Graeme Clark Collection
Permanent URI for this collection
Search Results
1 - 8 of 8
-
ItemElectrical stimulus induced changes in excitability of the auditory nerve( 1997)High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.
-
ItemEvoked responses in the auditory cortex of the congenitally deaf white cat following electrical stimulation of the cochlea [Abstract]( 1995)Knowledge of the functional status of central auditory structures is important when estimating possible benefits of cochlear implantation in the congenitally deaf. Thus far the performance of prelingually deaf adults following cochlear implantation has been disappointing. We have used congenitally deaf cats as a model for prelingual deafness. These animals are deaf from an early age as shown by longitudinal recordings of auditory brainstem responses. They were studied as adults (age 2 years). Under general anaesthesia the cochleae were electrically stimulated using the NUCLEUS-22 banded scala tympani electrode array. Recordings were made from the contralateral auditory cortex and inferior colliculus. Gross potentials, together with multi� and single-unit activities were recorded. Here we confine ourselves to gross potential recordings from the auditory cortex. The skull was opened over the auditory area and the cortex photographed. A computer-controlled 3-axes microdrive provided precise and reproducible positioning of the monopolar recording electrode. Gross potentials were evoked by electrical stimulation of the auditory nerve using bipolar electrodes 1/2, 7/8 or 1/8 (electrode 1 being the most apical). These potentials were recorded from both the cortical surface and at depths of up to 4 mm, amplified and band pass filtered (10 Hz to 10 kHz). The stimuli (0.2 ms biphasic pulses) evoked middle latency responses (10 - 20 ms) over the primary and secondary auditory areas. Thresholds were lowest using electrodes 1/8 (-24 dB re. 1 mApp). Narrower electrode configurations (1/2 and 7/8) were up to 15 dB less effective. The potentials evoked were mono-, bi- or triphasic in shape, depending on recording site. We observed little evidence of tonotopic cortical mapping of stimulation site (1/2 vs. 7/8). If present at all, potentials were considerably smaller when the recording electrode was placed outside the auditory areas. Moreover, threshold currents were far higher (40 dB). It is concluded that the auditory cortex of congenitally deaf animals receives specific information via stimulation of the auditory nerve.
-
ItemInvestigation of curved intracochlear electrode arrays [Abstract]( 1992)It has been demonstrated that the Melbourne/Cochlear multi-channel cochlear implant is safe and effective for use in profoundly-totally deaf patients. Recent studies have highlighted the importance of deaf insertion and placing the electrodes closer to the spiral ganglion neurons. In order to improve the electrode insertion depth and proximity to the modiolus, we have investigated curved electrode arrays. Prototypes of such arrays and their accessory inserter have been made. Trial insertions were performed on skeletonized cochleae of human temporal bones. The preliminary results showed that, when compared with conventional straight electrode arrays, the curved arrays could be inserted deeper and located closer to the modiolus. These findings indicate that the curved --.~ electrodes currently under investigation should result in a reduction in stimulus threshold and improve pitch perception and may also result in the use of more channels of stimulation.
-
ItemElectrical stimulation of the auditory nerve: comparison of half-band with full-band scala tympani bipolar electrodes( 1993)The Melbourne/Cochlear auditory prosthesis uses an intracochlear electrode array containing 22 circumferential full-band electrodes mounted on a Silastic carrier. It could be hypothesized that half-band electrodes, oriented towards the modiolus, would produce lower stimulus thresholds than conventional full-band electrodes. This hypothesis is based on the assumption that, compared with full-band electrodes, half-band electrodes would produce an electrical field in which a greater proportion of the current would excite a defined group of neurons. In order to verify this hypothesis we recorded electrically evoked auditory brainstem responses (EABRs) for both full- and half-band electrodes inserted in the scala tympani of deafened cats. EABR thresholds for half-band electrodes oriented towards the modiolus were not significantly different from thresholds evoked using full-band electrodes (p>0.05, paired t-test), whereas thresholds evoked using half-band electrodes oriented towards the outer scala wall were significantly higher (p<0.01) than either the modiolar half-band or the full-band electrodes. These physiological results suggest that the electrical field generated within the auditory nerve by modiolar oriented half-band electrodes does not differ significantly from that produced by full-band electrodes. On the basis of these results, together with the fact that half-band electrodes would have higher current densities and electrode impedances, and would require careful orientation during implantation, we consider that there is no benefit in incorporating half-band electrodes in the design of scala tympani electrode arrays.
