Brain Scanning Techniques

Ways of Studying the Brain

Understanding the relationship between brain structure or activity and psychological function requires specialised methods. Researchers have developed several complementary techniques — each with distinctive strengths and limitations — for studying the living brain and for examining post-mortem tissue. No single technique provides a complete picture; converging evidence from multiple methods is the gold standard.

Functional Magnetic Resonance Imaging (fMRI)

fMRI measures brain activity indirectly by detecting changes in blood oxygenation. Active neurons consume more oxygen, increasing blood flow to the active region (the haemodynamic response). fMRI detects the magnetic difference between oxygenated and deoxygenated haemoglobin — producing a BOLD signal (Blood Oxygen Level Dependent) — and generates a spatial map of activity across the whole brain with millimetre resolution.

Strengths: excellent spatial resolution (can localise activity to within ~1–2 mm); non-invasive and safe (no radiation); can image the whole brain. Limitations: poor temporal resolution — the haemodynamic response lags neural activity by 4–6 seconds, so fMRI cannot capture rapid neural events accurately. It also measures blood flow, not neural firing directly, and participants must remain very still in a noisy, claustrophobic scanner — artificial conditions that may not reflect real-world cognition. Expensive and requires specialist facilities.

Electroencephalography (EEG)

EEG records the summed electrical activity of large populations of neurons using electrodes placed on the scalp. It detects brainwaves — rhythmic oscillations in the electrical signal — that differ in frequency and amplitude across different states of consciousness (e.g. alpha waves during relaxed wakefulness, delta waves during deep sleep).

Strengths: excellent temporal resolution (millisecond precision) — captures the timing of neural events far better than fMRI; relatively inexpensive; participants can move naturally. Limitations: very poor spatial resolution — electrodes on the scalp cannot precisely localise the source of electrical activity within the brain. EEG detects activity from large brain regions rather than specific structures, and is particularly poor at detecting deep brain activity. Signal is susceptible to electrical artefacts (muscle movement, eye blinks).

Event-Related Potentials (ERPs)

ERPs are a refinement of EEG. By presenting a stimulus repeatedly and averaging the EEG signal across many trials, random neural noise is cancelled out, revealing the specific electrical response time-locked to the stimulus. ERPs provide precise information about the timing and sequence of neural processing stages — for example, the P300 component (a positive deflection approximately 300 ms after a rare, significant stimulus) is a well-studied index of attentional processing and decision-making.

ERPs inherit EEG's excellent temporal resolution and add greater specificity by isolating stimulus-evoked activity, but share the same poor spatial resolution. They are widely used in cognitive neuroscience and clinical assessment (e.g. detecting levels of awareness in non-responsive patients).

Post-mortem Studies

Post-mortem (PM) examination involves examining brain tissue after death — allowing detailed analysis of brain structure, cellular composition, and histological changes not visible in living brains. By comparing the brains of individuals who showed specific behavioural or cognitive characteristics during life with those who did not, researchers can identify structural correlates of function.

The classic examples are Broca's and Wernicke's cases: examination of the brains of patients who had shown specific language deficits revealed localised lesions — in the left inferior frontal gyrus (Broca, 1861) and left superior temporal gyrus (Wernicke, 1874) respectively — providing foundational evidence for language localisation. Strengths: allows histological (cellular-level) analysis impossible in living brains; no time pressure during examination. Limitations: retrospective and correlational — behaviour must be assessed during life and the brain examined later; the tissue may have changed between death and examination; causation cannot be established (lesions may be consequences rather than causes of behavioural differences); small samples limit generalisation.

Comparing the Techniques