Modelling firing regularity in the ventral cochlear nucleus: Mechanisms, and effects of stimulus level and synaptopathy

Double-pass consistency for amplitude- and frequency-modulation detection in normal-hearing listeners

Amplitude modulation (AM) and frequency modulation (FM) provide crucial auditory information. If FM is encoded as AM, it should be possible to give a unified account of AM and FM perception both in terms of response consistency and performance. These two aspects of behavior were estimated for normal-hearing participants using a constant-stimuli, forced-choice detection task repeated twice with the same stimuli (double pass). Sinusoidal AM or FM with rates of 2 or 20 Hz were applied to a 500-Hz pure-tone carrier and presented at detection threshold. All stimuli were masked by a modulation noise. Percent agreement of responses across passes and percent-correct detection for the two passes were used to estimate consistency and performance, respectively. These data were simulated using a model implementing peripheral processes, a central modulation filterbank, an additive internal noise, and a template-matching device. Different levels of internal noise were required to reproduce AM and FM data, but a single level could account for the 2- and 20-Hz AM data. As for FM, two levels of internal noise were needed to account for detection at slow and fast rates. Finally, the level of internal noise yielding best predictions increased with the level of the modulation-noise masker. Overall, these results suggest that different sources of internal variability are involved for AM and FM detection at low audio frequencies.

Sensorineural hearing loss impairs sensitivity but spares temporal integration for detection of frequency modulation

The effect of the number of modulation cycles (N) on frequency-modulation (FM) detection thresholds (FMDTs) was measured with and without interfering amplitude modulation (AM) for hearing-impaired (HI) listeners, using a 500-Hz sinusoidal carrier and FM rates of 2 and 20 Hz. The data were compared with FMDTs for normal-hearing (NH) listeners and AM detection thresholds (AMDTs) for NH and HI listeners [Wallaert, Moore, and Lorenzi (2016). J. Acoust. Soc. 139, 3088-3096; Wallaert, Moore, Ewert, and Lorenzi (2017). J. Acoust. Soc. 141, 971-980]. FMDTs were higher for HI than for NH listeners, but the effect of increasing N was similar across groups. In contrast, AMDTs were lower and the effect of increasing N was greater for HI listeners than for NH listeners. A model of temporal-envelope processing based on a modulation filter-bank and a template-matching decision strategy accounted better for the FMDTs at 20 Hz than at 2 Hz for young NH listeners and predicted greater temporal integration of FM than observed for all groups. These results suggest that different mechanisms underlie AM and FM detection at low rates and that hearing loss impairs FM-detection mechanisms, but preserves the memory and decision processes responsible for temporal integration of FM.