Interaction between electrically and acoustically evoked responses in the Cochlea of the Guinea Pig

Abstract

Recently, the audiometric criteria for cochlear implantation have been relaxed. At present, people with considerable residual low-frequency hearing have become eligible for implantation. The audiogram in this subpopulation is characterized by a large hearing loss in the high-frequency region, while hearing thresholds at low frequencies are only mildly raised or even normal. The high-frequency hearing loss, however, might be so severe that acoustical amplification has no benefit for speech understanding. These people might be treated by electrical stimulation of the high-frequency part of the cochlea and acoustical amplification of the low frequencies. Residual low-frequency hearing has proven to be useful for speech understanding with a cochlear implant. Especially in noisy environments, low-frequency hearing increases speech understanding. Due to this beneficial effect of low-frequency hearing, hybrid implants have been developed that combine an implant with a conventional hearing aid. Due to the increased interest in combined electrical and acoustical stimulation (EAS), research characterization of the interaction between electrical and acoustical stimulation in the cochlea is important. This thesis contains four different experiments that describe these interactions. The most important research questions were: “What are the effects of electrical stimulation on acoustically evoked responses in the auditory nerve”, and “What are the effects of acoustical stimulation on electrically evoked responses in the auditory nerve?”. The experiments conducted consisted of electrophysiological recordings of cochlear potentials in the cochlea of the guinea pig. Especially acoustically and electrically evoked compound action potentials were important recording parameters. These potentials represent the synchronised activity of many auditory-nerve fibres. We compared responses evoked with electro-acoustical stimulation to the responses evoked with acoustical or electrical stimulation alone. We systematically varied stimulus parameters such as acoustic frequency, acoustic level, electric current level, pulse width and pulse rate. We identified critical parameters that mainly determined electro-acoustical interaction. Low current levels and short pulse widths resulted in minimal interaction between electrical stimulation and low-frequency evoked acoustic responses. Therefore, low current levels and short pulse widths are advisable in EAS strategies to preserve acoustical responses. Furthermore, we found indirect evidence that high pulse rates and short electrodes are probably best to minimize interaction of electrical stimulation on acoustically evoked cochlear responses. We found that acoustic responses recover within milliseconds after electrical stimulation, which might find use in future EAS strategies. Regarding electrically evoked responses, loud acoustic stimuli suppressed electrically evoked auditory nerve activity. Hence, loud acoustic stimuli should probably be avoided in hybrid implants. However, desynchronizing effects of low-level (acoustic) noise may actually be beneficial to electric hearing. The findings that suppression of electrical auditory nerve responses decreased rapidly within a few milliseconds during noise presentation might be useful in future EAS strategy design. Last, the transient increase of the eCAP amplitude after noise offset might be important for EAS strategies