Question

EEG and MEG measure closely related, yet distinct phenomena. Explain what each technique measures and
the basic principles of how each works (physiology, physics, and acquisition hardware). Make clear how the
two types of data are related to each other. Explain the relative advantages of each, with examples of a
research question that would be best addressed with MEG, and one that could be answered using EEG,
where MEG would offer no advantages.

Answer 1

EG is also known as the electroencephalography, the EEG scan refers to the recording of the brains electrical signals, i.e. the firing of many neurons in the cortex of the brain. It can be used to record electrical activity for research purposes.

MEG is also known as the magnetoencephalography, the MEG scan is a technique used to record magnetic fields produced by the natural electrical activity in the brain. It's used for recording magnetic fields in the brain.

EEG:

• Records electrical potential changes related to the cortical activity. It is measured by summing the changes in action potentials between two electrode sites. The most activity comes from the post-synaptic dendritic activity of pyramidal cells.
- In unipolar recordings, one of these sites is a non-brain reference site such as the nose, ears, or mastoid.
- In bipolar recordings, both sites are active electrode sites on the scalp (e.g. right vs. left frontal area)
• Measures the activity of vertically arranged pyramidal cells across cortical layers.
•Researchers typically examine amplitude, (signal size) and frequency, (how fast the signal cycles)

Clinically it is used for epilepsy diagnosis and monitoring, sleep analysis, anaesthesia, and brain death/coma.

Spontaneous EEG waves :

•Delta waves: 0.5-4 Hz of higher amplitudes, associated with sleep, predominant wave in the first few years of life, also associated with certain types of pathology, like brain tumours
• Theta waves, 4-7 Hz, Associated with low alertness and sleepiness,
• Alpha waves: Defined by large, rhythmic waves of 8-12 Hz, Associated with relaxed wakefulness, lack of intense cognitive processes or stimulation, or closed eyes
If someone in an alpha rhythm is asked to perform a mental task, beta waves appear. This is considered an alpha blockade.
•Beta waves
-Defined by high-frequency, low amplitudes waves of 14-30 Hz, associated with the orienting response and physical and mental activity.
• Gamma waves, 40-80 Hz waves with smaller amplitudes, associated with meditation and "binding" of activity across cortical regions

MEG:

MEG detects very small magnetic fields generated by neural activity using SQUIDs (superconducting quantum interference devices). Specifically, it detects the depolarization of dendrites of pyramidal cells and postsynaptic potentials like EEG but measures more intracellular components than extracellular components.

In terms of the machine: No scalp electrodes or contact with the skin, but machine is expensive, bulky, requires a magnetically shielded room and a helium liquid supercooled 24 hours a day
• In terms of data: Excellent temporal resolution, better spatial resolution than EEG because magnetic signals are less distorted by the skull and scalp.

MEG offers an excellent temporal resolution, better spatial resolution than EEG because magnetic signals are less distorted by the skull and scalp like EEG can only measure the surface activity of the cortex, can only measure tangential activity (sulci) whereas EEG can also measure radial components.

EEG has a high temporal resolution, has a high spatial resolution. EEG has low sensitivity depth, and However, how is MEG? MEG has a high temporal resolution and a high spatial resolution, but it has low sensitivity depth. The spatial resolution and temporal resolution comparisons are below. These data are only obtained from some journals. In fact, the differences between these data are large.

EEG: spatial resolution (7-10 mm), temporal resolution (<1 ms)

MEG: spatial resolution (2-6 mm), temporal resolution (< 1 ms)

The result shows that MEG can replace EEG. Besides, some graphs compare the spatial resolution and temporal resolution among EEG, MEG.