Evidence for multiple mechanisms of cortical plasticity: a study of humans with late-onset profound unilateral deafness.

OBJECTIVE: To investigate 1: plasticity due to partial unilateral deafness of slow onset and 2: the time course of plasticity following abrupt, profound unilateral deafness in adult humans using cortical auditory evoked potentials. METHODS: Baseline data were measured from six participants with partial unilateral deafness due to an acoustic neuroma and compared with data from six controls. Further measurements were made in the unilaterally deaf group at 1-, 3- and 6-months post surgery for acoustic neuroma removal and consequent profound unilateral deafness. Data were recorded from 30 channels in response to pure tones presented to the intact ear. RESULTS: Baseline data revealed statistically higher amplitudes in unilaterally deaf participants but with normal hemispheric asymmetry. Longitudinal data revealed further increases in P1 amplitudes by 1-month post-surgery, and in N1 and P2 amplitudes by 6-months post-surgery, with statistically different scalp field topographies indicating reduced hemispheric asymmetries. CONCLUSION: Different patterns of plasticity occur following partial and profound unilateral deafness. Plasticity occurs both relatively rapidly and more gradually over at least 6-months post-surgery. SIGNIFICANCE: The different patterns of change over time are consistent with multiple physiological mechanisms of plasticity. Unravelling these mechanisms and their time course in humans may be relevant in understanding and, ultimately, influencing plasticity for therapeutic gain.