Sensitivity to a Break in Interaural Correlation in Frequency-Gliding Noises

This study was to investigate whether human listeners are able to detect a binaurally uncorrelated arbitrary-noise fragment embedded in binaurally identical arbitrary-noise markers [a break in correlation, break in interaural correlation (BIAC)] in either frequency-constant (frequency-steady) or frequency-varied (unidirectionally frequency gliding) noise. Ten participants with normal hearing were tested in Experiment 1 for up-gliding, down-gliding, and frequency-steady noises. Twenty-one participants with normal hearing were tested in Experiment 2a for both up-gliding and frequency-steady noises. Another nineteen participants with normal hearing were tested in Experiment 2b for both down-gliding and frequency-steady noises. Listeners were able to detect a BIAC in the frequency-steady noise (center frequency = 400 Hz) and two types of frequency-gliding noises (center frequency: between 100 and 1,600 Hz). The duration threshold for detecting the BIAC in frequency-gliding noises was significantly longer than that in the frequency-steady noise (Experiment 1), and the longest interaural delay at which a duration-fixed BIAC (200 ms) in frequency-gliding noises could be detected was significantly shorter than that in the frequency-steady noise (Experiment 2). Although human listeners can detect a BIAC in frequency-gliding noises, their sensitivity to a BIAC in frequency-gliding noises is much lower than that in frequency-steady noise.

Auditory midbrain representation of a break in interaural correlation

The auditory peripheral system filters broadband sounds into narrowband waves and decomposes narrowband waves into quickly varying temporal fine structures (TFSs) and slowly varying envelopes. When a noise is presented binaurally (with the interaural correlation being 1), human listeners can detect a transient break in interaural correlation (BIC), which does not alter monaural inputs substantially. The central correlates of BIC are unknown. This study examined whether phase locking-based frequency-following responses (FFRs) of neuron populations in the rat auditory midbrain [inferior colliculus (IC)] to interaurally correlated steady-state narrowband noises are modulated by introduction of a BIC. The results showed that the noise-induced FFR exhibited both a TFS component (FFRTFS) and an envelope component (FFREnv), signaling the center frequency and bandwidth, respectively. Introduction of either a BIC or an interaurally correlated amplitude gap (which had the summated amplitude matched to the BIC) significantly reduced both FFRTFS and FFREnv. However, the BIC-induced FFRTFS reduction and FFREnv reduction were not correlated with the amplitude gap-induced FFRTFS reduction and FFREnv reduction, respectively. Thus, although introduction of a BIC does not affect monaural inputs, it causes a temporary reduction in sustained responses of IC neuron populations to the noise. This BIC-induced FFR reduction is not based on a simple linear summation of noise signals.