Graeme Clark Collection
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ItemIntrinsic connections of the rat cochlear nucleus [Abstract]( 1996)In mammals three subdivisions of the cochlear nucleus can be distinguished: the dorsal (DCN), the posterior (PVCN) and the anteroventral (AVCN) cochlear nucleus (CN) I. The intrinsic connections between and within these areas have not been well defined. Wickesberg et al.2 revealed that projections from DCN to AVCN in the mouse are frequency specific and tonotopic. In contrast Synder and Leake in the cat revealed projections from AVCN to PVCN and DCN but only modest projections from PVCN and DCN to AVCN with no frequency specificity observed. These previous studies utilized the retrograde tracer horseradish peroxidase. We investigated this apparent contradiction further, using neurobiotin, a retrograde and anterograde tracer, to examine connections within the cochlear nucleus of the rat, with emphasis on the AVCN subdivision.
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ItemIntracellular responses of anteroventral cochlear nucleus neurones to intracochlear electrical stimulation in the rat [Abstract]( 1996)The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. Currently little is known about the intracellular physiological responses of neurones in the AVCN to electrical stimulation of the cochlea. We investigated the effect of cochlear electrical stimulation in the rat AVCN using in vivo intracellular recordings. Male rats were anaesthetised with urethane (1.3g/kg i.p), placed in a stereotaxic frame, the crania and dura removed and the cochlear nucleus exposed.
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ItemLateral inhibition in ventral cochlear nucleus chopper neurons: contribution to coding of a speech feature [Abstract]( 2002)Lateral inhibition in the auditory system enhances excitatory responses by suppressing off-best frequency (BF) neural activity. Previous work has suggested that lateral inhibition activated by high frequency frication noise associated with stop consonant plays a role in coding voice onset time (VOT), the period between consonant release and onset of the ensuing vowel.
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ItemInhibition underlies the encoding of short voice onset times in the ventral cochlear nucleus( 2001)Recent experiments in our laboratory have shown that voice onset time (VOT), the time between consonant release and the first glottal pulse of an ensuing vowel, is effectively encoded by neurons within the ventral cochlear nucleus (VCN). In this investigation we examined the possible neural mechanisms which may underlie this VOT encoding. In male rats anaesthetised with urethane (2.5g1kg i.p), microelectrodes containing 1M potassium acetate, were inserted into the VCN. Speech stimuli consisting of 3 syllables spoken naturally by a male and female were presented at double rate and 3 intensities (/bεt/, /dεt/, and /gεt/ at 45, 65 and 75 dB SPL). Intracellular recordings were made in 12 neurons, eight of which had a response to pure tones typical of spherical bushy neurons, responding in a primary-like (PL) fashion. The remaining cells were classified as either globular bushy (n=2) or stellate cells (n=2). In PL neurons, the VOT period was associated with hyperpolarisation. The duration and amplitude of this hyperpolarising influence was greater for female speech. These PL units showed better encoding of VOT than other cell types in which hyperpolarisation was less evident and action > potentials were often evoked during this period at the highest intensity level. We propose that this hyperpolarisation is due to stimulation of inhibitory sidebands by the high frequency frication noise within the VOT period. This inhibition reduces the probability of action potential generation during the VOT period and enhances the salience of the voice onset enabling more effective encoding of VOT than seen in the auditory nerve.
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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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ItemBrainstem encoding of short voice onset times in natural speech( 2001)An auditory nerve study has shown that short voice onset times (VOTs) in synthetic consonant-vowel syllables are not accurately encoded by the fibres' discharge rate. We have re-examined this issue within the ventral Cochlear nucleus (VCN), using natural speech and a fine-grain analysis of single unit responses. We recorded extracellularly from 93 VCN neurons in rats anaesthetised with urethane (2.5 g/kg ip). After identifying a cell's response type and best frequency (BF), 3 syllables spoken by a male were presented at double rate and 3 intensities (/bεt/, /dεt/, and /gεt/, at 45, 65, and 75 dB SPL). These three syllables differ in their VOTs (the interval between consonant release and the onset of glottal pulses associated with voicing) due to the different points of articulation of the three initial stop consonants. In many neurons (particularly onset cells), these syllables evoked a clear response to consonant release, followed by an interval of inactivity or reduced activity before the periodic response to the vowel's voicing frequency commenced. This interval of reduced or no activity corresponded to a given syllable's VOT. The responses of all cells (BFs: 0.9-19 kHz) to the 9 different syllable-SPL combinations were plotted as Grand Average post-stimulus time histograms. In 8/9 combinations, syllable onset was associated with a statistically significant peak in activity and the next significant peak in discharge rate occurred at the time of voice onset (± I ms). These results indicate that the prominent responses to consonant release and voice onset, produced by the synchronous firing of neurons with a wide range of BFs, accurately encode short VOTs.
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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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ItemResponses of bushy cells to tones: implications for place and temporal sound coding( 2000)The coding of sound requires both temporal and spatial information. The relative importance of temporal and place coding in the ventral cochlear nucleus (YCN) has not been well established. In male rats anaesthetised with urethane (1.3g/kg i.p), microelectrodes containing 1M KAc, were inserted into the YCN. Intracellular recordings were made in 26 neurons which had an intracellular response to pure tones typical of spherical bushy neurons. In response to tones at characteristic frequency (CF) these neurons responded in a primary-like (PL) fashion. The intracellular response was associated with sustained depolarisation to tones presented at CF. Action potentials were usually followed by hyperpolarisation, although hyperpolarisation was still present in their absence.
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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).
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ItemIntracellular responses of onset neurones in the ventral cochlear nucleus to acoustic stimulation [Abstract]( 1998)The ventral cochlear nucleus (VCN) contains a heterogeneous collection of cell types reflecting the multiple processing tasks undertaken by this nucleus. This in vivo study used intracellular recordings and dye-filling to examine membrane potential changes and firing characteristics of onset neurones to acoustic stimulation (50 ms pure tones, 5 ms r/f time, 0.2 Hz repetition). Using rats anaesthetised with urethane (1.3g/kg i.p), microelectrodes containing 1M KAc and 4% neurobiotin, were inserted into VCN. Stable impalements were made from 11 onset neurones, seven identified as multipolar cells. Neurones responded to characteristic frequency (CF) tones with sustained depolarisation at below spike threshold. Increasing stimulus intensity increased and sharpened the depolarisation occurring in the initial 5 ms of the response from which an onset spike was generated. With the exception of tones presented at the high frequency edge of the cells response area, which resulted in depolarisation and spike at both the onset and offset of the stimulus, off CF tones resulted in a broadening of the initial depolarisation with high stimulus intensities required to initiate an onset spike. The onset spike latency in response to a given frequency decreased with increasing intensity. Presentation of tones off CF resulted in longer latencies. The spike onset latency depended on the rise time of the depolarisation with its latency remaining constant across tones which is consistent with monosynaptic excitation from auditory nerve. Depolarisation rise times decreased with increasing stimulus intensity and increased off CP. These results suggest that multipolar cells receive convergent input from auditory nerve enabling intensity and frequency to be coded through changes in membrane responsiveness.