Graeme Clark Collection
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ItemReduction in excitability of the auditory nerve following electrical simulation at high stimulus rates. II. Comparison of fixed amplitude with amplitude modulated stimuli( 1997)We have previously shown that acute electrical stimulation of the auditory nerve using charge-balanced biphasic current pulses presented continuously can lead to a prolonged decrement in auditory nerve excitability (Tykocinski et al., Hear. Res. 88 (1995), 124-142). This work also demonstrated a reduction in electrically evoked auditory brainstem response (EABR) amplitude decrement when using an otherwise equivalent pulse train with a 50% duty cycle. In the present study we have extended this work in order to compare the effects of electrical stimulation using both fixed amplitude electrical pulse trains and amplitude modulated (AM) pulse trains that more accurately model the dynamic stimulus paradigms used in cochlear implants. EABRs were recorded from guinea pigs following acute stimulation using AM trains of charge-balanced biphasic current pulses. The extent of stimulus-induced reductions in the EABR were compared with our previous results using either fixed amplitude continuous, or 50% duty cycle pulse trains operating at 0.34 µC/phase (2 mA, 170 µs/phase) at 400 or 1000 pulses/s (Tykocinski et al., Hear. Res. 88 (1995) 124-142). The AM pulse train, operating at the same rates, was based on a I-s sequence of the most extensively activated electrode of a Nucleus Mini-22 cochlear implant using the SPEAK speech processing strategy exposed to 4-talker babble, and delivered the same total charge as the fixed amplitude 50% duty cycle pulse train. Two hours of continuous stimulation induced a significant, rate-dependent reduction in auditory nerve excitability, and showed only a slight post-stimulus recovery for monitoring periods of up to 6 hours. Following 2 or 4 h of stimulation using an otherwise equivalent pulse train with a 50% duty cycle or the AM pulse train, significantly less reduction in the EABR was observed, and recovery to pre-stimulus levels was generally rapid and complete. These differences in the extent of the recovery between the continuous waveform and both the 50% duty cycle and AM waveforms were statistically significant for both 400 and 1000 pulses/s stimuli. Consistent with our previous results, the stimulus changes observed using AM pulse trains were rate dependent, with higher rate stimuli evoking more extensive stimulus-induced changes. The present findings show that while stimulus-induced reductions in neural excitability are dependent on the extent of stimulus-induced neuronal activity, the use of an AM stimulus paradigm further reduces post-stimulus neural fatigue.
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ItemReduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates( 1995)While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 µC/phase. Charge density was held constant (~ 75 µC cm^-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation, EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 µC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 µC cm^-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.
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ItemElectrophysiologic effects following acute intracochlear direct current stimulation of the guinea pig cochlea( 1995)Auditory brain stem responses to both acoustic (auditory brain stem response [ABR]) and electrical (electrically evoked auditory brain stem response [EABR]) stimuli, as well as the frequency-specific compound action potential (CAP), were recorded before and periodically following continuous intracochlear DC stimulation (2, 7, and 12 µA) for 2 hours in normal-hearing guinea pigs, by means of a banded intracochlear electrode array. Click-evoked ABR, frequency-specific CAP, and the EABR input-output function remained generally unchanged following stimulation at 2 µA DC. However, following stimulation at 7and 12 µA, a significant decrement of the amplitude of the click-evoked ABR, frequency-specific CAP, and electrophonic component of the EABR was observed, while there was an increase in the amplitude of the EABR, associated with direct electrical stimulation of the auditory nerve.
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ItemAcute effects of high-rate stimulation on auditory nerve function in guinea pigs( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Furthermore, a number of safety studies using the Melbourne/Cochlear electrode array indicated that chronic electrical stimulation using charge-balanced biphasic current pulses and stimulus rates between 100 and 500 pulses per second (pps) do not result in additional spiral ganglion loss or general cochlear pathology.1-3 However, safe maximum levels for stimulus parameters (stimulus rate, charge per phase, charge density) have not yet been adequately defined.
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ItemDevelopment of safe and effective electrodes: a risk management approach [Abstract]( 1999)Results from studies on experimental animals, computer modelling and preliminary psychophysical studies with three patients, have confirmed the potential for subjective improvement with electrode arrays which lie closer to the modiolus than does the Nucleus straight array. Results of psychophysical studies with three cochlear implant patients, using developmental pre-curved arrays, confirm the feasibility of improving patients' performance through improvements in electrode design. In particular, it was found in psychophysical tests, with patients using a developmental pre-curved electrode array, that both maximum comfortable level and threshold reduced with decreasing distance of a stimulated electrode from the modiolus, and that the dynamic range increased. More intense neural excitation patterns were obtained with the closer electrodes. From this it is inferred that the development of more sophisticated electrode arrays, positioned closer to the modiolus than is currently the case with the Nucleus standard array, will enable the development of improved speech processing strategies. There are technical constraints in the design of a peri-modiolar array, and currently a number of approaches to this problem have been investigated. Whilst the goal of the design is that it be effective for sophisticated and variable manners of stimulus delivery, a primary constraint is safety.
