PLASTICITY, HEMISPHERIC ASYMMETRIES AND THE NEURAL REPRESENTATION OF SOUND

The mature central nervous system (CNS) has the capacity to reorganise when there is a change in sensory input. However, studies using the N1 cortical auditory evoked potential, or its magnetic homolog N1m, have not consistently demonstrated evidence of plasticity in adults with late onset unilateral deafness. In addition, little is known about the time course of experience-related plasticity in adults with unilateral deafness. The aim of the studies described in this thesis was to investigate plasticity in adults with unilateral deafness, using N1 auditory evoked potentials. Deafness occurred as a result of surgery for the removal of an acoustic neuroma. The stimuli were 500-Hz and 4-kHz tones presented monaurally to the intact ear, and the data were analysed using global field power and dipole source analysis. In the first study (Chapter 3), hemispheric asymmetries in the N1 response were measured in a group of 24 normally hearing adults at presentation levels of 40, 60 and 80 dB sensation level (SL). The results revealed that the mean hemispheric asymmetry was greater for the 4-kHz stimulus but there was no significant effect of presentation level. In addition, the results revealed that the magnitude of hemispheric asymmetry depended on the ear of stimulation; a trend for larger asymmetries was observed following stimulation of the left ear. The results of the study provide confidence that the methodology is suitable for measuring hemispheric asymmetries in individuals with unilateral deafness. The effect of stimulus level is important since this will vary in plasticity studies involving individuals with late onset unilateral deafness due to their pure tone sensitivity thresholds. Clarifying the effect of stimulus frequency in normally hearing adults is important since the effect of stimulus frequency on plasticity following unilateral deafness has not been reported previously.In the second study (Chapter 4), N1 responses were measured in 19 adults with unilateral deafness (10 and 9 right- and left-sided deafness respectively). Stimuli were typically presented at 60 dB SL. The results revealed that there was significantly greater mean activity and a shift towards reduced hemispheric asymmetries compared with 19 audiogram-matched controls. Similar changes were apparent after both right- and left-sided deafness, and for both 500-Hz and 4-kHz stimuli. Therefore the results reveal evidence of experience-related plasticity that mirrors the findings reported in animal models. The reduced hemispheric asymmetries were reflected in the dipole source model used in this thesis by changes in dipole strength, location and orientation. These findings may explain the inconsistencies reported in previous studies that have used N1 or N1m, where dipole location and orientation have not always been taken into adequate consideration.In the third study (Chapter 5), longitudinal measurements were made in six adults just prior to the onset of complete unilateral deafness, and at 1-, 3- and 6-months after the onset (4 right-sided and 2 left-sided deafness respectively). The results from the second study were further analysed by splitting the data into two groups: nine participants with <2 years deafness and 10 with ? 2 year? s deafness. The results from the longitudinal data revealed that there was a significant difference in mean activity across the four conditions. For both stimulus types an increase in mean activity occurred after the onset of deafness, and hemispheric asymmetries were reduced. The biggest changes occurred within 1-month, although further increases were noted in some individuals with ? 2 year? s duration of deafness. Changes that continue over this period of time suggest different physiological mechanisms for plasticity within the human central auditory system.