Categorical scaling of partial loudness in a condition of masking release

Intensity discrimination and neural representation of a masked tone in the presence of three types of masking release

Introduction

Hearing ability is usually evaluated by assessing the lowest detectable intensity of a target sound, commonly referred to as a detection threshold. Detection thresholds of a masked signal are dependent on various auditory cues, such as the comodulation of the masking noise, interaural differences in phase, and temporal context. However, considering that communication in everyday life happens at sound intensities well above the detection threshold, the relevance of these cues for communication in complex acoustical environments is unclear. Here, we investigated the effect of three cues on the perception and neural representation of a signal in noise at supra-threshold levels.

Methods

First, we measured the decrease in detection thresholds produced by three cues, referred to as masking release. Then, we measured just-noticeable difference in intensity (intensity JND) to quantify the perception of the target signal at supra-threshold levels. Lastly, we recorded late auditory evoked potentials (LAEPs) with electroencephalography (EEG) as a physiological correlate of the target signal in noise at supra-threshold levels.

Results

The results showed that the overall masking release can be up to around 20 dB with a combination of these three cues. At the same supra-threshold levels, intensity JND was modulated by the masking release and differed across conditions. The estimated perception of the target signal in noise was enhanced by auditory cues accordingly, however, it did not differ across conditions when the target tone level was above 70 dB SPL. For the LAEPs, the P2 component was more closely linked to the masked threshold and the intensity discrimination than the N1 component.

Discussion

The results indicate that masking release affects the intensity discrimination of a masked target tone at supra-threshold levels, especially when the physical signal-to-noise is low, but plays a less significant role at high signal-to-noise ratios.

Supra-threshold perception and neural representation of tones presented in noise in conditions of masking release

The neural representation and perceptual salience of tonal signals presented in different noise maskers were investigated. The properties of the maskers and signals were varied such that they produced different amounts of either monaural masking release, binaural masking release, or a combination of both. The signals were then presented at different levels above their corresponding masked thresholds and auditory evoked potentials (AEPs) were measured. It was found that, independent of the masking condition, the amplitude of the P2 component of the AEP was similar for the same stimulus levels above masked threshold, suggesting that both monaural and binaural effects of masking release were represented at the level of the auditory pathway where P2 is generated. The perceptual salience of the signal was evaluated at equal levels above masked threshold using a rating task. In contrast to the electrophysiological findings, the subjective ratings of the perceptual signal salience were less consistent with the signal level above masked threshold and varied strongly across listeners and masking conditions. Overall, the results from the present study suggest that the P2 amplitude of the AEP represents an objective indicator of the audibility of a target signal in the presence of complex acoustic maskers.

Benefit of binaural listening as revealed by speech intelligibility and listening effort

In contrast to the well-known benefits for speech intelligibility, the advantage afforded by binaural stimulus presentation for reducing listening effort has not been thoroughly examined. This study investigated spatial release of listening effort and its relation to binaural speech intelligibility in listeners with normal hearing. Psychometric functions for speech intelligibility of a frontal target talker masked by a stationary speech-shaped noise were estimated for several different noise azimuths, different degrees of reverberation, and by maintaining only interaural level or time differences. For each of these conditions, listening effort was measured using a categorical scaling procedure. The results revealed that listening effort was significantly reduced when target and masker were spatially separated in anechoic conditions. This effect extended well into the range of signal-to-noise ratios (SNRs) in which speech intelligibility was at ceiling, and disappeared only at the highest SNRs. In reverberant conditions, spatial release from listening effort was observed for high, but not low, direct-to-reverberant ratios. The findings suggest that listening effort assessment can be a useful method for revealing the benefits of spatial separation of sources under realistic listening conditions comprising favorable SNRs and low reverberation, which typically are not apparent by other means.