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Bio Technology : July 2009
AusBioFEATURE Improving hearing with the evolution of cochlear implants By Prof. Hugh McDermott The Australian bionic-ear manufacturer, Cochlear Ltd, has at least two claims to be the biggest in the world. First, it has the largest share in the global market for cochlear implants – electronic devices that can restore some hearing to deaf children and adults. Second, their implants provide the greatest number of individual electrodes among currently available devices. This is one practical reason for their pre-eminence in the industry. When the concept of a cochlear implant was initially explored experimentally in France in the 1950s, only a single electrode was used to stimulate the auditory nerve. Research in the 1960s showed that stimulating via an array of multiple electrodes could create a corresponding number of distinct hearing sensations. Today’s implants exploit the benefits of activating different sectors of the auditory nerve by placing between 12 and 22 separate electrodes within the cochlea (inner ear). Cochlear’s 22-electrode implants enable the majority of recipients to understand nearly all speech, provided that there is no background noise. However, their performance in more-complex acoustic situations, such as listening to music or speech in competing noise, is often unsatisfactory. To understand this problem, it might help to imagine an analogy. The deaf cochlea can be thought of as a blank canvas, while the function of a cochlear implant is to paint a picture on that canvas. The picture created is equivalent to hearing a recognisable sound. The 22 electrodes of the implant are represented by 22 spray-cans of paint. Because the electrodes are not actually in contact with the auditory nerves in the cochlea, the spray-cans can be imagined as located some distance from the canvas. What kind of picture can be painted in this way? It should be straightforward to create a cartoon-like sketch, which conveys symbolic meaning but not much else. This is analogous to understanding speech, in which the intended meaning is more important to perceive than the sound quality. Unfortunately, however, it would be impossible to paint an intricate landscape or detailed portrait, for example. This is not only because there are too few sources of paint, but also because the paint spreads out from the spray-cans, resulting in a blurry image. Future cochlear implants need to have many more electrodes if their users’ perception of complex sounds, including music, is to be greatly enhanced. Furthermore, those electrodes must be placed closer to the auditory nerves so that the stimulation they deliver can be focused adequately. At present, it is not clear how many electrodes 22 Australasian BioTechnology Volume 19 • Number 2 • July 2009 would be required if the aim is to approximate normal hearing. In a normal cochlea, the process of converting sound vibrations into neural activity is performed by many thousands of hair-cells, which are distributed along the length of the cochlear spiral. Taking this fact as a guide, it seems likely that the number of electrodes may need to be increased by a factor of at least ten – perhaps even a hundred – relative to today’s implants for large performance improvements to be achieved. In the meantime, research is also showing that the use of acoustic hearing in combination with cochlear implants is highly beneficial. This is significant, particularly because the high level of performance possible with the best existing devices is leading to a rapid increase in the number of recipients who are not totally deaf. When partially deaf implant users listen simultaneously via the device and one or two conventional hearing aids, they report that their understanding of speech in noisy conditions and their appreciation of music are both enhanced. Researchers are now developing improved sound-processing systems and fitting techniques to maximise the benefit available from such combined hearing devices. Prof. Hugh McDermott Professor of Auditory Communication and Signal Processing Audiology, Hearing and Speech Sciences, Department of Otolaryngology, The University of Melbourne VIC 3010 E-mail: firstname.lastname@example.org