Limited effect of binaural modulation on monaural modulation sensitivity

Speech can be degraded by the presence of interfering speech sources in daily life. The auditory system, however, shows great resilience towards these adverse conditions, and the spatial separation between two talkers is known to be beneficial. Within such acoustical settings, there is a possible interaction between the target speech modulations and spatial cues, which are important for speech intelligibility and for the spatial benefit of speech intelligibility, respectively. This study investigates the interaction between such cues with synthetic stimuli. Using a broadband noise stimulus that alternates periodically between two values of interaural time differences (ITD), we can show that listeners are able to accurately lateralize very brief noise segments of only 7.5 ms at the target ITD location and are able to discriminate between different binaural modulation rates. A primary question of this study was to determine the influence of this binaural modulation on the ability of listeners to detect a diotic amplitude modulation (AM). By imposing an AM envelope onto the binaural modulation, the interference of the binaural modulation on the ability to detect a diotic AM was investigated. Although AM detection thresholds were statistically significantly higher when the binaural modulation was present, thresholds were increased by only 1.7 dB. In a separate study, the ability to detect a binaural modulation in the presence of a diotic AM was tested. Across the population of listeners, nearly a doubling of the segment length needed to detect the binaural modulation was observed when an interfering, diotic AM was present. The weak interaction between the rapid interaural switching and AM detection could provide insight into how binaural and monaural processing mechanisms interact and aid in speech reception as a function of talker separation.