Alteration in auditory threshold and loudness scaling measurements during the 6 first months of auditory rehabilitation.

In addition to the significant effect of frequency, which simply means that subjects had better auditory thresholds for low frequencies than for high, a significant degradation in auditory threshold was observed in our subjects during their auditory rehabilitation at Fc+1/4 and Fc+1/8 octave. Obviously, this may have been induced not by the auditory rehabilitation but rather by presbyacusis, a natural progressive degradation of the inner hair cells due to aging. The hair cells encoding frequencies around the cut-off frequency appear to lie at the border between the damaged and undamaged hair cells, and are the most sensitive to aging. This loss of hearing may appear to be unduly rapid; but closer investigation revealed that in fact just two patients accounted for most of this statistical impairment (subjects J.R and M.M.).

A significant effect of frequency was also observed in the loudness scaling measurements, reflecting a loudness recruitment phenomenon, as observed in most presbyacusic subjects (Fowler, 1936; Steinberg and Garner, 1937). Loudness recruitment is linked to the reduced dynamic range of hearing in SNHI subjects: they may be able to hear loud sounds in the same way as normal-hearing listeners, but if these loud sounds are made even louder they become uncomfortable to listen to.

Furthermore, the alteration in loudness perception at Fc and Fc+1/8 octave during auditory rehabilitation is almost certainly linked to loudness recruitment. The significant correlation between change in auditory threshold and change in loudness showed that the greater the degradation in the subject’s auditory thresholds during rehabilitation, the more strongly loudness decreased. At Fc+1/8 octave, the dynamic range of hearing gradually reduced over these six months of testing and the recruitment phenomenon got correspondingly stronger.