Monaural and binaural loudness measures in cochlear implant users with contralateral residual hearing

Objective: The aim was to measure the loudness of monaural and binaural sti muli in a group of cochlear implant users who had residual hearing in the n onimplanted ear, and to consider the implications of these measures for a b inaural fitting consisting of a hearing aid and an implant in opposite ears , Three independent hypotheses were addressed: that the shapes of the elect ric and acoustic loudness growth functions would be similar, although the d ynamic ranges would differ; that standard implant and hearing aid fittings would result in substantial loudness mismatches between the acoustic and el ectric signals; and that loudness summation would occur for binaural combin ations of electric and acoustic signals. Design: A modified version of the "Loudness Growth in 1/2-Octave Bands" met hod (Allen, Hall, & Jeng, 1990) was used to measure loudness growth for eac h ear of nine subjects, At the time of the experiment, the subject group in cluded all implant users in Melbourne and Denver who were available for res earch and who also had sufficient residual hearing to use a hearing aid in the nonimplanted ear. Five acoustic frequencies and five electrodes were me asured for each subject. The same subjects also estimated the loudness of a set of stimuli including monaural and binaural signals chosen to cover the loudness range from very soft to loud. Results: The shapes of the averaged loudness growth functions were similar in impaired and electrically stimulated ears, although the shapes of iso-lo udness curves were quite different in the two ears, and dynamic ranges vari ed considerably. Calculations based on the psychophysical data demonstrated that standard fitting procedures for cochlear implants and hearing aids le ad to a complex pattern of loudness differences between the ears. A substan tial amount of loudness summation was observed for the binaural stimuli, wi th most summation occurring when the acoustic and electric components were of equal loudness. This is consistent with observations for subjects with n ormal hearing and subjects with bilaterally impaired hearing. Conclusions: These experiments provide data on which criteria and methods f or the binaural fitting of cochlear implants and hearing aids may be based. It is unlikely that standard monaural fitting methods for cochlear implant s and hearing aids will result in balanced loudness between the two ears ac ross a reasonably broad range of frequencies and levels. It is also likely that output levels of both devices will need to be reduced relative to a mo naural fitting to compensate for the binaural summation of loudness in some listeners.