-
ItemThe origin of electrophonic activity evoked by electrical stimulation of the cochlea( 1992)Electrophonic activation of auditory nerve fibres via electrical stimulation is only observed in cochleas with residual hair cells. While the generation of neural activity associated with this phenomenon is thought to ultimately occur at the inner hair cell synapse (1) it is unclear whether hair cells are activated directly by the electrical stimulus or mechanically via a travelling wave propagating along the basilar membrane. Support for the travelling wave hypothesis has recently come from a masking study using evoked potentials (2). To provide verification of these results, we measured the latency of the electrophonic activity recorded in single ventral cochlear nucleus (VCN) units of known characteristic frequency (CF). Stimulating electrodes were placed on, or just inside the round window of normal hearing anaesthetized cats (n=6). The response of single VCN units were recorded extracellularly and units exhibiting "primary like" activity (3) were analysed. Each unit's CF to acoustic stimulation and response properties to biphasic electric pulses were determined. Electrophonic activity was identified by its long latency (> 2.5 ms) and poor synchronization compared with the response evoked by direct electrical stimulation. Electrophonic activity was observed in 20 units -approximately 25% of the units isolated. These responses were more commonly recorded from cochleas in which the round window had not been opened. The latency of the electrophonic response varied inversely with CF, implying that the response is generated at the basilar membrane and results in a mechanical travelling wave. Finally, no electrophonic activity was observed in units with CFs greater than 10 kHz. Our data would predict that the latency of electrophonic activity in these units -if present -would be very short. Presumably its absence is a result of refractoriness within auditory nerve fibres following activity evoked by direct electrical stimulation.
-
ItemCochleotopic selectivity of the Melbourne/Cochlear multichannel scala tympani electrode array revealed using the 2-DG technique [Abstract]( 1991)Direct electrical stimulation of the cochlea has been used to provide auditory cues to deaf patients. Stimulating devices range from single channel implants located on the round window to multichannel scala tympani electrode arrays. This study investigated the cochleotopic selectivity of the multichannel bipolar Melbourne/Cochlear scala tympani electrode array using the 2-Deoxyglucose (2-DG) technique.
-
ItemChronic electrical stimulation of the auditory nerve in normal hearing kittens [Abstract]( 1991)Multichannel cochlear prostheses selectively stimulate discrete populations of residual auditory nerve fibres in order to provide profoundly deaf patients with pitch and temporal cues for speech discrimination. An important requirement for the success of these devices is that long-term intracochlear electrical stimulation must not have adverse effects on the spiral ganglion cell population or the cochlea in general. We have previously shown that chronic stimulation using charge balanced biphasic current pulses is safe in normal hearing adult animals (Shepherd et al., 1983). The present study was designed to examine the effects of chronic stimulation in the cochleas of young animals.
-
ItemChronic electrical stimulation of the auditory nerve in cats( 1982)One requirement for the success of a cochlear hearing prosthesis is that long-term electrical stimulation must not have adverse effects on the residual spiral ganglion cell population. Electrochemically 'safe' stimulation regimes have been defined for the cortex (Brummer &Turner, 1977). However, few investigators have examined the effects of long-term intracochlear electrical stimulation. Walsh et al (1980), stimulating with current densities greater than the 'safe' limits defined by Brummer &Turner (1977), for periods of up to 800 hours at current levels of 4.0-8.0 mA, recorded slight local neural degeneration adjacent to the electrodes.