Neuronal mechanisms of bilateral hearing in hearing impaired human subjects

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Single unit recording in the cochlear nucleus of owl have been done by Saberi and Takahashi (2002). We found that an appropriate time difference of input sound caused the excitation of cochlear neurons, but less excitation was resulted by different time sequences. Information of sound arrival difference was conducted into the superior colliculus and direction of sound source was detected.The benefit of binaural hearing has been extensively investigated in human subjects. However, little is known for such mechanisms in hearing impaired subjects. In the preliminary study, we examined the binaural hearing mechanisms in subjects who weared cochlear implant (CI) in one ear and the hearing aid (HA) on the other side (Iwaki and Kubo, 2000).Hearing in Noise Test (HINT), which was developed by Nilsson and Soli (1994), has been widely used in USA to examine the speech detection ability under quiet and noisy circumstances. We made this test using Japanese language with the cooperation of Soli (HINT-J: Shiroma et al., 2002). Using HINT-J the speech perception ability was examined under the quiet and noisy conditions. There was no significant difference in speech perception under quiet condition when CI alone and HA+CI conditions were compared. But a significant difference was found under noisy condition between these two conditions. Although characteristics of input sound should be different between two ears (CI vs HA), the additional effects was detected in our hearing impaired subjects.