Cognitive Connie
What are brain imaging techniques?
For most of history, knowledge of the living human brain was limited to what could be inferred from behaviour, from postmortem examination, or from rare patients with localised brain damage. The development of non-invasive brain imaging methods in the second half of the 20th century transformed neuroscience, allowing researchers to observe brain structure and activity in healthy volunteers in real time.
Key figures
Hans Berger
1873–1941German psychiatrist who recorded the first human EEG in 1924, measuring electrical brain activity through the intact skull. Identified the alpha wave (8–12 Hz, present during relaxed wakefulness with eyes closed) and demonstrated that EEG activity changed with mental states. Founded the field of electroencephalography.
Seiji Ogawa
1934–presentPhysicist at Bell Labs who in 1990 described the BOLD (Blood Oxygen Level Dependent) effect — the principle underlying fMRI. Demonstrated that deoxyhaemoglobin is paramagnetic and creates measurable MRI contrast, providing the basis for all subsequent functional MRI research.
Anthony Barker
1950–presentBritish physicist who developed the first practical transcranial magnetic stimulation (TMS) device in 1985, initially used to non-invasively stimulate the motor cortex and assess corticospinal conduction. TMS subsequently became a fundamental cognitive neuroscience tool for establishing causal brain-behaviour relationships.
Key concepts
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").
Test your knowledge
Frequently asked questions
What does fMRI actually measure, and why is this a limitation?+
fMRI does not directly measure neuronal firing. It measures the BOLD (Blood Oxygen Level Dependent) signal: when neurons become active, they demand more oxygen, triggering an increase in local blood flow ("neurovascular coupling") that delivers oxygenated haemoglobin to active regions. Deoxyhaemoglobin (which is paramagnetic) decreases; this changes the MRI signal, creating the BOLD response. This is a significant limitation for three reasons: 1. Temporal blurring: the haemodynamic response lags neural activity by 2–6 seconds and takes 10–12 seconds to return to baseline, making it impossible to track rapid neural dynamics. 2. Indirect measure: what causes the BOLD signal is debated. It likely reflects synaptic input to a region (including inhibitory inputs) rather than spiking output, meaning active suppression of neurons can produce a BOLD response. 3. Correlational: fMRI shows which regions are more active during a task vs a baseline condition. It cannot establish that a region is causally necessary — only TMS, lesion studies, or targeted interventions can do that.
Why do researchers use multiple imaging methods together?+
No single imaging method provides complete information, so combining methods is often essential. The key trade-off is between spatial resolution (where in the brain?) and temporal resolution (when in time?). fMRI has excellent spatial resolution (~2 mm) but poor temporal resolution (seconds). EEG/MEG have excellent temporal resolution (milliseconds) but poor/moderate spatial resolution. Combined EEG-fMRI simultaneously records both, providing temporal and spatial information — though technical challenges arise from the strong MRI magnetic field interfering with EEG electrodes. Similarly, TMS combined with EEG or fMRI allows causal disruption of a region while recording the resulting network effects. MEG + MRI (structural) is used in epilepsy localisation. PET remains essential for receptor density mapping that fMRI cannot provide. The convergence of multiple methods across multiple studies — triangulation — is the gold standard for establishing robust brain-behaviour relationships.
Sources
Last reviewed July 2025- 1.
Ogawa, S., Lee, T. M., Kay, A. R., & Tank, D. W. (1990). Brain magnetic resonance imaging with contrast dependent on blood oxygenation. Proceedings of the National Academy of Sciences, 87(24), 9868–9872. https://doi.org/10.1073/pnas.87.24.9868
+About this source
Original paper describing the BOLD effect — the blood oxygenation contrast that underlies all fMRI research.
- 2.
Logothetis, N. K. (2008). What we can do and what we cannot do with fMRI. Nature, 453(7197), 869–878. https://doi.org/10.1038/nature06976
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Critical analysis of fMRI strengths and limitations, including the relationship between BOLD signal and neural activity.
- 3.
Luck, S. J. (2014). An Introduction to the Event-Related Potential Technique (2nd ed.). MIT Press.
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Comprehensive guide to EEG/ERP methodology including temporal resolution advantages, spatial limitations, and major ERP components.