Influence of fine structure and envelope variability on gap-duration discrimination thresholds

The goal of the study was to investigate whether the temporal resolution of the auditory system is influenced by the variability of the stimulus envelope. To do so, the ability to detect an increment in the duration of a temporal gap (the test gap) was measured with an adaptive 3-IFC procedure. The stimulus consisted of a series of 10-ms broadband noise pulses. The pulses were separated by a 10-ms silent period, or temporal gap. In the main experiments, the test gap was either the first or the last gap in a series of 21 pulses. The variability in the stimulus' envelope was controlled directly by applying a jitter to the onset of the individual pulses in the pulse trains. Additionally, the stimuli were presented with different fine structure variabilities which also induced differences in the variability of the envelope. The gap-discrimination thresholds for the jittered noise pulse trains showed strong dependence on the amount of jitter as long as the jitter was applied randomly leading to a different pattern for every stimulus. When the jitter was applied as a frozen jitter resulting in a constant pattern of pulses, the thresholds did not increase significantly. A similar result was obtained for the different fine structure variabilities. A frozen fine structure led to thresholds about 1 ms lower than those obtained with random noise stimuli. A measure for the envelope variability was provided by calculating the variances of the envelope spectrum of the gammatone-filtered stimuli. The results of the calculations show a qualitative correspondence to the experimental results

Managing and resisting ‘degeneration’ in employee-owned businesses: a comparative study of two large retailers in Spain and the UK

Employee-owned businesses have recently enjoyed a resurgence of interest as possible ‘alternatives’ to the somewhat tarnished image of conventional investor-owned capitalist firms. Within the context of global economic crisis, such alternatives seem newly attractive. This is somewhat ironic because, for more than a century, academic literature on employee-owned businesses has been dominated by the ‘degeneration thesis’. This suggested that these businesses tend towards failure – they either fail commercially, or they relinquish their democratic characters. Bucking this trend and offering a beacon - especially in the UK - has been the commercially successful, co-owned enterprise of the John Lewis Partnership (JLP) whose virtues have seemingly been rewarded with favourable and sustainable outcomes. This paper makes comparisons between JLP and its Spanish equivalent Eroski – the supermarket group which is part of the Mondragon cooperatives. The contribution of this paper is to examine in a comparative way how the managers in JLP and Eroski have constructed and accomplished their alternative scenarios. Using longitudinal data and detailed interviews with senior managers in both enterprises it explores the ways in which two large, employee-owned, enterprises reconcile apparently conflicting principles and objectives. The paper thus puts some new flesh on the ‘regeneration thesis’

Synaptotagmin IV determines the linear Ca2+ dependence of vesicle fusion at auditory ribbon synapses

Mammalian cochlear inner hair cells (IHCs) are specialized for the dynamic coding of continuous and finely graded sound signals. This ability is largely conferred by the linear Ca2+ dependence of neurotransmitter release at their synapses, which is also a feature of visual and olfactory systems. The prevailing hypothesis is that linearity in IHCs occurs through a developmental change in the Ca2+ sensitivity of synaptic vesicle fusion from the nonlinear (high order) Ca2+ dependence of immature spiking cells. However, the nature of the Ca2+ sensor(s) of vesicle fusion at hair cell synapses is unknown. We found that synaptotagmin IV was essential for establishing the linear exocytotic Ca2+ dependence in adult rodent IHCs and immature outer hair cells. Moreover, the expression of the hitherto undetected synaptotagmins I and II correlated with a high-order Ca2+ dependence in IHCs. We propose that the differential expression of synaptotagmins determines the characteristic Ca2+ sensitivity of vesicle fusion at hair cell synapses

Tonotopic variation in the calcium dependence of neurotransmitter release and vesicle pool replenishment at mammalian auditory ribbon synapses

The mammalian cochlea is specialized to recognize and process complex auditory signals with remarkable acuity and temporal precision over a wide frequency range. The quality of the information relayed to the auditory afferent fibers mainly depends on the transfer characteristics of inner hair cell (IHC) ribbon synapses. To investigate the biophysical properties of the synaptic machinery, we measured changes in membrane capacitance (ACm) in low-frequency (apical region, - 300 Hz) and high-frequency (basal, - 30 kHz) gerbil IHCs maintained in near physiological conditions (1.3mm extracellular Ca2+ and body temperature). With maturation, the Ca2+ efficiency of exocytosis improved in both apical and basal IHCs and was more pronounced in the latter. Prehearing IHCs showed a similar Ca2+ cooperativity of exocytosis despite the smaller ACm in apical cells. After maturation, ACm in high-frequency IHCs increased linearly with the Ca2+ current, whereas, somewhat surprisingly, the relationship was significantly more nonlinear in low-frequency cells. This tonotopic difference seemed to be correlated with ribbon synapse morphology (spherical in apical and ellipsoid in basal IHCs) but not with the expression level of the proposed Ca2+ sensor otoferlin or the spatial coupling between Ca2+ channels and active zones. Repetitive stimulation of adult IHCs showed that vesicle pool refilling could become rate limiting for vesicle release, with high-frequency IHCs able to sustain greater release rates. Together, our findings provide the first evidence for a tonotopic difference in the properties of the synaptic machinery in mammalian IHCs, which could be essential for fine-tuning their receptor characteristics during sound stimulation

Influence of fine structure and envelope variability on gap-duration discrimination thresholds

The goal of the study was to investigate whether the temporal resolution of the auditory system is influenced by the variability of the stimulus envelope. To do so, the ability to detect an increment in the duration of a temporal gap (the test gap) was measured with an adaptive 3-IFC procedure. The stimulus consisted of a series of 10-ms broadband noise pulses. The pulses were separated by a 10-ms silent period, or temporal gap. In the main experiments, the test gap was either the first or the last gap in a series of 21 pulses. The variability in the stimulus' envelope was controlled directly by applying a jitter to the onset of the individual pulses in the pulse trains. Additionally, the stimuli were presented with different fine structure variabilities which also induced differences in the variability of the envelope. The gap-discrimination thresholds for the jittered noise pulse trains showed strong dependence on the amount of jitter as long as the jitter was applied randomly leading to a different pattern for every stimulus. When the jitter was applied as a frozen jitter resulting in a constant pattern of pulses, the thresholds did not increase significantly. A similar result was obtained for the different fine structure variabilities. A frozen fine structure led to thresholds about 1 ms lower than those obtained with random noise stimuli. A measure for the envelope variability was provided by calculating the variances of the envelope spectrum of the gammatone-filtered stimuli. The results of the calculations show a qualitative correspondence to the experimental results