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.

Keywords: MEG; brain stem atrophy; inferior colliculus; masking level difference.

Fig. 1.

Voxel-based morphometry of white matter loss in patients compared to controls, confirmed in our group of patients to be particularly severe in the brain stem including the midbrain, cerebral peduncles, and cerebellar tracts, displayed at P < 0.001, unc. and overlaid on the SUIT template. Mean peak white matter loss, in the region of the inferior colliculus (IC), is 36% (SD = 14%).

Fig. 2.

A: mean waveforms measured for the high-level (High) N₀S₀, medium-level (Medium) N₀S₀, low-level (Low) N₀S₀, and low-level (Low) N₀Sπ tones for the patient and control groups. There is a clear N1m peak response in patients and controls for all tones except the low-level N₀S₀ tone. The P2m peak for the low-level N₀Sπ tone is clear for the controls but significantly reduced in patients. Significant differences between patients and controls on a point by point basis are indicated by black and grey horizontal lines, for right and left hemisphere, respectively. B: sLORETA images of the N1m and P2m time window for controls and patients contrasting the high-level N₀S₀ vs. low-level N₀S₀, and low-level N₀Sπ vs. low-level N₀S₀. Dipole locations are indicated with red and blue spheres.