INSTRUCTIONS

UNDERSTANDING EEG WAVEFORMSSIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION

BackgroundThis simulation demonstrates how the EEG signal varies as the orientation of the neural source to the electrodes changes. In this simulation, we are looking at an imaginary situation in which we are able to measure the response of an isolated group of neurons with EEG methodology using a neutral reference point (eg. average reference).

CONCLUSION

BACKGROUND

SIMULATION

CREDITS

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. 

The dipole symbolThe dipole produced from a group of neurons can be represented by a symbol like the one shown to the right. In this simulation, the round side of the symbol represents the negative (blue) charge in the extra cellular fluid at the site of excitatory post synaptic potentials. The straight side represents the positive (red) charge in the extracellular fluid surrounding the opposite end of the cells.

Scalp distributionDipoles produce an electric field that spreads to the scalp. The strength and polarity at a particular scalp location can be measured by placing an active electrode at the scalp location, and comparing its voltage to that recorded by a reference electrode placed at a different location. This is the EEG signal.

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.

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.  With respect to the EEG signal recorded at the scalp, a waveform with a negative polarity means that the voltage recorded by the electrode is more negative than its reference.

ConclusionThe strength and polarity of the signal recorded at each electrode depends on the electrode's location with respect to the source of the neural activity. The signal is strongest for electrodes that are close to the dipole and aligned with the dipole's orientation.  The polarity of the signal is influenced by the orientation of the dipole. Assuming a neutral reference, the polarity of the signal will reflect the polarity of the end of the dipole closest to the electrode. 

Created by: Cindy Bukach Nadia Bukach Kendall Stewart Llewellyn Hensley Jane Couperus Cathy Reed Special thanks to: Eliz Akin Jake Cardwell

The cortex is the outermost layer of the brain and contains a thin layer of cell bodies. This is where the majority of pyramidal neurons contributing to the EEG signal can be found.

A dipole is pair of equal positive and negative charges separated by a small distance.

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.

Dipoles in the brainThe dipole from a single neuron is generally too weak to be measured at the scalp. Several neurons must be active at the same time and in a similar orientation in order to produce a dipole strong enough for EEG to measure. Pyramidal neurons in the cortex are ideally situated to produce a dipole that can be measured with EEG.

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.

EEG graphsEach electrode senses voltage data which are displayed on the labeled graphs. Press PLAY to start the simulation. You can PAUSE to freeze the simulation and RESTART to watch again. The simulation starts on the next page.

C4

T7

T8

C3

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.

Electrode configurationElectrodes placed on the scalp measure voltage changes with millisecond precision, resulting in wave-like timecourse data. Electrodes can be placed on the scalp in several standardized locations. In this simulation we use five active electrodes in the standard locations named T7, C3, CZ, C4, and T8.

Cz

Scalp voltage The distribution of positive and negative charge will be displayed in two scalp views to the left and right of the coronal section controls. In this simulation, voltage values are simplified into 8 discrete colors. In reality these voltage values are not discrete, they are a continuous gradient of voltage strengths.

T7

C4

C3

Cz

T8

Instructions Turn Electrodes ON/OFF: Click on an electrode to turn it ON or OFF.Move the Dipole: Click on a box within the coronal slice to move the dipole.Rotate the Dipole: Click on the rotate 45 degrees boxes to change the dipole’s orientation relative to the scalp.