Biofeedback Glossary

Dictionary of Biofeedback / Neurofeedback Terms

*Items with a star (*) before the name were provided by the EEG Division of the Association for Applied Psychophysiology and Biofeedback (AAPB). They were compiled by Thomas F. Collura, Ph.D.; David Kaiser, Ph.D.; Joel Lubar, Ph.D.; and James Evans, Ph.D. We appreciate Dr. Collura's permission to post this list of terms on AAPB's web site.

*Active - one of the two electrodes in a differential measurement. Generally on the head, it is the location you are interested in measuring and training.

*Amplitude - a measure of the size of a signal. Various metrics are used, including peak-to-peak (P-P) and root-mean-­squared (RMS). pop is a measure of the signal's size measured from peak to valley. RMS is a measure of the signal's energy. For pure sinewaves, there is a fixed ratio between Pop and RMS measurements (2.8). For other waveforms (square, triangle, EEG-like), the ratio will vary, depending on the wave shape. EEG amplitude is generally expressed in microvolts.

*Bipolar - an EEG measurement taken between two (generally 10-20) locations on the scalp, designed to measure the brain activity in the regions near and between the two locations

*Channel - a single EEG channel is a waveform recording taken either bipolar or monopolar. For one channel of recording, you need an active electrode, a reference electrode, and a ground. For two channels of recording, you need two active electrodes, two references (you can use the same reference for both channels), and a ground.

*Coherence - a measure of how similar in frequency two signals are. Reflects the amount of information shared between (or common to) two site, or between two sites and a third (or more) site(s). This is computed as a correlation coefficient between the channels, and varies from 0 to 1.0, or from 0 to 100%. Coherence training is important, as it allows neurofeedback to be used to train the amount of coupling or connection between brain locations, as the brain functions.

*Cortex - the outer layer of the brain, consisting of enfolded layers. The human Neocortex is the most recent brain development, consisting of the Frontal, Temporal, Parietal, and Occipital lobes.

*DC - direct current. A signal containing energy at 0 Hz, i.e. a steady potential. Since the potential is considered to slowly change, a working bandwidth of 0-0.1 Hz, for example, may be considered "DC."

*Differential amplifier - an electronic amplifier that measures the difference in potential between two electrode locations.

*EEG - electroencephalogram - the electrical activity of the brain as measured from the scalp using high gain amplifiers.

*Electrode - a metallic device used to make a connection to the scalp or body, to measure electrical potential.

*Electrolyte - a gel or liquid used to allow electrical potential to be measured from the body surface, using a metallic device. The electrolyte actually conducts the electrical signal from the skin surface to the electrode surface.

*Frequency - the rate at which a signal is changing or vibrating. EEG frequency is measured in cycles per second. Typical frequency ranges are Theta: 4-7, Alpha: 8-12, SMR or Low Beta: 12-15, Beta: 15-20, and High Beta: 20-30. Exact frequency ranges used for EEG training vary with the practitioner and protocol.

*Monopolar - an EEG measurement from an active (generally 10-20) location on the scalp, to a "neutral" reference such as an ear.

*Neurofeedback - a form of biofeedback training that uses the EEG (Electroencephalogram), also known as the "brain wave" as the signal used to control feedback. Sensors applied to the trainee's scalp record the brainwaves, which are converted into feedback signals by a human/machine interface using a computer and software. By using visual, sound, or tactile feedback to produce operant conditioning of the brain, it can be used to induce brain relaxation through increasing alpha waves. A variety of additional benefits, derived from the improved ability of the CNS (central nervous system) to relax, may also be obtained.

*Phase - a measure of the temporal relationship between two signals. Reflects the speed of information sharing between two sites, or between two sites and a third (or more) sites. Two sinewave signals are said to be "in phase" (have zero phase difference) when their peaks and valleys are aligned in time. Phase is generally measured in degrees, ranging from 0 degrees (in phase) to 180 degrees (out of phase).

*Protocol - a set of controls, based upon a treatment plan or regimen that determines how neurofeedback training is done. This includes the enhance and inhibit settings, frequency bands, decision criteria, feedback signals, threshold adjustments, and decision points in the training.

*Reference - the second of two electrodes in a differential measurement. Generally an ear or linked ears for monopolar (or "referential") measurement, or can be a second scalp location for bipolar (or "differential") measurement.

*SCP - slow cortical potential. An EEG signal typically considered to range between 0.05 Hz and 5 Hz. SCPs are seen to vary with mental tasks, particularly planned movement. The Bereitzschaftpotential is a movement-related SCP.

*Silver Chloride - AgCl, a material often used for EEG or similar sensors, especially useful for low-noise and/or SCP or DC recordings.

*Site - a specific EEG placement, e.g. T3 or C4.

*Synchrony - a measure of how time-locked two signals are. When training synchrony, this may also be referred to as "phase synchrony"

*Ten- Twenty (10-20) System - A system of locating positions (sites) on the scalp, for standard EEG recording. They are based on 10% and 20% of the distance across the head, hence the name. Standard charts and descriptions of the locations are freely available.

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