Graeme Clark Collection
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ItemThe relationship between the output synchrony of cochlear nucleus neurons and the site of stimulation in the cochlea( 2001)A model has been developed to determine the relationship between the output synchrony of cochlear nucleus neurons and the site of stimulation in the cochlea. This is an Integrate and Fire Neuron Model in which noisy periodic synaptic inputs to the neuron are summed and a spike is generated when the membrane potential reaches threshold. The model describes the stochastic input that auditory nerve fibres provide to a cochlear nucleus neuron and the corresponding stochastic output. To investigate the relationship between the output synchrony of cochlear nucleus neurons (namely globular bushy cells) and the site of stimulation in the cochlea, phase differences between the periodic inputs of the model were incorporated, in order to mimic how the travelling wave consecutively activates auditory nerve fibres originating over a spatial spread of the basilar membrane. Analysis of the model found that output synchrony decreased with an increase in frequency and spatial spread. Furthermore, enhancement of the output synchrony relative to the input synchrony occurred for small spatial spreads of the basilar membrane over which input primary afferent fibres originate. Adding noise helped to make the model more realistic. As a result enhancement of synchrony occurred with a spatial spread of less than 1.25 mm and 0.75 mm for 0.5 kHz and I kHz respectively, while for the higher frequencies analysed (2 kHz and 5 kHz) enhancement of synchrony did not occur. This research has implications for the design of electrode arrays in cochlear implants. The number and geometry of the electrodes and the stimulus patterns to be used will depend on the degree of convergence of fibres and how phase information is processed by neurons in the brainstem.
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ItemEffects of sensorineural hearing loss on the refractory properties of auditory nerve fibers( 2000)We hypothesised that the loss of the peripheral processes and the partial demyelination of auditory nerve fibres (ANFs) following a sensorineural hearing loss would increase their refractory properties. Normal control, and long-term (2.5 months) systemically deafened rats were anaesthetised (urethane, 1.3 g/kg i.p.), a bipolar stimulating electrode was implanted into the scala tympani and glass microelectrodes (30-80 MΩ) used to record single ANF activity. Stimuli (pairs of 100 µs/phase charge balanced biphasic pulses with interpulse intervals (IPIs) of 0.34-10 ms) were presented at 6 dB above threshold using a repetition interval of 250 ms. Absolute refractory period (ARP) was defined as the IPI at which the probability of eliciting a spike to the second stimulus was 0.1. In the present results, based on recordings from 62 fibres, ANFs were distinguished from cochlear nucleus (CN) neurones by their significantly shorter median latencies (AN: 0.575ms vs CN: 1.137ms; Whitney-Mann Rank Sum, p<0.0001). There were no significant differences between minimum ANF latencies from normal and deafened animals. Although the median ARP was greater in deafened versus normal animals, this difference was not statistically significant (normals: median0.658ms, interquartile range 0.554-0.913ms; deafened: 0.772ms and 0.616-1.073ms; p=0.16). Finally, the spike latency associated with the second pulse of a pair systematically increased with decreasing IPI, contrasting with the stable latency of the response to the leading pulse. Although pathological changes to ANFs may increase their refractory properties, at this duration of deafness these changes were not significant.
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ItemDelay analysis in an investigation of auditory temporal coding( 2000)Delay analysis is a method for analysing phase-locked responses to periodic stimuli which is widely used in the study of auditory cells, as it provides an estimate of the delay present in a system from steady-state data. While the usual formulation utilises the assumption that the delay is constant across frequencies, in the auditory system delay varies with frequency. In this paper two new formulations of delay analysis are introduced, and are applied to the analysis of auditory temporal coding. In rats anaesthetised with urethane (1.3g/kg i.p.), in vivo extracellular recordings were made in the auditory nerve, cochlear nucleus and trapezoid body using glass microelectrodes filled with 1M potassium acetate (50-70MΩ).