Cortical Auditory Evoked Potentials and the neural processing of speech stimuli in cochlear implant users

Abstract

With the current implementation of newborn hearing screening in New Zealand, rising numbers of prelingual children receiving hearing aids and cochlear implants are expected. An objective measure for speech perception would be invaluable clinically for these children and other difficult-to-test populations. Cortical Auditory Evoked Potentials (CAEPs) offer insight into detection and processing in the cortices, and the potential for evaluation of hearing aids and cochlear implants (Dillon, 2005; Kelly, Purdy, & Thorne, 2005). Thus CAEPs from speech stimuli have been investigated (Agung, Purdy, McMahon, & Newall, 2006), with the hope that it can be useful in the assessment of speech detection and processing abilities. The current study recorded CAEPs in response to /m/, /g/, and /t/, spanning the speech frequency spectrum, in adults (part I) and children (part II) from the New Zealand Northern Cochlear Implant programme. The aim was to investigate differences in the neural processing of these phonemes, and their relationship with speech perception tests. The change CAEPs were recorded in the children as they adapted to their implant over 3 months. Repeated measures analyses explored the interactions between effects of stimuli, speech perception, and adaptation to the implant over time, as a function of chronological age and CI experience. Furthermore, CAEPs were recorded with two evoked potential systems to compare artefact minimisation strategies. Different CAEPs were observed across stimuli for N1 waveforms in the adults and P1 waveforms in the children. The CAEPs for low frequency /m/ was later and larger than the high frequency /t/ stimulus, suggesting neural differentiation between the most spectrally diverse phonemes. Longer P2 latency was correlated with better speech perception, and this warrants further investigation of the relationship between behavioural phoneme discrimination and P2. CAEPs changes over time appear to be larger in younger children, for the high frequency /t/ stimulus. This is supportive of the sensitive period of maximal plasticity and the urgency in early intervention. Future research should consider that age of implantation was joined by other interacting factors such as length of auditory deprivation, deafness characteristics and CI experience in determining P1 maturation.