Placement Of Electrodes For Activation Of 8th Nerve

The goal of this study is to supplement recent attempts to use electrical stimulation as a hearing prosthesis in cases of sensorineural hearing loss by examining various placements for the stimulating electrodes. In the basal turn of an anesthetized guinea pig, two electrodes were placed in scala tympani and two were placed in scala vestibuli. A fifth electrode was placed in a neck muscle. These electrodes were used five times in each possible pairing to deliver bipolar pulses with a 0.5-msec duration. The order of use was random. The threshold in microamperes for an evoked potential recorded from the contralateral auditory cortex and the threshold for a twitch in the facial muscles were obtained in a double blind manner. When two stimulating electrodes in the cochlea were employed, the threshold for an evoked potential was 156 μA and 3610 μA were needed before a muscle twitch could be detected. Different placements of the two electrodes within the cochlea did not significantly affect their performance. When a single intracochlear electrode and a muscle electrode were used for stimulation, 92 μA were needed to evoke a cortical potential and 557 μA caused a muscle of the face to contract. Because of the larger range, bipolar is preferable to monopolar stimulation of the auditory system. [Supported in part by the Deafness Research Foundation and the National Institute of Neurological Diseases and Stroke.]