Department of Linguistics
SHLRC Seminar
| Seminar Title: | The mechanisms of auditory neuropathy and the implications for cochlear implant fitting. |
| Presenter(s): | Dr Catherine McMahon |
| Time: | 1:00-2:00 pm, Monday 28th August 2006 |
| Place: | Linguistics Seminar Room, W5C 221 |
Auditory neuropathy (AN) is defined by normal cochlear mechanical activity but absent synchronous neural responses (measured using the auditory brainstem response, ABR). Despite this, most AN subjects show only a mild to moderate hearing loss (Starr et al., 2000). Although the term "auditory neuropathy" was coined almost 10 years ago (Starr et al., 1996), the physiological mechanisms underlying this remain unclear. This is largely because the measurement techniques used to diagnose AN are not sensitive enough to provide detailed information about hair-cell and residual neural currents generated in the cochlea (the site of disruption). Typically, broadband click stimuli are used which stimulates a broad region of the cochlea, thereby reducing neural and hair cell synchrony. Furthermore, resultant waveforms are measured with far-field scalp electrodes, reducing the amplitude (and possibly the integrity) of the waveform due to electrotonic decay.
We have used frequency-specific round window electrocochleography (ECoG) to measure local hair-cell and neural currents generated within the cochlea to more accurately assess the site-of-lesion in 14 children with AN and have compared these responses with those obtained in 2 normally-hearing children. As a cascade of electrical events precedes the generation of an action potential, we have used the presence (or absence) of hair-cell and neural potentials to differentially diagnose the site-of-lesion all subjects.
The results of this study demonstrate that, in the majority of cases, two patterns of ECoG waveforms can be identified. These indicate a pre-synaptic and a post-synaptic mechanism of auditory neuropathy exist. These results have important implications in the management of these children, particularly for determining the success of cochlear implantation (from a physiological perspective) which is increasingly being used in this population.