Binaural speech processing in individuals with auditory neuropathy

Auditory neuropathy disrupts the neural representation of sound and may therefore impair processes contingent upon inter-aural integration. The aims of this study were to investigate binaural auditory processing in individuals with axonal (Friedreich ataxia) and demyelinating (Charcot-Marie-Tooth disease type 1A) auditory neuropathy and to evaluate the relationship between the degree of auditory deficit and overall clinical severity in patients with neuropathic disorders. Twenty-three subjects with genetically confirmed Friedreich ataxia and 12 subjects with Charcot-Marie-Tooth disease type 1A underwent psychophysical evaluation of basic auditory processing (intensity discrimination/temporal resolution) and binaural speech perception assessment using the Listening in Spatialized Noise test. Age, gender and hearing-level-matched controls were also tested. Speech perception in noise for individuals with auditory neuropathy was abnormal for each listening condition, but was particularly affected in circumstances where binaural processing might have improved perception through spatial segregation. Ability to use spatial cues was correlated with temporal resolution suggesting that the binaural-processing deficit was the result of disordered representation of timing cues in the left and right auditory nerves. Spatial processing was also related to overall disease severity (as measured by the Friedreich Ataxia Rating Scale and Charcot-Marie-Tooth Neuropathy Score) suggesting that the degree of neural dysfunction in the auditory system accurately reflects generalized neuropathic changes. Measures of binaural speech processing show promise for application in the neurology clinic. In individuals with auditory neuropathy due to both axonal and demyelinating mechanisms the assessment provides a measure of functional hearing ability, a biomarker capable of tracking the natural history of progressive disease and a potential means of evaluating the effectiveness of interventions.