Apparatus for the electrical stimulation of nerves

Apparatus primarily for stimulating the auditory nerve in the vicinity of the cochlea of deaf persons to give a sensation indicative of the pitch of speech is described. The stimulus which is in the form of an alternating current of varying period dependent on the pitch of speech is applied to an electrode positioned in the vicinity of the cochlea. The current is non-zero for most of the duration of most stimulation periods and in order to elicit a stable auditory sensation d.c. baseline fluctuation is prevented by the use of a controlled current generator. The period of speech sounds is found using a specially devised circuit and a voltage related to the frequency of such sounds is determined by a period-to-voltage converter and a reciprocal circuit, this voltage being supplied as a control signal for the current generator. An alternative microcomputer version is also described. In order to give an indication of the presence of voiceless sounds, a detector capable of distinguishing between voiced and voiceless sounds is also included together with a noise source which is coupled to the stimulation electrode when the detector indicates the presence of voiceless sounds.