Neurons are electrical in function, thus their electrical activity can be recorded. Such recordings can be intra-cellular or extra-cellular.
Intra-cellular: With a glass micro-electrode inside the cell recording changes in the membrane potential/current.
Extra-cellular: With external micro-electrodes recording Field Potential and single “unit” spikes.
EEG records potentials over the scalp and EcoG records potentials over the brain.
Intra-cellular recordings are used to measure the voltage across membranes of electrical currents passing through cell membranes. This is done by impaling the cell with a glass microelectrode to measure the membrane potential.
The Resting Membrane Potential of a neuron is = -60 to -80 mV. During an Action Potential, it can reach +40mV.
Extra-cellular recordings use an electrode inserted into living tissue to measure electrical activity from adjacent neurons. If the electrode tip size is about 1μ, the electrode will usually detect the activity of an adjacent neuron or a single unit.
The action potential is very similar to intra-cellular action potentials but much smaller.
There are four different types of well-defined brain microcircuits:
Motor System Microcircuits: For body movements, locomotion, eye movement, respiration, etc.
Striatum Microcircuits: The largest input of the Basal Ganglia, They are important for motor function and cognition.
Olfactory Microcircuits: Smell.
Neocortical Microcircuits: Cognitive functions rely heavily on the neocortex. These microcircuits play an important role in scalp-recorded EEG and ERP signals.
The Human Brain is made up of outer grey matter and inner white matter. The grey matter contains most of the brain’s neuronal cell bodies. EEG originates here, from post-synaptic potentials, not action potentials.
All neuronal cell membranes have clouds of positive and negative ions over their surfaces. There is an excess of positive charges on the outside of the cell membrane and negative chargers on the inside. This separation of charges gives rise to the Resting Membrane Potential (Vm).
EEG is commonly used in medical and research areas to record the bio-potentials from the brain placing electrodes on the scalp.
It reflects the electrical activity of the brain.
It gives high temporal resolution (milliseconds) and low spatial resolution.
Vladimirovich (1912) recorded EEG from an animal model (dog).
Cybulski (1914) first recorded EEG signal of induced seizures.
Berger (1924) recorded the first EEG signal from a human and coined the term electroencephalogram.
Single neuron activity produces too small a signal to record by an EEG. Rather, an EEG reflects the summation of the synchronous activity of many neurons with similar spatial orientations.
Brain waves are commonly measured as peak to peak voltage and normally range from 0.5 to 100μV in amplitude. The raw EEG signal is derived using the Fourier transform power spectrum.
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