Technique	What it measures	Spatial resolution	Temporal resolution	Invasive?
fMRI	Blood flow and oxygenation (haemodynamic response)	High	Poor (~5s lag)	Non-invasive
EEG	General electrical activity via scalp electrodes	Poor	Very high (ms)	Non-invasive
ERPs	Electrical response to a specific stimulus	Poor	Very high (ms)	Non-invasive
Post-mortem	Physical brain structure after death	High (direct)	Not applicable	Invasive (after death)

A useful way to hold this together:

• Need to know where activity happens? fMRI gives the best spatial resolution of the living-brain methods.
• Need to know when activity happens? EEG and ERPs give millisecond timing.
• Need to examine deep structure in detail? A post-mortem lets you look directly, at the cost of losing any link to living behaviour.

None of the four is "best" outright. The right technique depends on whether the research question is about location, timing, or structure — that judgement is exactly what an evaluation question rewards.

1. Confusing spatial resolution with temporal resolution

Spatial resolution is about where activity occurs; temporal resolution is about when. Mixing these up reverses the strengths of every technique. Say the words out loud when you revise: spatial = space = where, temporal = time = when.

2. Saying fMRI measures electrical activity

fMRI measures blood flow and oxygenation — the haemodynamic response — not electrical activity. The techniques that measure electrical activity are EEG and ERPs. This is one of the most common errors on this topic.

3. Claiming EEG can localise activity precisely

EEG has poor spatial resolution and cannot pinpoint the exact source of activity. Crediting it with precise localisation contradicts its main limitation. Its strength is temporal resolution, not spatial.

4. Forgetting that ERPs are derived from EEG

ERPs are not a separate recording device. They are extracted from EEG data by averaging many trials of the same stimulus to filter out background noise. An answer that treats them as unrelated to EEG misses how they are produced.

5. Treating post-mortems as able to prove causation

A post-mortem can reveal a correlation between brain damage and a behaviour, but the damage may have had another cause, or may have occurred during or after death. Writing that a post-mortem proves a structure caused a behaviour overstates what the method can show.

6. Listing strengths and limitations with no comparison

Higher-mark evaluation questions reward comparing techniques, not just listing facts about one. Contrast the spatial–temporal trade-off between fMRI and EEG, and weigh the access to deep structure that post-mortems give against their causation problems.