Doctor of Philosophy
Health and Rehabilitation Sciences
Dr. David Purcell
Dr. Susan Scollie
Auditory processing disorder (APD) is characterized by difficulty listening in noisy environments despite normal hearing thresholds. APD was previously thought to be restricted to deficits in the central auditory system. The current work sought to investigate brainstem and peripheral mechanisms that may contribute to difficulties in speech understanding in noise in children with suspected APD (sAPD). Three mechanisms in particular were investigated: cochlear tuning, efferent function, and spatial hearing. Cochlear tuning was measured using stimulus frequency otoacoustic emission (SFOAE) group delay. Results indicate that children suspected with APD have atypically sharp cochlear tuning, and reduced medial olivocochlear (MOC) functioning. Sharper-than-typical cochlear tuning may lead to increased forward masking. On the contrary, binaural efferent function probed with a forward masked click evoked OAE (CEOAE) paradigm indicated that MOC function was not different in typically developing (TD) children and children suspected with APD. A third study with multiple OAE types sought to address this contradiction. Despite numerically smaller MOC inhibition in the sAPD group, MOC function was not significantly different between the two groups. Finally, spatial release from masking, localization-in-noise and interaural time difference thresholds were compared in TD and children with sAPD. Results indicate no significant difference in spatial hearing abilities between the two groups. Non-significant findings at group level in these studies may be related to the large heterogeneity in problems associated with APD. Fragmentation of APD into deficit specific disorders may facilitate research in identification of the specific anatomical underpinnings to listening problems in APD. Prior to conducting studies in children, three studies were conducted to optimize stimulus characteristics. Results of these studies indicate that the MOC may not be especially sensitive to 100 Hz amplitude modulation, as previously reported. Click stimulus presentation rates >25 Hz activate the ipsilateral MOC reflex in typical MOC assays, contaminating contralateral MOC inhibition of CEOAEs. Finally, localization-in-noise abilities of TD children are on par with adults for a white noise masker, but not for speech-babble. This finding suggests that despite maturation of physiological mechanisms required to localize in noise, non-auditory factors may restrict the ability of children in processing complex signals.
Boothalingam, Sriram, "Role of The Cochlea and Efferent System in Children with Auditory Processing Disorder" (2014). Electronic Thesis and Dissertation Repository. 2235.