What is Impedance?
Impedance is opposition to alternating current (AC) flow, and it has two components,
Resistance by itself is opposition to direct current (DC) flow, and in the context of impedance is a frequency-independent opposition to AC current flow. A volume control on a radio, for example, is typically a device that creates a variable resistance.
Reactance is a combination of capacitance and inductance, which oppose AC current flow in a manner that depends on the frequency content of the AC current. Because the EEG contains a strong AC signal, ERP researchers measure impedance rather than merely resistance.
Factors causing impedance
- In the context of EEG recordings, impedance is typically measured by passing a small 10-Hz current between two or more electrodes and measuring the opposition to the flow of this current.
- The goal is to measure the impedance between the electrode and the highly conductive living skin tissue that immediately overlies the skull (i.e., the electrode impedance).
- The living skin tissue is covered by a layer of dead skin cells, and these dead skin cells provide a relatively high impedance interface between the electrode2 and the living skin tissue.
- When impedance is measured between two electrodes, the measured value reflects the impedance of everything between the two electrodes, which includes the impedance between each electrode and the living skin tissue lying right beneath it.
Impact the S/N ratio of EEG recordings
Electrode impedance might be expected to impact the S/N ratio of EEG recordings. Two key issues are commonly raised in this context, namely
- Common mode rejection
- Cephalic skin potentials
- Common mode rejection
- It refers to the ability of a recording system to reject noise that is in common to the active and reference electrodes. That is, any noise sources that are identical in the active and reference recording electrodes are attenuated in a differential amplifier, because the output of the amplifier subtracts the voltage measured at the reference electrode from the voltage measured at the active electrode.
- This primarily eliminates noise induced by electrical devices in the recording environment rather than biological noise generated by the subject.
- As the common mode rejection of an amplifier increases, the contribution of noise signals decreases and the S/N ratio
- The ability of an EEG amplifier to accomplish this depends, in part, on the ratio of the electrode impedance to the amplifier’s input impedance.
- The amplifier’s input impedance is its tendency to oppose the flow of current from the electrodes through the amplifier, and it is determined by the electronics used in the amplifier.
- The amplifier’s input impedance is a fixed value.
- The electrode impedance, in contrast, is determined by the properties of the skin, which can vary considerably across individuals, across electrode sites, and across time.
- As the electrode impedance increases, the common mode rejection of the system decreases, and the S/N ratio of the recording decreases.
1.1 High impedance eeg recording system
- With traditional low-impedance EEG recording systems, this problem is typically solved by cleansing and abrading the skin.
- Abrasion of the skin reduces impedance by disrupting the external layer of dead skin cells, providing a more direct contact with the underlying living skin tissue.
- To deal with the problem of decreased common mode rejection with high electrode impedances, it is possible to use amplifiers with higher input impedance, thus yielding the same ratio of electrode impedance to input impedance.
2 Cephalic Skin Potentials
- High electrode impedances may lead to a second problem that cannot be solved by means of changes to the amplifier’s input impedance, namely an increase in skin potential artifacts.
- Skin potentials arise because of the standing electrical potential that is normally present between the inside and the outside of the skin.
- The magnitude of this potential depends on the conductance of the skin, which in turn depends on factors such as the thickness of the skin, the number of sweat glands and hair follicles, and the degree of skin hydration.
- When the voltage is recorded between two electrodes on the surface of the skin, any differences in the conductance of the skin under these two electrodes will lead to a different voltage offset for each electrode, which creates an electrical potential between the two electrodes.
- This potential will vary over time if the conductance of the skin under one electrode varies over time in a different manner than the conductance of the skin under the other electrode.
IMPEDENCE IN EEG
It is always recommended to have minimum impedence of 10 kilo ohms. Greater impedence leads to voltage drop and attenuation of bio-signal. Lower impedence wouldn’t allow transduction of biosignal.
- Low impedance in electrodes used for bio-potential recordings such as EEG and EMG,
- It is required to provide impedance matching with the environment you are measuring from,
- It maximizes the power transfer and prevents voltage drop.
Noises in EEG