- Structural MRI
- Magnetic resonance imaging that produces high-resolution three-dimensional images of brain anatomy. Uses the magnetic properties of hydrogen nuclei (in water and fat) to generate contrast between different tissue types: grey matter (neuronal cell bodies), white matter (myelinated axons), and cerebrospinal fluid. Spatial resolution is excellent (~1 mm). Used to detect tumours, lesions, atrophy, and structural abnormalities. Does not measure brain activity.
- Functional MRI (fMRI)
- Measures brain activity indirectly via the BOLD (Blood Oxygen Level Dependent) signal — changes in the ratio of oxygenated to deoxygenated haemoglobin that occur when neurons fire and demand more oxygen. Spatial resolution is good (~2–3 mm) but temporal resolution is poor (~2–6 seconds, limited by the sluggish haemodynamic response). The most widely used method in cognitive neuroscience. Correlational: it shows which regions are more active during a task, not whether they are necessary for that task.
- Electroencephalography (EEG)
- Records electrical activity (voltage fluctuations) generated by synchronised postsynaptic potentials in pyramidal neurons using electrodes placed on the scalp. Excellent temporal resolution (milliseconds); poor spatial resolution (~centimetres), as the skull and scalp blur the signal. Cannot reliably locate sources deep in the brain. Used to study oscillatory rhythms (alpha, beta, theta, gamma waves), sleep stages, epilepsy, and event-related potentials (ERPs).
- Event-Related Potentials (ERPs)
- Averaging many EEG recordings time-locked to a stimulus to extract the brain's consistent electrical response to that stimulus, cancelling out random background noise. Different ERP components (N100, P300, N400, MMN) reflect specific cognitive processes: N100 = early auditory/visual detection; P300 = attention and target detection; N400 = semantic incongruity; MMN (mismatch negativity) = automatic detection of deviant stimuli. ERPs provide millisecond-precision evidence about cognitive processes.
- Magnetoencephalography (MEG)
- Measures the tiny magnetic fields generated by neuronal electrical activity using arrays of superconducting quantum interference devices (SQUIDs) in a magnetically shielded room. Like EEG, MEG has excellent temporal resolution (milliseconds); unlike EEG, magnetic fields are not distorted by the skull, giving somewhat better spatial resolution. Expensive and rare; primarily used in epilepsy presurgical mapping and high-resolution cognitive neuroscience research.
- Positron Emission Tomography (PET)
- A nuclear medicine imaging technique that uses radioactive tracers injected into the bloodstream. The tracer emits positrons that annihilate with electrons to produce gamma rays detected by the scanner. Depending on the tracer, PET can measure blood flow (H₂¹⁵O), glucose metabolism (FDG), dopamine receptor density, or amyloid plaques (amyloid PET for Alzheimer's diagnosis). Good spatial resolution but poor temporal resolution, radiation exposure, and requires a cyclotron on site. Largely replaced by fMRI for functional studies but still essential for receptor imaging and metabolic studies.
- Transcranial Magnetic Stimulation (TMS)
- Delivers a rapidly changing magnetic field through a coil held above the scalp, inducing electrical currents in underlying cortical neurons. Can be used to temporarily disrupt a cortical region ("virtual lesion") to test whether it is causally necessary for a task, or to facilitate activity. Essential methodological complement to fMRI (which is correlational): TMS can establish causal necessity. Repetitive TMS (rTMS) has therapeutic applications in depression (left prefrontal stimulation). Single-pulse TMS can suppress visual cortex activity (causing phosphenes or suppression).
- Diffusion tensor imaging (DTI)
- An MRI technique that measures the direction and degree of water diffusion in white matter tracts, allowing reconstruction of the major fibre pathways (tractography). Water diffuses preferentially along axons (anisotropic diffusion), so DTI can map connections between brain regions. Used to study white matter integrity (disrupted in traumatic brain injury, multiple sclerosis, and schizophrenia) and to produce maps of structural connectivity (the "structural connectome").