Tissue impedance and current flow in the implanted ear. Implications for the cochlear prosthesis.

Tissue impedance was measured in the cochleas of monkeys and guinea pigs implanted with electrode arrays. The impedances were measured within the scala tympani and between the scala tympani and the internal auditory meatus of the modiolus. The measurements revealed several impedance properties. 1) The impedances inside and outside of the scala tympani are resistive for frequencies between 8 Hz and 12.5 kHz. 2). The impedances measured between the scala tympani and the internal auditory meatus were larger in magnitude than the impedances measured in the scala tympani. 3) Impedance was not affected significantly by changing stimulus current flow from 0.45 to 45 microA rms. 4) Impedance magnitude increased rapidly by different factors inside and outside the scala tympani after sacrifice of the subject animals. Measurements of the tissue impedance made in conjunction with measurements of threshold of response to electrical stimulation revealed that the threshold of response to electrical stimuli was lowest when the stimulating currents were highest outside the scala tympani. We conclude that the excitable elements of the auditory nerve are being stimulated within the modiolus rather than within the scala tympani.