UNDERSTANDING EEG WAVEFORMS
SIMULATION 1: ELECTRODE POSITION
BackgroundThis simulation demonstrates how EEG waveforms are affected by the position of electrodes.
In this simulation, we are looking at an imaginary situation in which we are able to measure the response of a single neuron with EEG methodology.
How do dipoles form?When a PSP is received by a pyramidal neuron, the changes in ion concentrations surrounding the cell creates a dipole. In this simulation we are modeling an excitatory PSP. The resulting dipole consists of a negative (blue) polarity in the extracellular fluid at the dentrites where the PSP occurred, and a positive (red) polarity surrounding the cell body.
Pyramidal neurons have a pyramidal-shaped cell body. They have a long set of dendrites at the top of the cell body, shorter dendrites at the base, and a long axon. They are found in various brain regions, such as the cerebral cortex, hippocampus, and amygdala.
Dendrites are branching structures that carry information from other neurons.
The neuron's cell body contains the nucleus and other organelles.
Polarity is the property of having a positive or negative charge. With respect to the dipole, negative polarity is simply the negative end of the dipole.
An excitatory PSP is a post synaptic potential that makes the inside of the cell more positive, moving the cell closer to the threshold for producing an action potential.
Understanding EEG recordingsChanges in the strength and polarity of dipoles are measured by electrodes. Each electrode records data with millisecond precision. These wavelike timecourse data are measured in voltage units and presented in the graph to the right.
Voltage is a measure of the difference in electrical potential between two points. In EEG, we measure the difference in electric potential between the active electrode of interest, and a reference electrode. EEG data are typically measured in microvolts.
Timecourse data are a measure of change over time. In EEG, we measure variations in electrical brain signals recorded at the scalp over time. Voltage (microvolts) is plotted on the y-axis, and time is plotted on the X-axis.
EEG electrodes are small metal sensors that conduct electricity and are used to record brain activity. They are either placed on the scalp with adhesive, or are embedded in a cap for easier application. They do not penetrate the scalp.
In this simulation, there are three electrodes, each placed near a different part of the pyramidal neuron. You can click each electrode to turn it on or off.
ConclusionThe position of the electrode with respect to the neural source affects the signal recorded by that electrode. Electrodes placed at the negative end of a dipole record a negative signal, while electrodes placed at the positive end of a dipole record a positive signal. Electrodes placed perpendicular to a dipole are not sensitive to the activity from the dipole.
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A dipole is pair of equal positive and negative charges separated by a small distance.
What do EEGs measure?Electroencephalograms (EEGs) measure the electrical activity produced by post synaptic potentials (PSPs) during neural communication. The foundation of this electrical activity is the formation of dipoles.
A post synaptic potential (PSP) is an incoming signal to a neuron that results in a small local change in electrical potential that disperses towards the cell body. It can be either excitatory, making the inside of the cell more positive compared to the outside, or inhibitory, making the inside of the cell more negative.
An electroencephalogram (EEG) is a direct measure of electrical activity in the brain using sensors (electrodes) placed on the scalp. Changes in brain activity measured by EEG recordings produce wave-like data reflecting the changes in the strength and polarity of electrical signals over time.
Graphs display dataEach electrode senses voltage data which are displayed on the graphs next to the electrode.
The PLAY button begins the PSP and displays the resulting dipole. You can PAUSE at any point or RESTART.