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ItemInsertion study using new peri-modiolar electrode array designs [Abstract]( 1999)Intracochlear multi-channel cochlear implants have been shown to successfully provide auditory information for profoundly deaf patients by electrically stimulating discrete populations of auditory nerve fibers via a scala tympani (ST) electrode array. Histological and radiological examination of implanted human temporal bones showed that the current straight Nucleus® array is usually positioned against the outer wall of the ST. An electrode array close to the modiolus could be expected to reduce stimulation thresholds and result in a more localized neural excitation pattern.
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ItemPhysiological and histopathological effects of chronic monopolar high rate stimulation of the auditory nerve [Abstract]( 1999)Speech processing strategies based on high rate electrical stimulation have been associated with improvements in speech perception among cochlear implant users. The present study was designed to evaluate the electrophysiological and histopathological effects of long-term intracochlear monopolar stimulation at the maximum stimulus rate of the current Nucleus Cochlear implant system (14493 pulses/s) as part of our ongoing investigations of safety issues associated with cochlear implants.
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ItemChronic electrical stimulation of the auditory nerve using high-surface area platinum electrodes [Abstract]( 1999)Electrical stimulation using high surface area (HiQ) platinum (Pt) electrodes exhibit a lower electrode impedance, polarisation and direct current (DC) in vitro compared with standard (ST) Pt electrodes of the same geometric size. In the present study we investigated whether HiQ electrodes maintain these advantages in vivo. This could be important for the development of new arrays with an increased number of smaller electrodes. Under general anaesthesia (Ketamine 20 mg/kg and Xylazine 4mg/kg i.p.) five normal hearing cats were implanted bilaterally with a two-channel Pt scala tympani electrode array (4 HiQ, I ST array). Chronic electrical stimulation using charge balanced biphasic current pulses was delivered unilaterally via a transcutaneous leadwire connected to a backpack-stimulator for periods of up to 2400 hours. DC, stimulus current and electrode voltage waveforms were monitored twice daily and access resistance (Ra), electrode impedance (Zc) and polarisation (Zc-Ra) calculated (kΩ). Mean HiQ data were compared to ST data using Students t-test (*=p<0.05, **=p<0.001). Immediately following implantation both HiQ and ST-electrodes exhibited low impedance values (Ra: 1.06 vs 1.00, Zc: 1.24 vs 2.12*, and Zc-Ra: 0.18 vs 1.08**). Subsequently impedance increased, largely due to a rise in Ra (5.35 vs 6.8: Zc: 6.96 vs 9.80, and Zc-Ra: 1.61 vs 3.00*). At the end of the experiment the array was explanted and tested in saline (Ra: 0.63 vs 0.74*, Zc: 0.73 vs 1.90**, Zc-Ra: 0.10 vs 1.16**). These initial results suggests that intracochlear tissue growth increased the access resistance of both electrode designs. However, HiQ electrodes maintained not only a significantly lower polarisation, they also showed a lower average residual DC (23 vs 130 nA**) throughout the experiment.
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ItemElectrical stimulation of the auditory nerve: chronic monopolar stimulation using very high stimulus rates [Abstract]( 1998)Speech processing strategies based on high rate electrical stimulation have been associated with recent improvements of speech perception among cochlear implant users. In the present study we investigated the effects of chronic monopolar stimulation using very high rates (14493 pulses\s). Under general anaesthesia (ketamine (20 mg/kg) and xylazine (3.8 mg/kg) i.p.) six normal hearing cats were implanted bilaterally with a three channel platinum (Pt) scala tympani electrode array, while a return Pt-electrode was placed outside the bulla. Chronic electrical stimulation using charge-balanced biphasic current pulses was delivered unilaterally via a transcutaneous leadwire connected to a backpack-stimulator for up to 2000 h. The animals hearing status was periodically monitored using acoustically evoked compound action potentials (CAP's) and brainstem responses (ABR's). In addition the electrically evoked ABR (EABR) was also recorded to ensure that the chronic stimulus was above threshold. Stimulus current and electrode voltage waveforms were monitored twice daily and access resistance (Ra) and electrode impedance (Zc) calculated. ABR and CAP thresholds were elevated immediately following implantation, but generally showed evidence of partial recovery (0-40 dB). Further deterioration of thresholds on the stimulated side (10-30 dB) was subsequently observed, while control-thresholds remained more stable. Ra (1.3-1.8 kΩ) and Zc (2.2-3.8Ω) typically increased in the first few weeks of electrical stimulation up to Ra:5.6 kΩ and Zc:8.1 kΩ, before decreasing slightly to a constant plateau. These initial results indicate changes in the electrode-tissue interface and tissue growth within the cochlea. They also indicate that chronic stimulation at these high rates may decrease residual hearing.
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ItemDecrement in auditory nerve function following acute high rate stimulation using various stimulus paradigms in guinea pigs [Abstract]( 1996)Previous experimental studies have shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) do not adversely affect the adjacent spiral ganglion population. More recently psychophysical trials have indicated that speech processing strategies based on high pulse rates (1000 pps or more) can improve speech perception in cochlea implant patients. In this paper we summarize the results following acute high rate stimulation using different stimulus paradigms.