Current advances in cochlear implants
21 March 2002
Hugh McDermott PhD, Principal Research Fellow at the Department of Otolaryngology, The University of Melbourne, discusses current advances in cochlear implant technology
Cochlear implant technology has developed consistently and rapidly since devices with multiple electrodes first came into widespread use about 20 years ago. Advances have occurred, and are continuing to occur, in many aspects of implant design and application.
Understanding speech
The developments which have probably resulted in the greatest increases in benefit for people with a hearing impairment are technical improvements in the way sounds are processed to deliver most information via the restricted capacity of the implanted electrodes. For example, with early versions of multiple-electrode devices, few implant users could understand more than about 20% of speech in the absence of visual cues. In contrast, an average user of a current multiple-electrode implant could expect to understand over 80% of speech when listening in ideal conditions.
Most of this improvement is attributable to increases in the amount of acoustic information that is available through the sound processor. In an experimental multiple-electrode implant developed in Melbourne, Australia in the 1970s, only very limited information about speech was extracted from sound signals picked up by the microphone. This information was presented in a highly processed form that was intended to be used mainly as an aid to lip-reading. However, experiments studying how people perceive sounds with electric stimulation of the cochlea and advances in signal processing techniques led to progressive increases in the amount of information about sounds that could be delivered by the implant. The latest sound processing systems provide useful information about a wider variety of sounds. Users of these devices are generally able to converse without relying on lip-reading and can often communicate easily using the telephone.
Cochlear implants are less successful in unfavourable listening conditions. In particular, background noise disrupts speech understanding for most implant users even at low levels that would have little effect on listeners with normal hearing. Although understanding speech in noisy surroundings can also be difficult with conventional acoustic hearing aids, the problem is generally worse with a cochlear implant. This is probably because the implant’s sound processor is attempting to convey information about both the speech and the noise through an array of no more than 22 electrodes in existing devices. This small number of electrode channels makes it hard for listeners to resolve different parts of the speech and noise. Inadequate separation of speech from simultaneously present noise results in poor speech understanding.
In future, this problem might be solved by providing much larger numbers of electrodes. In the meantime, other methods for removing noise from speech are being developed. These include using two or more microphones to focus the maximum sensitivity of the sound processor on the particular speech signal to which the implant user wishes to pay most attention. Such techniques have been shown to provide large increases in the ability of hearing-impaired people to understand speech in noisy situations.
Restoring natural hearing
Another technical problem that is gradually being overcome is the lack of sensitivity of implant systems to soft sounds. Up to a point, improvements in sound sensitivity are worthwhile, because they enable listeners to understand more speech, and to understand speech better when the speaker is distant or has a relatively weak voice. On the other hand, increasing sensitivity can result in the microphone picking up too much background noise, thus reducing speech clarity.
Recent advances in sound processor design have sought a compromise in which speech is easier to understand in quiet conditions, while background noises are kept at a comfortable level.
Future developments may lead to a processor that brings sensitivity closer to that of normal hearing, while also enabling implant users to hear the loudness of different sounds in a similar way to the loudness perception of normally hearing listeners. In combination with the continuing progress in noise-reduction techniques, these improvements may lead to an enhanced representation of a wide range of sounds, as well as speech that is easier to understand.
Many people who presently rely on cochlear implants to meet their communication needs are satisfied with their ability to understand speech in most everyday situations. Nevertheless, much work remains to be done to improve perception of music and various other non-speech sounds. Several research studies have shown that nearly all implant users experience some difficulty in detecting changes in the pitch of sounds. Consequently, melodies are not always easily recognised. As with speech, tunes and songs tend to be heard less clearly when other sounds, including musical accompaniment, are present simultaneously. Implant users often find that musical instruments are hard to identify by their sound alone.
Current research is aimed at increasing the complexity of the electrical signals delivered to the cochlea by the electrodes so that additional details of sound signals can be represented more accurately.
Eligibility criteria for implants
Developments in cochlear implants over time have resulted in such large improvements in performance that the criteria for implantation applied in some clinics have gradually been relaxed. Initially, only people with total, or near-total deafness in both ears were considered suitable for an implant. At present, some clinics in Australia and elsewhere place more emphasis on a person’s ability to understand speech while using conventional hearing-aids than on their basic hearing sensitivity when making recommendations about implantation. For example, clinicians at a major centre in Melbourne would probably suggest that a person consider the option of a cochlear implant when their ability to understand spoken sentences while using well-fitted aids is less than about 40% of words correctly recognised with the poorer ear, and less than 70% with the better ear. However, for a variety of reasons including the estimated cost-effectiveness of cochlear implantation, clinics in other parts of the world may have different criteria for the selection of candidates.
The relaxation of selection criteria in some clinics means that it is now not unusual for people with a cochlear implant to have usable acoustic hearing in one ear, or (less commonly) in both ears. Generally, these people have a hearing impairment that is characterised by decreasing sensitivity for sounds with increasing frequencies (or higher pitches). Interestingly, this seems to be broadly compatible with the way cochlear implants work. The sounds generated artificially by implants are often described as having a relatively high pitch. If an implant user can also hear some sounds after amplification by a conventional hearing-aid, those sounds are likely to provide a lower pitch quality that may mix with the implant’s signals in a beneficial way. For example, musical sounds might be experienced as more pleasant and easier to recognise when heard via a combination of acoustic and electric signals.
There is also increasing evidence that cochlear implants may work more effectively when the people who receive them either have some residual acoustic hearing, or have had hearing recently before the implant operation. This is probably because the auditory nerve tends to degenerate progressively over a long period of time when it is deprived of stimulation. Better performance of a cochlear implant seems to be associated with more complete survival of the auditory nerve.
Looking to the future
A small number of researchers are currently pursuing the exciting possibilities now being opened up by the use of combined acoustic and electric stimulation. In a few cases, people with considerable hearing sensitivity at low frequencies, but a total loss of hearing at high frequencies, have received a modified cochlear implant in which the electrode array is much shorter than usual. The short electrode stimulates only those regions of the cochlea that are normally responsive to high-frequency sounds. Improved surgical techniques employed when the short electrode is inserted may reduce the likelihood that damage will occur to the low-frequency regions of the cochlea where most hearing sensitivity remains.
Although this research is too new for definite conclusions to be drawn at this time, it does appear feasible to implant a short electrode while substantially preserving existing hearing. If continuing research confirms the benefits of combining acoustic and electric hearing in this way, the number of potential cochlear implant recipients may grow dramatically.
In the foreseeable future, it is likely that many people with a less than total hearing impairment who find conventional hearing aids unsatisfactory will be able to obtain additional information about sounds by choosing to receive a cochlear implant. Thus the overall outlook for people with a hearing impairment is optimistic, even in the short term. Conventional acoustic hearing aids will continue to meet the needs of people with less-severe impairment, new devices that deliver both acoustic and electric stimulation may be suitable for people with moderately severe impairment, while cochlear implants will remain the most appropriate treatment for those with the greatest hearing loss.
