Electroencephalogram
EEG Is a record of the electric signal generated by the cooperative action of brain cells, or more precisely, the time course of neuronal extracellular field potentials generated by their synchronous action.
- EEG recorded in the absence of stimuli is called spontaneous EEG;
- Brain electric field generated as a response to an external or internal stimulus is called an event-related potential (ERP).
EEG can be measured through electrodes placed on the scalp or directly on the cortex.
Electrocorticography (ECoG) : Electrodes placed directly on the surface of the brain (under the skull).
- Invasive (requires surgery)
- Much higher spatial resolution than EEG
- High temporal resolution
Local Field Potentials (LFP) : Electric fields measured intracortically with electrodes implanted in the brain structures were named local fields potentials (LFP). Signals recorded from electrodes inserted inside brain tissue. It measures Electrical activity from neurons in a very small local region (synaptic activity + nearby neurons).
The amplitude of EEG of a normal subject in the awake state, recorded with the scalp electrodes, is 10–100 μV. In the case of epilepsy, the EEG amplitudes may increase by almost an order of magnitude. An epileptic seizure happens when many neurons in the brain start firing together abnormally. This synchronized electrical activity disrupts normal brain function
Making EEG electrodes bigger doesn’t significantly change what they record. Scalp potentials are mostly independent of electrode size due to severe space averaging by volume conduction between the brain and scalp.
MEG A variable electric field generates a magnetic field as follows from the Maxwell equations. The recording of the magnetic field of the brain is called a magnetoencephalogram (MEG). The magnetic field produced by the brain is incredibly small.The MEG’s amplitude is less than 0.5 picotesla (pT).
| Source | Magnetic field |
|---|---|
| Earth | ~50 µT (microtesla) |
| MRI scanner | 1.5–7 T (tesla) |
| Brain (MEG) | <0.5 pT (picotesla) |
| The MEG technique’s significant advantage is that the magnetic field generated inside the brain is affected to a much lesser extent by the conductivities of the skull and scalp than the electric field generated by the same source. |
Magnetic evoked fields (EF) are a counterpart of ERP. The purpose of analyzing EFs is source localization, identifying the brain region that generated the response.
Both EEG and MEG measure the same underlying neuronal currents, but from different perspectives. EEG records the electrical voltages created by those currents and is best for radial sources, while MEG records the accompanying magnetic fields, is best for tangential sources.
In the investigation of brain processes, especially perception and cognitive functions usually ERPs are analyzed since they supply information on the brain’s reaction to specific external or internal stimuli. ERPs are also used for testing the sensory pathways in psychiatric diseases and developmental disorders, e.g., in dyslexia diagnosis.
Typical frequencies include:
- Delta (0.5–4 Hz)
- Theta (4–8 Hz)
- Alpha (8–13 Hz)
- Beta (13–30 Hz)
- Gamma (>30 Hz)
In the brain, there are ~86 billion neurons (nerve cells). Each of them is synaptically connected with up to other neurons.
- Some neurons: a few thousand synapses
- Purkinje cells (cerebellum): can have 100,000+ synapses
- Others: much fewer
Neurons communicate via both electrical synapses (gap junctions), which allow direct ionic current flow between cells, and chemical synapses, which generate postsynaptic potentials (PSPs). PSPs are graded and local, whereas action potentials are all-or-none electrical spikes that propagate along axons.