Francis X. Smith1, Bob McMurray2,3, Ruth Litovsky4, Inyong Choi1,3
1Department of Otolaryngology, University of Iowa, Iowa City, United States
2Department of Psychology and Brain Sciences, University of Iowa, Iowa City, United States
3Department of Communication Sciences and Disorders, University of Iowa, Iowa City, United States
4Department of Communication Sciences and Disorders, University of Wisconsin-Madison, Madison, United States
The normal auditory system has remarkable mechanisms that integrate sounds from the two ears, enabling listeners to utilize binaural information for achieving critical tasks in their everyday communication. Such binaural benefits include localization (that depends on the computation of binaural disparity) and detecting weak sounds (based on binaural redundancy processing and summation). For patients with single sided deafness (SSD), a cochlear implant (CI) helps to improve sound localization and speech recognition in some noisy scenarios. However, it is not yet known whether CI recovers binaural summation mechanisms for detecting weak sounds in SSD listeners. In this study, the binaural benefit of soft-speech recognition was tested in six SSD CI patients and thirty normal hearing (NH) listeners. SSD CI patients were tested in a four-alternative forced choice speech recognition task using soft speech in three conditions: using only their acoustic ear, only their electrical stimulation ear, or using both ears. NH subjects performed the same speech recognition task in four conditions: one ear with normal speech, one ear with noise vocoded speech, a combination of normal speech to one ear and vocoded speech to the other, or normal speech played to both ears. Electroencephalographic recordings were obtained from all participants as they completed the task. As expected, we observed an accuracy benefit for speech recognition when listening with both ears, even when the input to one ear is CI electrical stimulation / noise vocoded compared to listening with only one ear. Surprisingly, however, the early auditory response observed in the N1P2 complex was lower in amplitude in the combined conditions compared to listening with either one or two normal acoustic inputs. This suggests that the binaural summation benefit in SSD CI users is not solely due to enhanced encoding during early auditory processing, but due to late cognitive processes.