Effects of Talker Sex Differences on Binaural Summation in Cochlear Implant Users and Normal Hearing Listeners

The benefit of binaural summation ("summation") is often measured in co-located steady-state noise; summation for competing speech is less understood. The goal of the present study was to evaluate how the target-masker acoustic properties and binaural listening mode affect summation for speech-in-speech recognition. Recognition of target speech in two-talker speech maskers was measured with each ear alone and with both ears in bilateral CI users (BiCI), CI users with asymmetric hearing loss (AHL), CI users who were single-sided deaf (SSD), and normal-hearing listeners (NH). Target and masker speech was co-located; the talker sex of the maskers was either the same as or different from the target. With same-sex maskers, significant binaural summation was observed only in the BiCI group. In contrast, with different-sex maskers, significant summation was observed only in the SSD and NH groups, where binaural summation was significantly greater with different-sex than with same-sex maskers. A significant but weak correlation between interaural performance asymmetry and binaural summation was observed in some within-group comparisons, but not in across-group comparisons. For the BiCI group, summation was significantly correlated with SRTs from the better-performing CI ear, but not with performance asymmetry. The findings can be partly accounted for by psychometric-function slope and performance asymmetry within a probability-summation model. However, the probability-summation model generally overestimated the observed summation, suggesting that the assumption of independent processing across ears may not hold for CI users, possibly due to the variability in binaural fusion across individuals.