An investigation of central auditory nervous system plasticity following amplification

A common clinical observation in first-time hearing aid users is the initial report of poor speech perception with hearing aids; however, after an extended period of hearing aid use these same individuals report improved speech perception. In the literature, this finding is referred to as the auditory acclimatization effect (Arlinger et al., 1996). The underlying mechanism of the changes perceived by the hearing aid wearer might be the result of plasticity in the central auditory nervous system (CANS). Self-rating scales and behavioral measures have been used to investigate the acclimatization effect; however, a paucity of electrophysiologic data exists. Of particular interest in this study, is determining the extent to which auditory evoked potentials might reveal physiological changes in the CANS related to hearing aid use and acclimatization. Subjects were hearing-impaired individuals between the ages of 49-71 years and were recruited from audiological centers in Storrs, Connecticut and Lexington, Kentucky. The experimental group consisted of ten first-time hearing aid wearers who were evaluated on the day of the initial hearing aid fitting and then again six to eight weeks later. The control group consisted of ten age-matched and hearing loss-matched individuals who did not wear amplification. Three different measures were utilized to assess plasticity in the CANS including: the Abbreviated Profile of Hearing Aid Benefit (APHAB), the nonsense syllable test (NST), and the late auditory evoked potentials (N1 and P2). The NST and late auditory evoked potentials were completed during the pre-test session and the post-test session. ^ Results indicated no significant differences between pre- and post-test sessions for the NST, N1 amplitude, P2 amplitude, and P2 latency between the control and experimental groups; however, statistically significant differences did exist for the change in N1 latency measure between the two groups. The change in N1 latency was significantly greater for the experimental group compared to the control group. Due to good test-retest reliability for N1 latency and evidence of plastic changes in animals, as well as humans, following alterations in the acoustic environment, it seems likely that the changes seen in the N1 latency were due to plasticity in the CANS following amplification.