Cortical representation of the combination of monaural and binaural unmasking

Abstract

The audibility of a target tone is improved by introducing either amplitude modulations that are coherent across different frequency channels of the masker (comodulation masking release, CMR) or interaural phase differences that are different for target and masker (binaural masking-level difference, BMLD). Although the two effects are likely to be based on different processing strategies, they both result in improved figure-background decomposition for a target-in-noise situation. In this study, we analyzed the combination of CMR and BMLD for a target tone in a masker with six 48-Hz-wide noise bands, distributed over a wide frequency range from 216 Hz to 2.78 kHz. Psychoacoustical detection thresholds for the tones in noise were determined for two masker conditions (comodulated or unmodulated bands) and two interaural phase differences of the target tone (0 or 180). The mean results indicate that the effects of unmasking add independently. The lowest thresholds are found for the dichotic signal embedded in a modulated masker with an overall threshold difference of about 16 dB compared to the unmodulated condition with no binaural cues. Based on the psychoacoustic results, a set of 12 signal-masker configurations was selected individually to explore the representation of the audibility of the test tone in brain activation maps by means of auditory functional MR imaging. The comparison of the results for the combination of CMR and BMLD with the results for the separate effects indicates a large overlap of the activated brain regions, where a largely extended area is activated, covering primary auditory cortex and adjacent regions. The result is in agreement with previous fMRI studies on auditory masking, identifying specific regions in the auditory cortex representing a change of the audibility of a target tone in a noise masker, irrespective of the overall sound pressure level of the stimulus. © Springer Science+Business Media New York 2013.