Evaluation of binaural functions in bilateral cochlear implant users

Abstract

Binaural abilities are difficult to assess, yet important to understand, in the course of rehabilitating bilateral cochlear implantees. One purpose of this research was to develop a binaural assessment methodology using direct electrical input to the cochlear implant, ''direct connect assessment'', pre-processed by appropriate headrelated transfer functions (HRTFs) to simulate the binaural cues for spatial release from masking and sound localization. A second purpose was to create and evaluate new, modified rules for adaptively measuring the speech reception threshold in noise with the Hearing In Noise Test (HINT) for use with cochlear implant subjects. A third purpose was to develop a new sound localization protocol whose difficulty is adjusted to the sound localization ability of the individual cochlear implant subject. These new protocols were combined with the direct connect method and evaluated in acoustic hearing subjects and cochlear implant subjects. HINT thresholds and sound localization scores measured in the sound field and with the direct connect method did not differ significantly and were highly correlated in both groups. Measures of spatial release from masking, head shadow effects, binaural squelch, and binaural summation derived from the thresholds also were comparable for both methods. The alternative adaptive rules for HINT were found to measure different points on the same performance-intensity function. These rules allowed all cochlear implant subjects in the study to be tested adaptively and their thresholds to be compared to norms. Sound localization was measured successfully in all subjects with a sector localization protocol whose difficulty is to be adjusted to the localization ability of the subject. This protocol reduced by one-third to one-half the time required to complete the assessment of the localization ability.