Hearing Loss Clinical Trial
Official title:
Cortical Activity in Subjects With Unilateral Cochlear Implants and Preserved Ipsilateral Residual Hearing and the Evolution of Temporal Cortex Changes in New Recipients of Cochlear Implants.
Investigators hope to study patients with 1 cochlear implant and who have had hearing preservation surgery and compare the changes in the brain of these patients with normal hearing patients by doing a scan of the brain when listening to speech. The information investigators collect may help investigators to understand how the brain learns to hear with a cochlear implant and whether it uses the same areas in the brain to listen as in normal hearing patients as well as predict which patients will do well with a cochlear implant as a very specialised operation is needed to insert the cochlear implant in to the ear and is done by very few surgeons in the country. After the operation, patients need several months of teaching to help them learn how to hear through the implant. Having a cochlear implant requires a lot of cooperation and time from the patient and if investigators could understand how the brain learns to hear with a cochlear implant and predict which patients do well and why then we may be able to avoid unnecessary procedures.
Cochlear implants are an established treatment for individuals with hearing loss where
conventional hearing aids fail to provide appropriate hearing amplification and result in
poor speech and language perception.
Although a large proportion of patients with cochlear implants are expected to do well with
an implant, there is still uncertainty why some individuals do poorly.
Furthermore investigators have limited knowledge about how the brain interacts with a
cochlear implant to process speech. Evidence suggests that the underlying central mechanisms
responsible for processing speech using an implant are related to neuroplasticity. That is to
say that the brain establishes new cortical networks between the vital auditory centres and
the cochlear implant thus enabling processing of speech.
Investigators have recently completed a study comparing the cortical changes that occur in
subjects with bilateral cochlear implants to normal hearing controls. In this study both
groups of patients were subjected to binaural simultaneous auditory stimuli. Analysis
suggests that subjects with bilateral cochlear implants and postlingual hearing loss seem to
recruit similar areas of the temporal lobes to process speech as their normal hearing
counterparts.
The implant recipients activations were larger and more diffuse but did not differ greatly
from those described previously in normal hearing. This study provides a good comparison of
hearing using bilateral cochlear implants to that of normal unaided hearing. This study
provides a more realistic comparison of cortical activations due to hearing in cochlear
implantees and normal hearing subjects. This is in contrast to other studies involving
monaural stimulation of a unilateral cochlear implant to monaural stimulation of normal
hearing subject with the contralateral normal hearing ear blocked off. Investigators are only
aware of one additional study where subjects with bilateral cochlear implants have been
compared to normal hearing controls.
In addition investigators did not observe any recruitment of the visual cortices in our
bilaterally implanted subjects. This finding was also present in Strelnikov's study. However
previous work has suggested subjects with cochlear implants rely on visual clues to process
speech as a result of the degraded signal provided by the implant. It is possible that
because the subjects in this study and Strelnikov's study were experienced implant users and
benefited from binaural auditory stimulation, they were not dependant on audio-visual
integration to process speech.
There has been an increasing trend towards hearing preservation surgery for patients
undergoing cochlear implantation. Literature supports improved speech and pitch perception in
quiet and noise for subjects who combine the use of their implant with any residual preserved
natural hearing compared with using the implant alone. Once again the the underlying
mechanisms for this are unknown. It would be of scientific interest to investigate the
interaction between natural residual hearing and hearing via a cochlear implant. It is likely
that this cohort of subjects process speech similarly to those with normal hearing.
Finally, studies have monitored the speech progression of implantees from before surgery to
several years post implant switch on. It has been suggested that the predominant improvement
in speech perception occurs in the first year of implant use. This would suggest that the
neuronal networks responsible for speech processing are established very early on however,
investigators are not aware of any studies which have attempted to track the evolution of the
temporal cortices in new recipients of cochlear implants.
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