The primary differences between Electroencephalography (EEG), functional Magnetic Resonance Imaging (fMRI), and Magnetoencephalography (MEG) lie in their measurement methods, spatial resolution, and depth sensitivity when studying brain activity.
Understanding the Techniques
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EEG (Electroencephalography): EEG measures the electrical activity of the brain using electrodes placed on the scalp. It primarily detects changes in voltage generated by the flow of electrical current within neurons.
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fMRI (functional Magnetic Resonance Imaging): fMRI measures brain activity by detecting changes in blood flow. Specifically, it monitors the blood oxygenation level-dependent (BOLD) signal, which reflects neuronal activity.
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MEG (Magnetoencephalography): MEG measures the magnetic fields produced by electrical currents in the brain. It uses extremely sensitive magnetometers to detect these tiny fields outside the scalp.
Comparing EEG, fMRI, and MEG
| Feature | EEG | fMRI | MEG |
|---|---|---|---|
| Measurement | Electrical activity on the scalp | Blood flow changes (BOLD signal) | Magnetic fields produced by brain |
| Spatial Resolution | Centimeters, limited to surface | Millimeters, not limited to cortex | Millimeters at cortex, less precise for deep sources |
| Depth Sensitivity | Limited to cortical surface | No limitation | Less precise for deep sources |
Key Differences Explained
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Spatial Resolution: The spatial resolution refers to how accurately each method can pinpoint the location of brain activity. EEG has a lower spatial resolution, meaning it's difficult to identify the precise origin of activity with high accuracy. The reference shows that EEG resolution is in centimeters and limited to the cortical surface. fMRI and MEG have better spatial resolution in the millimeter range, with fMRI being able to examine activity throughout the brain while MEG is more accurate when focused at the cortex.
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Depth Sensitivity: Depth sensitivity is the ability of the method to detect activity from structures deep within the brain. EEG is mostly limited to detecting surface brain activity, per the reference. fMRI has good depth sensitivity and isn't limited to the cortical surface. MEG has better surface detection resolution than EEG, and while it can measure activity from deeper sources, its accuracy diminishes with depth.
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Measurement Type: EEG measures electrical activity directly, fMRI measures changes in blood flow indirectly related to activity, and MEG measures magnetic fields produced by electrical brain activity.
Practical Insights
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EEG is relatively inexpensive and has excellent temporal resolution, making it useful for studying rapid changes in brain activity like those seen during seizures or sleep stages. However, its limited spatial resolution and depth sensitivity are downsides.
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fMRI provides detailed spatial information and is used to map functional brain areas. It's used frequently in cognitive neuroscience. However, it is costly and has poor temporal resolution, as the BOLD response changes relatively slowly.
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MEG is a non-invasive and relatively direct measure of neural activity with good temporal resolution, and higher spatial resolution compared to EEG. However, it is very expensive and sensitive to external electromagnetic interference, thus requiring highly shielded environments to perform tests.
Summary Table
| Feature | EEG | fMRI | MEG |
|---|---|---|---|
| Primary Focus | Electrical Activity | Blood Flow | Magnetic Fields |
| Spatial Resolution | Centimeters, Cortical | Millimeters, Whole Brain | Millimeters, Cortex |
| Depth Sensitivity | Surface Limited | Deep Brain | Deep Activity, Limited Precision |
| Temporal Resolution | Excellent | Poor | Good |
| Cost | Low | High | Very High |
