EVALUATION OF AUDITORY CORTICAL PLASTICITY FROM FIRST AMPLIFICATION TO ONE YEAR OF HEARING AID USE: THE RELATIONSHIP BETWEEN AIDED CORTICAL AUDITORY EVOKED POTENTIALS (ACAEPs) AND SPEECH PERCEPTION OUTCOMES AMONG HEARING-IMPAIRED ADULT PATIENTS

Over the last decade, aided cortical auditory evoked potentials (ACAEPs) have continued to be a focus of interest due to the lack of adequate tools to objectively assess cortical auditory activity in response to amplified stimuli. The majority of authors have investigated the direct relationship between behavioral thresholds and ACAEPs and the evolution of ACAEP waves among children with sensorineural hearing loss (SNHL) undergoing rehabilitation. In contrast, scarce data are available regarding changes in ACAEPs over time in adult hearing aid users, particularly in relation to speech perception outcomes. The main goal of this project was to investigate the relationship between ACAEPs and speech perception capability over time in post-lingual SNHL adult patients who were first-time hearing aid users. We hypothesized that, in patients with better speech understanding, a modification of the P1-N1-P2 complex could be expected as a result of neuroplastic changes due to hearing aid amplification. A longitudinal prospective clinical study was conducted on 72 new hearing aid users suffering from symmetrical, sloping SNHL. Patients were assessed at three different time points: baseline (T0), 6 months after the initial assessment (T6), and 12 months after the initial assessment (T12). All the participants went through the same evaluation protocol, which included pure-tone audiometry, speech audiometry tests, ACAEPs recorded with two different stimuli (1000 Hz and 2000 Hz) and questionnaires assessing hearing aid benefit. Analysis of amplitude values at the three different time points demonstrated an increasing tendency for all waves in both experimental conditions (p<0.01). Latencies seemed to become shorter from T0 to T12 for each wave and in the case of 1 kHz and 2 kHz stimuli. (p<0.05). Linear regression analysis found that only P2 amplitude showed a statistically significant increase in its variation while matrix sentence test (MST) and speech intellection threshold (SIT) decreased in both experimental conditions, even when the analysis was adjusted for age and daily hearing aid use (p<0.05). The data collected in this study provide new evidence regarding the relationship between ACAEPs and the speech recognition capability of adults who are new hearing aid users. In both experimental conditions, we observed larger P2 amplitude in patients with better speech perception outcomes. It should be underlined that, even though P2 may reflect auditory processing beyond sensation, its increase could be an expression of neural activity associated with the acquisition process driven by exposure to sounds and speech. The observation that P2 amplitude tended to improve as SIT and MST scores decreased might be, in the future, a further object of investigation to assess its reliability as a marker of speech perception improvement; it may assist hearing aid dispensers and audiologists as a source of feedback in the evaluation of listening benefits in hard-to-test patients.