Graeme Clark Collection
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ItemEffects of chronic electrical stimulation on cochlear nuclear neuron size in deaf kittens [Abstract]( 1996)It is now well recognized that normal afferent innervation is necessary for the development. This study investigated the effect of chronic electrical stimulation of the auditory nerve on the maturation of cochlear nucleus soma area of the neonatally deafened kittens. Eight kittens were deafened using kanamycin and ethacrynic acid, received a stimulated intracochlear implant in the left side and a dummy implant in right side, and classified two groups - the stimulated and the control group.
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ItemDecrement in auditory nerve function following acute high rate stimulation in guinea pigs [Abstract]( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Psychophysical studies suggested that speech processing strategies based on stimulus rates of up to 1000 pulses per second (pps) may lead to an improvement in speech perception, due to a better representation of the rapid variations in the amplitude of speech. However, "neural fatigue" has been known to occur following brief periods of electrical stimulation at rates high enough to ensure that stimuli occur within the neurons relative refractory period, and has been shown to depend on stimulus duration and rate of the evoked neural activity. Prolonged electrical stimulation at these high stimulus rates could, therefore, have an adverse effect on the neurons metabolism and result in cellular energy depletion.
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ItemMusical pitch perception by a user of the Nucleus 22-electrode cochlear implant [Abstract]( 1995)The study of how musical sounds are perceived by users of cochlear implants is both interesting and rewarding. In particular, it has the potential to provide detailed information on the perception of pitch with electrical stimulation of the auditory nerve. However, it is difficult to find subjects whose understanding of musical terminology and memory of conventional musical pitch relationships are adequate for this research. In our Melbourne laboratory, we have been fortunate to locate one implant user who received several years' formal training in the tuning of musical instruments before he lost his hearing. This subject is capable of estimating musical intervals in an open-set context, and can also adjust the interval between two "notes" to match a given target. Experiments involving both estimation and production of musical intervals have been carried out. Several parameters of the electrical stimulation have been varied to create notes of different pitches. These include the rate of steady pulse trains delivered to a fixed electrode position; the place of stimulation with a constant pulse rate; selected combinations of rate and place; and the frequency of sinusoidally amplitude-modulated (SAM) pulse trains delivered to a fixed electrode position. The results show that when pulse rate is the variable parameter, the rate ratio for a given musical interval closely approximates that for acoustic signals, provided that the electrode used is in the apical region of the cochlea. When the stimulation is moved to more-basal positions, the rate ratios become larger than the corresponding acoustic frequency ratios. Changing the place of stimulation with a constant pulse rate causes pitch changes that are comparable with the presumed place-to-characteristic frequency mapping of the subject's electrode array. Combinations of place and rate variation result in more complex effects, in which the pitch associated with the place of stimulation generally dominates, but is affected by the rate. The pitch of SAM pulse trains, although probably weaker than that of unmodulated low-rate pulse trains, is related to the modulation frequency. The ratio of modulation frequencies required for a given musical interval is larger than the corresponding ratio of acoustic frequencies in normal hearing.