Exploring the adaptive capabilities of the brain following unilateral conductive hearing loss

Thesis

Plasticity in adulthood potentially allows the recovery of perceptual abilities following sensory impairments. Training-dependent recovery of sound localisation accuracy following unilateral conductive hearing loss has been widely used to study the adaptability of the auditory system. Previous studies have implicated the inferior colliculus (IC) in this plasticity, but have not examined whether neural correlates of training-induced plasticity are found in this midbrain structure.

To explore this question, we performed chronic electrophysiological recordings in the IC of ferrets while they were trained to adapt to unilateral hearing loss, induced by reversibly plugging one ear. We found that IC responses were initially disrupted by monaural hearing loss, which coincided with impaired localisation behaviour. With daily training, however, adaptive changes occurred in the spatial response properties of IC neurons and in the animals’ localisation behaviour. Neural correlates of training-induced adaptation are therefore found in the IC, supporting its role in auditory spatial plasticity. These results also provide insight into how sound azimuth is encoded by IC neurons.

Demonstrating that training-dependent adaptation has potential therapeutic value for people with hearing disorders requires an understanding of the persistence and context dependence of this plasticity, and of the extent to which adaptation generalises to untrained sounds. By measuring the behavioural effects in ferrets of repeated periods of monaural occlusion in the same ear and then of the opposite ear, we show that adaptation leaves a “memory trace” that determines how localisation accuracy is affected when unilateral hearing loss is experienced again. To evaluate the generalisation of learning, we measured localisation accuracy after adaptation using different bandwidth stimuli presented in silence or against background noise. Our results show a positive relationship between generalisation and the degree of adaptation achieved. Finally, a virtual reality setup was developed for investigating these questions in human participants.

Authors: Sanchez Jimenez, A

• DOI: 10.5287/ora-eoyv48px8
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: monaural hearing loss; sound localisation; earplug; plasticity; electrophysiology; ferret
• Subjects: Neurosciences