E, Instantaneous filter outputs for the stimuli in A. D, Third-order Butterworth lowpass filter with f c/ f 1 = 0.54. C, Instantaneous normalized MET currents for the stimuli in A. B, First-order Boltzmann function relating the instantaneous stimulus pressure to the normalized MET current (or, equivalently, to the open probability of the MET channels), with M 0 = 0.20 and b = 2743 Pa −1. The 13 stimuli range from 30 to 78 dB SPL in steps of 4 dB. A, One cycle of the instantaneous pressure for each stimulus. Input, transfer function, and output of each stage of the model for one stimulus frequency and several stimulus levels. These findings highlight the significant impact of peripheral filtering mechanisms on phase locking.īoltzmann function auditory nerve lowpass filter modeling phase locking ribbon synapse. The estimated cutoff frequency varies with spontaneous rate, revealing a synaptic contribution to lowpass filtering. In addition to attenuating the AC component, the filter approximately recovers the sinusoidal waveform of the stimulus. This apparent gain control can be accounted for by a static saturating transducer function followed by a lowpass filter. Period histograms typically retain an approximately sinusoidal shape across stimulus levels, with the peripheral auditory system operating as though its overall transfer function is an exponential function whose slope decreases with increasing stimulus level. SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing. These findings advance our understanding of ANF phase locking by highlighting the role of peripheral filtering mechanisms in shaping responses of individual ANFs. Notably, the estimated cutoff frequency is lower for low- than for high-spontaneous-rate ANFs, implying a synapse-specific contribution to lowpass filtering. The model also accounts for changes in the maximum and minimum instantaneous spike rates with changes in stimulus level. Using responses to tones of ANFs from cats of both sexes, we show that, for a given ANF, the period histograms obtained at all stimulus levels for a given stimulus frequency can be described using one set of level-independent model parameters. Here we show that these findings can be accounted for by a model consisting of a static Boltzmann transducer function yielding a clipped output, followed by a lowpass filter and a static exponential transfer function. The mechanism underlying this apparent gain control is unclear but is distinct from mechanical compression, is independent of refractoriness and spike-rate adaptation, and is apparently instantaneous. The operating points and slopes of these functions change with stimulus level. Previous work has shown that phase-locked period histograms are often well described by exponential transfer functions relating instantaneous stimulus pressure to instantaneous spike rate, with no observed clipping of the histograms. Phase locking of auditory-nerve-fiber (ANF) responses to the temporal fine structure of acoustic stimuli, a hallmark of the auditory system's temporal precision, is important for many aspects of hearing.
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