title: The binaural masking level difference: cortical correlates persist despite severe brain stem atrophy in progressive supranuclear palsy.
abstract: Under binaural listening conditions, the detection of target signals within background masking noise is substantially improved when the interaural phase of the target differs from that of the masker. Neural correlates of this binaural masking level difference (BMLD) have been observed in the inferior colliculus and temporal cortex, but it is not known whether degeneration of the inferior colliculus would result in a reduction of the BMLD in humans. We used magnetoencephalography to examine the BMLD in 13 healthy adults and 13 patients with progressive supranuclear palsy (PSP). PSP is associated with severe atrophy of the upper brain stem, including the inferior colliculus, confirmed by voxel-based morphometry of structural MRI. Stimuli comprised in-phase sinusoidal tones presented to both ears at three levels (high, medium, and low) masked by in-phase noise, which rendered the low-level tone inaudible. Critically, the BMLD was measured using a low-level tone presented in opposite phase across ears, making it audible against the noise. The cortical waveforms from bilateral auditory sources revealed significantly larger N1m peaks for the out-of-phase low-level tone compared with the in-phase low-level tone, for both groups, indicating preservation of early cortical correlates of the BMLD in PSP. In PSP a significant delay was observed in the onset of the N1m deflection and the amplitude of the P2m was reduced, but these differences were not restricted to the BMLD condition. The results demonstrate that although PSP causes subtle auditory deficits, binaural processing can survive the presence of significant damage to the upper brain stem.
authors.affiliation: Department of Clinical Neurosciences,University of Cambridge, Cambridge, United Kingdom; Cognition and Brain Sciences Unit, Medical Research Council, Cambridge, United Kingdom; laura.hughes@mrc-cbu.cam.ac.uk.
authors.forename: Laura E
authors.initials: LE
authors.lastname: Hughes
authors.affiliation: Department of Clinical Neurosciences,University of Cambridge, Cambridge, United Kingdom; Cognition and Brain Sciences Unit, Medical Research Council, Cambridge, United Kingdom; Behavioural and Clinical Neuroscience Institute, Cambridge University, Cambridge, United Kingdom;
authors.forename: James B
authors.initials: JB
authors.lastname: Rowe
authors.affiliation: Wessex Neuroscience Centre, Southampton, United Kingdom; and.
authors.forename: Boyd C P
authors.initials: BC
authors.lastname: Ghosh
authors.affiliation: Cognition and Brain Sciences Unit, Medical Research Council, Cambridge, United Kingdom;
authors.forename: Robert P
authors.initials: RP
authors.lastname: Carlyon
authors.affiliation: School of Psychological Sciences, Manchester Academic Health Science Centre, University of Manchester, Manchester, United Kingdom.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Plack
authors.affiliation: Cognition and Brain Sciences Unit, Medical Research Council, Cambridge, United Kingdom;
authors.forename: Hedwig E
authors.initials: HE
authors.lastname: Gockel
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001303:Auditory Cortex / Q000502:physiology / Q000503:physiopathology*; D001307:Auditory Perception / Q000502:physiology*; D001933:Brain Stem / Q000473:pathology*; D005072:Evoked Potentials, Auditory; D005260:Female; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D008875:Middle Aged; D010470:Perceptual Masking / Q000502:physiology*; D013494:Supranuclear Palsy, Progressive / Q000473:pathology / Q000503:physiopathology*
keywords: MEG; brain stem atrophy; inferior colliculus; masking level difference