Masking of sounds by a background noise – cochlear mechanical correlates

Key points•  Basilar membrane responses to tone and noise stimuli presented simultaneously were measured in chinchillas and a gerbil. Overall responses increase monotonically with stimulus level in a compressive manner. •  Response components elicited by the weaker stimulus, whether tone or noise, were reduced by the stronger one. That is, the suppressor stimulus could be either the tone or the noise. Suppression by noise stimuli occurred only when the tone frequency was in the non‐linear region of the basilar membrane recording site. •  Basilar membrane responses to click and noise stimuli were also recorded in some experiments. The click response component was suppressed by the noise. •  Mutual suppression, meaning the simultaneous suppression of the tone and noise response components, was also observed under certain stimulus conditions. •  Detection thresholds of the tone stimuli increased in a near‐linear fashion with noise level increments.Abstract  In the search for cochlear correlates of auditory masking by noise stimuli, we recorded basilar membrane (BM) vibrations evoked by either tone or click signals in the presence of varying levels of background noise. The BM vibrations were recorded from basal regions in healthy cochleae of anaesthetized chinchilla and gerbil. Non‐linear interactions that could underpin various aspects of psychophysical masking data, including both compression and suppression at the BM level, were observed. The suppression effects, whereby the amplitude of the responses to each stimulus component could be reduced, depended on the relative intensities of the noise and the tones or clicks. Only stimulus components whose frequencies fell inside the non‐linear region of the recording site, i.e. around its characteristic frequency (CF), were affected by presentation of the ‘suppressing’ stimulus (which could be either the tone or the noise). Mutual suppression, the simultaneous reduction of the responses to both tones and noise components, was observed under some conditions, but overall reductions of BM vibration were rarely observed. Moderate‐ to high‐intensity tones suppressed BM responses to low‐intensity Gaussian stimuli, including both broadband and narrowband noise. Suppression effects were larger for spectral components of the noise response that were closer to the CF. In this regime, the tone and noise stimuli became the suppressor and probe signals, respectively. This study provides the first detailed observations of cochlear mechanical correlates of the masking effects of noise. Mechanical detection thresholds for tone signals, which were arbitrarily defined using three criteria, are shown to increase in almost direct proportion to the noise level for low and moderately high noise levels, in a manner that resembles the findings of numerous psychophysical observations.