Cortical responses to speech and complex tonal stimuli in adults with normal hearing and sensorineural hearing loss

Abstract

Aims: Aims of this thesis were to: 1) investigate speech stimuli and background-noisedependent changes in cortical event related potentials (ERPs) in unaided and aided conditions, and determine amplification effects on ERPs, 2) examine behavioural and neural processing of pitch cues in adults with normal hearing (NH) and adults with sensorineural hearing loss (SNHL), and 3) investigate the effects of auditory training on pitch processing using behavioural and electrophysiological approaches in adults with SNHL. Method: In Study 1 P1, N1, and P2 responses to naturally produced syllables in quiet and in multi-talker babble were recorded, with and without a hearing aid in the right ear. Acoustic characteristics of the hearing-aid-transduced stimuli were measured using inthe- canal probe microphone measurements. In Study 2 behavioural pitch discrimination abilities were tested using the monaural TFS1 test (Moore & Sek, 2009a). Cortical potentials (N1, P2 and acoustic change complex, ACC) were recorded in response to frequency shifted (deltaF) tone complexes in an ‘ABA’ pattern in adults with mild and high frequency SNHL. In Study 3 N1, P2 and P3 ERPs and their related behavioural measures of discrimination (d-prime sensitivity and reaction time) were recorded using an active oddball paradigm. Behavioural pitch discrimination abilities were tested using the monaural (right ear) TFS1 (Temporal Fine Structure 1) test. All tests were conducted during pre-training and post-training sessions. Training consisted of discrimination of complex tones varying in pitch using custom software (Vandali et al., 2015) Results: The first study revealed that CAEP latencies and amplitudes showed significant effects of speech contrast, background noise and amplification. N1 and P2 components varied differently across conditions. Hearing-aid induced spectral and temporal changes to the speech stimuli affected P1-N1-P2 components. The second study showed that the SNHL group performed more poorly than the NH group for the TFS1 test and hence had poorer discrimination of fine structure cues, despite having normal or mild hearing loss in the frequency region of the stimulus. P2 (latency and amplitude) was more reflective of pitch differences between the complexes than N1. The presence of the acoustic change complex in response to the TFS transitions in the ABA stimulus varied with deltaF (and hence with pitch salience). Acoustic change complex amplitudes were reduced for the group with SNHL compared to controls. The third study demonstrated stimulus-specific ERP changes after training with no significant improvement in behavioural discrimination performance. In Study 3 P2 amplitude was more sensitive to training mastery (progress on the auditory training task) than behavioural discrimination abilities. Conclusion: Cortical ERPs reflect spectral and temporal characteristics of speech and complex-tonal stimuli and changes induced by background noise, amplification and training.