A lexicon, from “AC” to “Watt”, of often used terms in the fields of neurofeedback and biofeedback.

ALTERNATING CURRENT (ac.) A flow of electricity which reaches maximum in one direction, decreases to zero, then reverses itself and reaches maximum in the opposite direction. The cycle is repeated continuously. The number of such cycles per second is the frequency. The average value of voltage during any cycle is zero.

AMPERE (A or amp) A unit of electrical current or rate of flow of electrons. One volt across one ohm of resis- tance causes a current flow of one ampere.

AMPLIFIER A device that enables a relatively small input signal to control a larger amount of power, so that the output of the device is an enlarged reproduction of the input signal. An ideal amplifier produces this increase in amplitude without changing any other characteristic of the signal (e.g. distortion, frequency, etc.).

AMPLITUDE A measure of the “strength” of an electrical signal, normally measured in volts or amperes.

BAND-PASS FILTER A filter designed to transmit signals in a given band of frequencies and to substantially decrease the amplitude of all others.

BANDWIDTH The range within the limits of a band. The width of a bandpass filter is generally taken as the limits between which its decrease in amplitude is not more than 3.0 decibels greater than its average decrease in amplitude throughout its passband.

BINARY A term used to indicate a signal which has only two allowable states, (e.g. “1” = on “0” = off) having voltage levels of approximately 5 and 0 volts.

BIPOLAR A term which implies the use of a differential amplifier, two active electrodes and a ground electrode.

COMMON MODE A measure of how well a differential amplifier ignores REJECTION asignal which appears simultaneously and in phase at both input terminals.

CURRENT The movement of electrons through a conductor. Measured in amperes, and its symbol is I.

DIFFERENTIAL Eliminates signals common to both active inputs and AMPLIFIER amplifies the difference usually by ten thousand. Desireable features include an input amplifier at the patient end of input cable for improved artifact rejection.

DIRECT CURRENT (dc.) An essentially constant value current that flows in only one direction.

FILTER A selective network, or circuit, which passes alternating currents of desired frequencies and substantially reduces all others.

FREQUENCY (F) The number of recurrences of a periodic phenomena, such as alternating current or voltage, per unit time. frequency is commonly measured in HERTZ (Hz), the number of times per second that a periodic waveform is repeated.

GAIN Any increase in power when a signal is transmitted from one point to another.

IMPEDANCE (Z) The total opposistion of a circuit to the flow of an alternating current. The unit of measure is the OHM.

INTEGRATOR A device which measures the total activity represented by a result is a value equal to the area under the curve of the waveform during the time period the measurement is made. It is commonly reported in microvolts-seconds.

MHO The unit used to measure conductance. An emf of one volt across a conductance of one mho results in a current flow of one ampere.

MONOPOLAR Implies the use of an amplifier having only one active input with respect to a neutral or ground connection. This type of design does not reject artifact as well as a bipolar system.

NOTCH FILTER A filter which greatly attenuates alternating current signals over a very narrow band of frequencies while passing all frequencies outside this band. It is commonly used to ignore 60 Hz interference which has been detected along with a biological signal.

OHM A unit in which resistance and impedance are measured. A potential difference of one volt applied to a conductor having a resistance of one ohm will cause a current of one ampere to flow.

OPTIC ISOLATOR A safety interface which provides a physical break between the EMG and external devices which may be lined powered. PEAK TO PEAK The maximum difference between the most positive and the most negitive excursion of a signal waveform.

RESISTANCE The opposition of a substance to the flow of direct current. A conductor has a unit resistance of one ohm when a potential difference of one volt across it produces a current flow of one ampere.

ROOT-MEAN-SQUARE (rms) The effective value of an ac quantity which is independent of waveform. Mathematically it is the square root of the average of the squares of the instantanious amplitudes taken over a cycle of that quantity.

MONOPOLAR Implies the use of an amplifier having only one active input with respect to a neutral or ground connection. This type of design does not reject artifact as well as a bipolar system.

VOLT (V) The unit of measurement of electromotive force. It is equivalent to the force required to produce a current of one ampere through a resistance of one ohm.

WATT (W) The practical unit of electric power. A watt is the power absorbed by a circuit through which a current of one ampere flows when a potential of one volt is impressed.