Cochlear implant (CI) users often find sound localization difficult. For single-sided deaf (SSD) subjects, CI implantation does not necessarily restore localization ability. Hemisphere specific changes are a potential mechanism behind loss of binaural integration in the auditory system when compared to bilateral deafness (BD). We compare the effects of SSD versus BD on interaural time difference (ITD) coding in the auditory midbrain of gerbils.
Adult gerbils were unilaterally deafened using ototoxic drugs. After 15 days, ITD responses were recorded in the IC using bilateral CI stimulation. Normal hearing (NH) and BD adult animals served for comparison. Tuning metrics of rate-ITD functions, such as best ITD and the ITD at maximum slope (ITDms), as well as neural discrimination thresholds (NDTs) were evaluated.
Both SSD and BD groups revealed a lower incidence of peak-shaped responses when compared to NH animals. Best ITDs and ITDms in BD subjects shifted away from the physiologically relevant range of ITDs in gerbil, whereas there were no shifts of best ITD nor ITDms in SSDs. However, SSD animals presented asymmetric increases in NDTs, with substantially higher thresholds on the hearing side.
Results show degradations in ITD coding in both BD and SSD subjects. However, assuming that sound localization is based on the comparisons of overall (population) neural activity between hemispheres (two-channel model), hemisphere specific changes in ITD coding as seen in the SSD animals likely contribute to degraded ITD discrimination performance. A similar mechanism may explain poor sound localization abilities in human SSD subjects. These data provide the basis for studying ITD coding in juvenile SSD gerbils as a model for prelingual SSD.
