Absolute auditory threshold: testing the absolute

Temporal integration of infrasound at threshold

Infrasounds are signals with frequencies below the classical audio-frequency range, i.e., below 20 Hz. Several previous studies have shown that infrasound is audible as well, provided that the sound level is high enough. Hence, the sound pressure levels at threshold are much higher than those in the classical audio-frequency range. The present study investigates how the duration and the shape of the temporal envelope affect thresholds of infrasound stimuli in quiet. Two envelope types were considered: one where the duration of the steady state was varied (plateau bursts) and one where the number of consecutive onset-offset bursts was varied (multiple bursts). Stimuli were presented monaurally to human listeners by means of a low-distortion sound reproduction system. For both envelope types, thresholds decrease with increasing duration, a phenomenon often referred to as temporal integration. At the same duration, thresholds for plateau-burst stimuli are typically lower than those for multiple-burst stimuli. The data are well described by a slightly modified version of a model that was previously developed to account for temporal integration in the classical audio-frequency range. The results suggest similar mechanisms underlying the detection of stimuli with frequencies in the infrasound and in the classical audio-frequency range. Since the model accounts for the effect of duration and, more generally, the shape of the envelope, it can be used to enhance the comparability of existing and future datasets of thresholds for infrasounds with different temporal stimulus parameters.

How to define thresholds for level and interaural-level-difference discrimination: Insights from scedasticities and distributions

Sensitivity to changes in the stimulus level at one or at both ears and to changes in the interaural level difference (ILD) between the two ears has been studied widely. Several different definitions of threshold and, for one of them, two different ways of averaging single-listener thresholds have been used (i.e., arithmetically and geometrically), but it is unclear which definition and which way of averaging is most suitable. Here, we addressed this issue by examining which of the differently defined thresholds yielded the highest degree of homoscedasticity (homogeneity of the variance). We also examined how closely the differently defined thresholds followed the normal distribution. We measured thresholds from a large number of human listeners as a function of stimulus duration in six experimental conditions, using an adaptive two-alternative forced-choice paradigm. Thresholds defined as the logarithm of the ratio of the intensities or amplitudes of the target and the reference stimulus (i.e., as the difference in their levels or ILDs; the most commonly used definition) were clearly heteroscedastic. Log-transformation of these latter thresholds, as sometimes performed, did not result in homoscedasticity. Thresholds defined as the logarithm of the Weber fraction for stimulus intensity and thresholds defined as the logarithm of the Weber fraction for stimulus amplitude (the most rarely used definition) were consistent with homoscedasticity, but the latter were closer to the ideal case. Thresholds defined as the logarithm of the Weber fraction for stimulus amplitude also followed the normal distribution most closely. The discrimination thresholds should therefore be expressed as the logarithm of the Weber fraction for stimulus amplitude and be averaged arithmetically across listeners. Other implications are discussed, and the obtained differences between the thresholds in different conditions are compared to the literature.

Towards a unifying basis of auditory thresholds: Thresholds for multicomponent stimuli

Sounds consisting of multiple simultaneous or consecutive components can be detected by listeners when the stimulus levels of the components are lower than those needed to detect the individual components alone. The mechanisms underlying such spectral, spectrotemporal, temporal, or across-ear integration are not completely understood. Here, we report threshold measurements from human subjects for multicomponent stimuli (tone complexes, tone sequences, diotic or dichotic tones) and for their individual sinusoidal components in quiet. We examine whether the data are compatible with the detection model developed by Heil, Matysiak, and Neubauer (HMN model) to account for temporal integration (Heil et al. 2017), and we compare its performance to that of the statistical summation model (Green 1958), the model commonly used to account for spectral and spectrotemporal integration. In addition, we compare the performance of both models with respect to previously published thresholds for sequences of identical tones and for diotic tones. The HMN model is similar to the statistical summation model but is based on the assumption that the decision variable is a number of sensory events generated by the components via independent Poisson point processes. The rate of events is low without stimulation and increases with stimulation. The increase is proportional to the time-varying amplitude envelope of the bandpass-filtered component(s) raised to an exponent of 3. For an ideal observer, the decision variable is the sum of the events from all channels carrying information, for as long as they carry information. We find that the HMN model provides a better account of the thresholds for multicomponent stimuli than the statistical summation model, and it offers a unifying account of spectral, spectrotemporal, temporal, and across-ear integration at threshold.

Comparing and modeling absolute auditory thresholds in an alternative-forced-choice and a yes-no procedure

Detecting sounds in quiet is arguably the simplest task performed by an auditory system, but the underlying mechanisms are still a matter of debate. Threshold stimulus levels depend not only on the physical properties of the sounds to be detected but also on the experimental procedure used to measure them. Here, thresholds of human subjects were measured for sounds consisting of different numbers of bursts using both an alternative-forced-choice and a yes-no procedure in the same experimental sessions. Thresholds measured with the yes-no procedure were typically higher than thresholds measured with the alternative-forced choice procedure. The difference between the two thresholds decreased as stimulus duration increased. It also varied between subjects and varied with the probability of false alarms in the yes-no procedure. It is shown that a previously proposed model of detection (Heil et al., Hear Res 2017) can account for these findings better than other models. It can also account for the shapes of the psychometric functions. The model is consistent with basic concepts of signal detection theory but is based on a decision variable that follows Poisson statistics. It also differs from other models of detection with respect to the transformation of the stimulus into the decision variable. The findings in this study further support the model.

Role of the temporal window in dolphin auditory brainstem response onset

Auditory brainstem responses (ABRs) to linear-enveloped, broadband noisebursts were measured in six bottlenose dolphins to examine relationships between sound onset envelope properties and the ABR peak amplitude. Two stimulus manipulations were utilized: (1) stimulus onset envelope pressure rate-of-change was held constant while plateau pressure and risetime were varied and (2) plateau duration was varied while plateau pressure and risetime were held constant. When the stimulus onset envelope pressure rate-of-change was held constant, ABR amplitudes increased with risetime and were fit well with an exponential growth model. The model best-fit time constants for ABR peaks P1 and N5 were 55 and 64 μs, respectively, meaning ABRs reached 99% of their maximal amplitudes for risetimes of 275-320 μs. When plateau pressure and risetime were constant, ABR amplitudes increased linearly with stimulus sound exposure level up to durations of ∼250 μs. The results highlight the relationship between ABR amplitude and the integral of some quantity related to the stimulus pressure envelope over the first ∼250 μs following stimulus onset-a time interval consistent with prior estimates of the dolphin auditory temporal window, also known as the "critical interval" in hearing.

Inconsistent effects of stochastic resonance on human auditory processing

It has been demonstrated that, while otherwise detrimental, noise can improve sensory perception under optimal conditions. The mechanism underlying this improvement is stochastic resonance. An inverted U-shaped relationship between noise level and task performance is considered as the signature of stochastic resonance. Previous studies have proposed the existence of stochastic resonance also in the human auditory system. However, the reported beneficial effects of noise are small, based on a small sample, and do not confirm the proposed inverted U-shaped function. Here, we investigated in two separate studies whether stochastic resonance may be present in the human auditory system by applying noise of different levels, either acoustically or electrically via transcranial random noise stimulation, while participants had to detect acoustic stimuli adjusted to their individual hearing threshold. We find no evidence for behaviorally relevant effects of stochastic resonance. Although detection rate for near-threshold acoustic stimuli appears to vary in an inverted U-shaped manner for some subjects, it varies in a U-shaped manner or in other manners for other subjects. Our results show that subjects do not benefit from noise, irrespective of its modality. In conclusion, our results question the existence of stochastic resonance in the human auditory system.