The auditory midbrain implant (AMI), which is designed for stimulation of the inferior colliculus (IC), is now in clinical trials. The AMI consists of a single shank array (20 contacts) and uses a stimulation strategy originally designed for cochlear implants since it is already approved for human use and we do not yet know how to optimally activate the auditory midbrain. The goal of this study was to investigate the effects of different pulse rates and phase durations on loudness and pitch percepts because these parameters are required to implement the AMI stimulation strategy. Although each patient was implanted into a different region (i.e. lateral lemniscus, central nucleus of IC, dorsal cortex of IC), they generally exhibited similar threshold versus phase duration, threshold versus pulse rate, and pitch versus pulse rate curves. In particular, stimulation with 100 μs/phase, 250 pulse per second (pps) pulse trains achieved an optimal balance among safety, energy, and current threshold requirements while avoiding rate pitch effects. However, we observed large differences across patients in loudness adaptation to continuous pulse stimulation over long time scales. One patient (implanted in dorsal cortex of IC) even experienced complete loudness decay and elevation of thresholds with daily stimulation. Comparing these results with those of cochlear implant and auditory brainstem implant patients, it appears that stimulation of higher order neurons exhibits less and even no loudness summation for higher rate stimuli and greater current leakage for longer phase durations than that of cochlear neurons. The fact that all midbrain regions we stimulated, which includes three distinctly different nuclei, exhibited similar loudness summation effects (i.e. none for pulse rates above 250 pps) suggests a possible shift in some coding properties that is affected more by which stage along the auditory pathway rather than the types of neurons are being stimulated. However, loudness adaptation occurs at multiple stages from the cochlea up to the midbrain.
|Original language||English (US)|
|Number of pages||11|
|State||Published - Jun 12 2008|
Bibliographical noteFunding Information:
We would like to thank Dr. Joerg Pesch for technical assistance; Frank Risi and Peter Gibson for AMI development; and Drs. Madjid Samii and Amir Samii along with their neurosurgical team at the International Neuroscience Institute (Hannover, Germany) for successful implantation of the AMI in our patients. We would also like to thank the two anonymous reviewers for their helpful suggestions that have improved the overall manuscript. This study was supported by Cochlear Ltd. (Lane Cove, Australia).
- auditory prosthesis
- deep brain stimulation
- inferior colliculus
- rate pitch