ABSTRACT
It has long been known that electric fields generated by action potentials accompanying the conduction of nerve impulses, and the depolarization of muscle membranes can propagate through the various volume conductor layers of the body and be sensed on the skin surface. Such potentials are given names according to their sources: from muscles, we have the
electromyogram
(EMG); from the brain, the
electroencephalogram
(EEG) and evoked cortical potentials (ECPs); from the eyes, the
electrooculogram
(EOG) and the
electroretinogram
(ERG); from the ear, the
electrocochleogram
(ECocG); and from the heart, the
electrocardiogram
(ECG or EKG). All of these potentials have frequency spectra that basically span the audio range
of frequencies, and, in some cases, very low, subsonic frequencies as well. Their amplitudes are basically in the range of 10s of microvolts to 10s of millivolts. All bioelectric signals are noisy, i.e., they are recorded with bioelectric background noise, as well as measurement noise from electrodes, amplifiers and the surrounding electrical environment. Signal averaging is often used to improve the signal-to-noise ratio of evoked transient signals such as ECPs and ERGs.
