Otolaryngology - Theses
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ItemTemporal processing in the deafened auditory cortex( 2012)The cochlear implant (CI) has proven highly successful in treating sensorineural hearing loss, however we remain unable to explain much of the variability in recipient outcomes. Studies of language impairment in normal hearing, in addition to more limited studies in CI recipients, suggest that the temporal processing capacity of the central auditory pathway may be a limiting factor in some CI users. In particular, the detection of amplitude modulation (AM) appears to be a good predictor of speech perception ability. The central aim of this thesis is to better understand the neural correlates of AM ICES detection, and identify what changes occur following deafness and chronic ICES experience. We recorded multiunit neural responses in the primary auditory cortex (AI) of anaesthetised rats that were either acutely (n=4) or long-term deaf (n=4) from adulthood. While previous reports show that temporal processing declines following neonatal long-term deafness, we found that the representation of AM intracochlear electrical stimulation (ICES) is maintained, or even improves, following adult-onset long-term deafness. We trialed a novel fully-implantable stimulator in seven animals, which was capable of chronically delivering AM ICES. While the stimulator itself generally proved reliable, functioning for up to five months in vivo, the intracochlear electrode array failed in all but one animal. The effects of chronic ICES in that subject were similar to that of LT deafness, although there was some evidence that phase locking was enhanced at high modulation frequencies, which justifies further study. Using a spike train classifier, we explored the temporal resolution necessary to detect AM ICES in the acutely-deafened auditory cortex. Modulated stimuli could be readily detected using both spike timing and rate. These results suggest that AM detection remains robust following deafness, and is supported by multiple redundant neural codes, reinforcing the notion that envelope information is privileged by the auditory system in order to support speech perception.