Frequency Analyzer for Bioelectric Potentials in the Sub‐Audio Range

An analyzer for sub‐audio bioelectric potentials, particularly the human electroencephalogram, is described. The instrument is coupled to the output of a standard EEG amplifier and records twenty frequencies by means of individual filters each coupled to a two‐inch cathode‐ray oscilloscope. A 35‐mm slow moving camera provides a time base while simultaneously photographing, the filter deflections, a cathode‐ray trace of the unanalyzed slow wave and one‐second time markers produced by a clock driven flashing light. The system employs amplitude modulation of a 1200 cycle carrier; the filters being selective to the lower sideband frequencies which are recorded as horizontal deflections of the oscilloscopes. The present analyzer, in contrast to others, has a one‐half‐second time constant and is thus a low ``Q'' system responsive to activity of short duration. The frequency spectrum obtained is necessarily broad but detailed in the time domain, an aspect of physiological importance, hitherto neglected.