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

Comodulation Masking Release in Tinnitus Frequency and One Lower Octave of Tinnitus Frequency

Inhibitory function is the basis of many perceptual and non-perceptual abilities in the auditory system. In people with tinnitus, decreased inhibitory function in the central auditory system has been proven. This disorder is caused by an increase in neural activity caused by an imbalance between stimulation and inhibition. The aim of this study was to evaluate and compare inhibitory function in persons who had tinnitus, at and one octave lower than the tinnitus frequency. Studies show that inhibition has very important role in comodulation masking release. According to inhibitory dysfunction in people with tinnitus, in this study we assessed comodulation masking release in tinnitus frequency and one lower octave. Participants were divided into two groups. Group 1 consisted of 7 individuals with unilateral tonal tinnitus at 4 kHz and group 2 included 7 individuals with unilateral tonal tinnitus at 6 kHz. Paired test, in each group separately, showed that the comodulation masking release and Across Frequency comodulation masking release differed significantly between tinnitus frequency and one octave lower in each group (p < 0.05). In fact, the disinhibition in the area around the frequency of the tinnitus seems to be greater than the tinnitus frequency area. It seems that the results of CMRs can be used in planning and managing the treatment of people with tinnitus (such as sound therapy, etc.).

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.