Page 28 - Spring 2015

Matthew J. Goupell
Postal:
Department of Hearing and Speech Sciences University of Maryland College Park, Maryland 20742 USA
Email:
goupell@umd.edu
Pushing the envelope
of Auditory Research with Cochlear Implants
The cochlear implant has advanced our understanding of hearing in novel ways, but still challenges our understanding in many regards.
The function and Importance of Hearing,
either Acoustically or electrically
Acoustic information is typically transferred to the brain by the collection of sound by the pinna (external ear), the amplification of frequencies important for speech understanding by the ear canal and middle ear bones, and the transduction of sound to the nervous system by the hair cells in the inner ear (Figure 1A). The inner ear, or cochlea, operates like a frequency analyzer, where hair cells near the base of the cochlea are tuned to and transmit high-frequency acoustic information (≈20,000 Hz) and where hair cells near the top of the cochlea are tuned to and transmit low-frequency acoustic information (≈20 Hz). The hair cells are an in- dispensable part of the transduction process because they start the electrochemi- cal reactions that cause the spiral ganglia and auditory nerve to pass the acoustic information as electrical spikes that the brain interprets as sound. Hair cells are damaged or die because a person might have a genetic disposition, need to take an ototoxic medicine that saves their life but destroys their hearing, are exposed to
Figure 1. A: diagram of the pinna, ear canal, middle ear, and inner ear (i.e., cochlea). A co- chlear implant (CI) bypasses these structures with a behind-the-ear microphone attached to a speech processor, a frequency-modulated (FM) transmitter (outside the skin) and receiver (under the skin), and the CI, which is placed in the cochlea. A CI mimics the frequency organi- zation of the cochlea by sending different frequency information to different places. B: a Med- El SYNCHRONY CI System, with the SONNET and RONDO speech processors. C: close-up view of the coil of wire that is the FM receiver and the electrode array, which is curved to follow the shape of the cochlea (a two-and-a-half-turn snail shell shape). Two types of CIs are shown: from Med-El SONNET (top) and from Advanced Bionics HiRes 90K Advantage (bottom). D: electrode contacts of the Advanced Bionics HiRes 90K Advantage CI. Images provided courtesy of Advanced Bionics and Med-El.
26 | Acoustics Today | Spring 2015 , volume 11, issue 2 ©2015 Acoustical Society of America. All rights reserved.