SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
This 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).
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
The 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
The 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
Dipoles 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
Electrodes 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
Each 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
The 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.
SIMULATION 4: ELECTRODE POSITION AND DIPOLE DISTANCE AND ORIENTATION
Special thanks to:
Eliz Akin
Jake Cardwell