Brain Imaging

The ability to observe the structure and activity of the human brain without surgery transformed cognitive neuroscience in the latter half of the twentieth century. Brain imaging techniques range from purely structural (MRI, CT) to functional (fMRI, PET, EEG, MEG), and from purely observational (correlational) to causal (TMS, tDCS). Each technique involves fundamental trade-offs — particularly between temporal resolution (how quickly changes can be detected) and spatial resolution (how precisely they can be located) — that determine which research questions each method can answer.

Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radiofrequency pulses to produce detailed images of brain anatomy, using the magnetic properties of hydrogen atoms in water. Structural MRI reveals grey and white matter volume, cortical thickness, and anatomical anomalies. Diffusion tensor imaging (DTI) tracks water diffusion along white matter tracts, revealing structural connectivity. Functional MRI (fMRI), developed in the early 1990s following Ogawa et al.'s (1990) description of the blood-oxygenation-level-dependent (BOLD) contrast, measures the haemodynamic response to neural activity: activated neurons increase local oxygen consumption; the resulting influx of oxygenated haemoglobin (which is diamagnetic, unlike deoxygenated haemoglobin which is paramagnetic) changes the local T2* signal. This indirect proxy for neural activity has high spatial resolution (~1–3 mm³) but poor temporal resolution (~2–6 seconds) due to the sluggish haemodynamic response.

Electroencephalography (EEG), first applied to humans by Hans Berger in 1929, records electrical potentials from populations of synchronously active cortical neurons via electrodes placed on the scalp. Its defining advantage is millisecond temporal resolution — adequate to track the rapid dynamics of cognitive processing. Its limitation is poor spatial resolution: electrical signals are smeared by skull and scalp (the "inverse problem" of source localisation). Event-related potentials (ERPs) are EEG signals time-locked to specific events (stimuli or responses) and averaged across trials to reveal component waveforms (N400, P300, MMN) that index specific cognitive processes. Magnetoencephalography (MEG) measures the tiny magnetic fields generated by synchronous neural currents using superconducting quantum interference devices (SQUIDs). Magnetic fields are not distorted by the skull, giving MEG better spatial resolution than EEG with similarly excellent temporal resolution; but MEG is expensive (~£3 million for a system) and requires magnetic shielding.

Transcranial magnetic stimulation (TMS) is a technique for establishing causal relationships rather than correlations. A rapidly changing magnetic field induces electrical currents in superficial cortical regions, temporarily disrupting local neural processing — creating a "virtual lesion." By applying TMS while participants perform cognitive tasks, researchers can ask: is this specific region necessary for this function? Single-pulse TMS can time-stamp the role of a region within milliseconds; repetitive TMS (rTMS) produces longer-lasting changes in cortical excitability. PET (positron emission tomography) uses radioactively labelled compounds to measure cerebral blood flow, glucose metabolism, or neurotransmitter receptor binding — it was the dominant functional imaging technique before fMRI but is now used primarily for neurochemical imaging.