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A large amount of variability in speech perception outcome exists among CI users. Factors contributing to these variations can be attributed to differences in amplitude, temporal and spectral resolution processing capabilities which are necessary to detect ongoing changes in the incoming complex speech signals.
Psychophysical measures can be used to study performance variability. Unfortunately, these measures are confounded by numerous developmental and task-related variables especially in children.
Cortical auditory evoked potential is one of the objective tests that reflect the neural detection and/or discrimination of sound underlying speech perception. Therefore, it is beneficial to assess outcome variability with CI. CAEPs include obligatory evoked potentials such as P1, N1, and P2, and discriminative potentials such as Acoustic change complex (ACC), mismatch negativity (MMN) and P300.
ACC is preferred over other discriminative CAEPs in assessing children. Because, ACC shows good test retest reliability, has less testing time and doesn’t require active participation.
The current study was designed to investigate the value of implementing CAEP as a tool for auditory discrimination (which is an integral part in speech perception). Hopefully, this will aid in addressing the variability in performance in CI children and to aid in evaluating children who cannot be tested using behavioral measures.
The study group children were unilaterally CI children. All children in the study group were submitted to full history taking, age based hearing evaluation with their CI, language age based speech perception tests and CAEPs to different stimuli. Stimuli used to evoke CAEP included tonal and speech stimuli, tonal was 1000 Hz tone with no change and two change conditions were used; including frequency changes (from 2Hz to octave changes) and gap-in-tone (from 5 msec to 200 msec gap durations). Speech stimuli included vowel with no change /i/ and two change conditions /i-u/ and /u-i/.
Psychophysical AFT used to assess temporal resolution; this was conducted on 33 children only who could understand the test. Electrical- CAEP was elicited from 12 subjects who were implanted with cochlear sound processor.
The results of the study pointed that ACC could be successfully recorded in CI users using relatively short stimuli (500 msec long). However; the percent detection of ACC was significantly lower than NH children and varied across stimuli used. There was large range of variability in ACC response parameters across individuals tested. Implantation age, hearing age and duration of aural rehabilitation were variables affecting ACC response parameters. In relation to speech perception, ACC percent detection was the only consistent and significant parameter related to speech perception. In relation to tests for temporal resolution, ACC to gap-in-tone thresholds were significantly positively correlated to behavioral test for temporal resolution (Auditory fusion test- revised).
In conclusion, the clinical application of the onset response in predicting performance is limited in children with CI. On the contrary, ACC may help explain individual differences in speech perception by CI users. ACC percent detection and thresholds are better response parameters recommended to be used clinically than latency and amplitude. ACC to gap-in-tone can serve as a useful objective tool to assess temporal resolution. However, ACC is not suitable to replace psychophysical tests. Lastly, it is recommended to use our proposed clinical protocol after assessing it’s applicability.