Martin J. Spencer

Broadband Onset Inhibition Can Suppress Spectral Splatter in the Auditory Brainstem

In vivo intracellular responses to auditory stimuli revealed that, in a particular population of cells of the ventral nucleus of the lateral lemniscus (VNLL) of rats, fast inhibition occurred before the first action potential. These experimental data were used to constrain a leaky integrate-and-fire (LIF) model of the neurons in this circuit. The post-synaptic potentials of the VNLL cell population were characterized using a method of triggered averaging. Analysis suggested that these inhibited VNLL cells produce action potentials in response to a particular magnitude of the rate of change of their membrane potential. The LIF model was modified to incorporate the VNLL cells’ distinctive action potential production mechanism. The model was used to explore the response of the population of VNLL cells to simple speech-like sounds. These sounds consisted of a simple tone modulated by a saw tooth with exponential decays, similar to glottal pulses that are the repeated impulses seen in vocalizations. It was found that the harmonic component of the sound was enhanced in the VNLL cell population when compared to a population of auditory nerve fibers. This was because the broadband onset noise, also termed spectral splatter, was suppressed by the fast onset inhibition. This mechanism has the potential to greatly improve the clarity of the representation of the harmonic content of certain kinds of natural sounds.

An investigation of dendritic delay in octopus cells of the mammalian cochlear nucleus.

Octopus cells, located in the mammalian auditory brainstem, receive their excitatory synaptic input exclusively from auditory nerve fibers (ANFs). They respond with accurately timed spikes but are broadly tuned for sound frequency. Since the representation of information in the auditory nerve is well understood, it is possible to pose a number of questions about the relationship between the intrinsic electrophysiology, dendritic morphology, synaptic connectivity, and the ultimate functional role of octopus cells in the brainstem. This study employed a multi-compartmental Hodgkin-Huxley model to determine whether dendritic delay in octopus cells improves synaptic input coincidence detection in octopus cells by compensating for the cochlear traveling wave delay. The propagation time of post-synaptic potentials from synapse to soma was investigated. We found that the total dendritic delay was approximately 0.275 ms. It was observed that low-threshold potassium channels in the dendrites reduce the amplitude dependence of the dendritic delay of post-synaptic potentials. As our hypothesis predicted, the model was most sensitive to acoustic onset events, such as the glottal pulses in speech when the synaptic inputs were arranged such that the models dendritic delay compensated for the cochlear traveling wave delay across the ANFs. The range of sound frequency input from ANFs was also investigated. The results suggested that input to octopus cells is dominated by high frequency ANFs.

LOCO at the Australian Synchrotron

LOCO has been used during the commissioning of the Australian Synchrotron storage ring with a number of benefits. The LOCO (linear optics from close orbits) method compares a model response matrix to the real machine response matrix. Using this approach we are able to adjust the machine to match the ideal model. Results presented here show that LOCO has provided a high degree of control over a wide range of machine parameters.

Storage ring turn-by-turn BPMS at the Australian Synchrotron

The Australian synchrotrons Storage Ring is equipped with a full compliment of 98 Libera electron beam position processors from I-tech (EBPPs) [1]. The EBPPs are capable of measuring beam position data at turn-by-turn (TBT) rates and have long history buffers. TBT data from the EBPPs has been used to determine the linear optics of the storage ring lattice using techniques developed at other facilities. This is a useful complement to other methods of determining the linear optics such as LOCO. Characteristics of the EBPPs such as beam current dependence have been studied during commissioning and will also be presented.

Final commissioning results from the injection system for the Australian Synchrotron project

Danfysik has delivered a full-energy turn-key injection system for the Australian Synchrotron. The system consists of a 100 MeV linac, a low-energy transfer beamline, a 130 m circumference 3-GeV booster, and a high energy transfer beamline. The booster lattice was designed to have many cells with combined-function magnets (dipole, quadrupole and sextupole fields) in order to reach a very small emittance. The injection system has been commissioned and shown to deliver a beam with an emittance of less than 30 nm, and currents in single- and multi-bunch mode in excess of 0.5 and 5 mA, respectively, fulfilling the performance specifications. The repetition frequency is 1 Hz. Results from the commissioning of the system will be presented.

Lifetime contribution measurements at the Australian Synchrotron

There are always a number of factors that contribute to the lifetime of a stored particle beam. Measurements presented here show the relative importance of these effects during the commissioning of the Australian synchrotron storage ring.

Measurements of impedance and beam instabilities at the Australian Synchrotron

In this paper we present the first measurements of machine impedance and observed beam instabilities at the Australian Synchrotron. Impedance measurements are made by studying the single bunch behaviour with beam current, using optical and X-ray diagnostic beamlines. An observed coupled-bunch instability, its cause and cure is also discussed.

Spiking models of interaural level difference encoding - beyond the rate subtraction code

Interaural Level Difference (ILD) provides an important cue for the location of a sound source in the azimuthal plane. Typically, ILD decoding in the brainstem is modeled as a subtraction of spike rates, with inhibitory inputs from one ear subtracted from the excitatory inputs from the other [1-3]. The inferior colliculus (IC) is known to receive input from this circuit, and to encode the spatial location of sounds. Recent experimental evidence suggests that inhibitory input for ILD doesn’t provide subtraction, but instead provides a gain adjustment [4]. In addition, the exact mechanism of the creation of spatial receptive fields in the inferior colliculus remains unclear, and may also be a gain mechanism [5]. The excitatory input to the IC from neurons that decode ILD may contain spike timing cues for location. These spike-timing cues may be initiated in the ILD encoding cells even if the cues are absent from the inputs from the cochlear nucleus. In this study we used a spiking neuron model to recreate and model the full circuit of ILD sensitivity, and explore both the issue of ILD decoding, and the representation of sound source location in the IC. The auditory periphery was modeled as a gammatone filterbank which provided inputs directly to a leaky integrate-and-fire model representing the cells of the cochlear nucleus. These cells are known to lock to the envelope of the sound stimulus, and this behavior was recreated by low-pass filtering of the gammatone filterbank inputs to the cells, and use of a dynamic spike threshold mechanism [6]. The ILD sensitive cells and IC cells were both modeled as simple leaky integrate-and-fire neurons. The model was able to recreate important experimental results regarding ILD encoding cells, particularly the variation of sensitivity with source intensity, and successfully created spatial receptive fields like those found in the IC. The results will be helpful in the future understanding of the binaural mechanisms of the auditory brainstem.