EXAM MEG

1. Basic principles (no formula)

Magnetoencephalography (MEG) is a functional neuroimaging technique that measures the magnetic fields generated by neural activity in the brain. These magnetic fields arise primarily from synchronous postsynaptic currents in large populations of cortical pyramidal neurons.

MEG does not measure single neurons or action potentials. Action potentials are too brief and not synchronised enough to produce a detectable external magnetic field. Instead, MEG reflects summed postsynaptic potentials, which last longer (tens of milliseconds) and can synchronise across thousands of neurons.

The signal detected by MEG is strongest when neurons are:

  • spatially aligned

  • temporally synchronised

  • close to the sensors

Because magnetic fields pass through skull and scalp with minimal distortion, MEG provides a direct and temporally precise measure of brain activity.

MEG is functional, not structural. Anatomical localisation requires a structural MRI for source modelling.

2. Terminology (must know)

Postsynaptic potentials
Slow electrical currents generated when neurons receive input; main contributor to MEG signals.

Pyramidal neurons
Large cortical neurons aligned perpendicular to the cortical surface; their geometry allows magnetic fields to sum.

Evoked responses (ERFs)
Time-locked and phase-locked responses to stimuli; visible after trial averaging.

Induced responses
Task-related activity not phase-locked to stimulus onset; revealed using time–frequency analysis.

Oscillations
Rhythmic activity of neural populations, grouped into frequency bands (delta, theta, alpha, beta, gamma).

Source space vs sensor space
Sensor space refers to signals recorded at the sensors.
Source space refers to estimated brain locations after modelling.

Inverse problem
The problem that multiple brain source configurations can explain the same sensor data; source localisation is therefore model-based.

Beamforming
A source localisation technique that estimates activity at specific brain locations while suppressing others.

3. Applications and interpretation in clinical practice

Epilepsy (key clinical use)

MEG is used to:

  • localise epileptogenic foci

  • identify interictal spikes

  • guide presurgical planning

MEG is particularly valuable when:

  • MRI appears normal

  • EEG localisation is unclear

  • surgery risks eloquent cortex (language, motor areas)

Concordance between MEG findings and clinical outcomes is associated with better post-surgical seizure control.


Presurgical functional mapping

MEG maps:

  • motor cortex

  • somatosensory cortex

  • language networks

This helps surgeons avoid functionally critical regions during resection.

Research and emerging clinical roles

  • Neurodevelopmental disorders

  • Dementia subtyping

  • Subtle functional changes before behavioural deficits

  • Lifespan brain development

Interpretation must always consider task design, developmental stage, and source modelling assumptions.

4. Normal versus abnormal findings

Normal MEG findings

  • Strong alpha rhythms in occipital cortex at rest

  • Beta rhythms in motor cortex at rest

  • Event-related responses with expected timing and lateralisation

  • Age-appropriate oscillatory frequencies

Abnormal MEG findings

  • Delayed evoked responses

  • Reduced or excessive synchrony

  • Abnormal lateralisation (e.g. bilateral language representation)

  • Focal high-frequency activity (especially relevant in epilepsy)

  • Slowed dominant frequencies (e.g. in neurodegeneration)

Important:
Abnormal MEG does not always mean structural abnormality. Functional changes can occur before MRI abnormalities appear.

5. Key developmental features and relevance to diagnosis

Brain oscillations and functional networks change across development.

Key developmental points:

  • Dominant frequencies increase with age (e.g. alpha slows in infancy, speeds up with maturation)

  • Functional networks become more specialised and lateralised over childhood

  • Prefrontal and associative networks mature later than sensory systems

Clinical relevance:

  • Reduced activation may reflect immaturity, not pathology

  • Atypical lateralisation may indicate neurodevelopmental risk

  • MEG is especially valuable in:

    • infants

    • non-verbal children

    • developmental disorders

Interpretation must always be age-referenced, not adult-norm referenced.

6. Strengths and limitations

Strengths

MEG:

  • is non-invasive

  • directly measures neural activity

  • has millisecond temporal resolution

  • has good spatial localisation

  • is minimally distorted by skull and scalp

  • is well suited for epilepsy and cognitive timing studies

Limitations

MEG:

  • is less sensitive to deep brain structures

  • requires magnetically shielded rooms

  • is expensive and limited in availability

  • relies on model-based source localisation

  • is sensitive to environmental and movement artefacts

High-yield exam takeaway sentence

MEG is a non-invasive functional neuroimaging technique that directly measures synchronised postsynaptic neural activity with high temporal resolution, making it particularly valuable for epilepsy localisation, developmental neuroscience, and the study of fast cognitive processes, while interpretation must account for modelling assumptions and developmental stage.


What is OPM-MEG and how does it work?

Optically Pumped Magnetometers (OPMs) are a newer type of sensor used in MEG to measure the magnetic fields generated by neural activity in the brain.

OPMs work using laser light and atomic vapour (commonly rubidium). The magnetic field produced by brain activity changes the energy state of the atoms, which alters how much laser light passes through the vapour. This change is detected and converted into a magnetic field signal.

Unlike conventional MEG sensors, OPMs do not require cryogenic cooling and can be placed directly on the scalp, allowing the sensors to sit much closer to the brain.

Strengths of OPM-MEG compared to conventional MEG

OPM-MEG:

  • allows on-scalp recording, increasing signal strength

  • improves spatial sensitivity by reducing sensor–brain distance

  • is wearable, moving with the head

  • is more tolerant of movement

  • fits all head sizes, making it ideal for children and infants

  • avoids liquid helium and large fixed helmets

Limitations of OPM-MEG compared to conventional MEG

OPM-MEG:

  • still requires magnetically shielded environments

  • currently has fewer sensors than standard MEG systems

  • has less established clinical standardisation

  • is more sensitive to environmental magnetic noise

  • is not yet widely available clinically