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.
Highlights
Human MEG identified a cortical neurophysiological correlate of the BMLD bilaterally at ϳ150 ms, resulting from the increased audibility of the out-of-phase tones, which corresponded to an increase in level of at least 9.4 dB
There is a strong association of the IC with the BMLD in comparative animal models, but the cortical signature persists in PSP despite severe subcortical neurodegenerative pathology in the region of the IC
The delayed N1m and impaired P2m are suggestive of auditory perceptual impairments in the patients but were not specific to the BMLD
Summary
SubjectsFourteen patients with PSP (mean age: 68; range: 58 – 83; 4 female) were recruited from the specialist clinic at Addenbrooke’s Hospital, The paradigm used to study the BMLD response was implemented using E-prime 2.1 software (www.pstnet.com) in Windows XP (www.microsoft.com). In the main MEG experiment, a 64-dB SPL white noise masker, low-pass filtered at 2.5 kHz, was presented in phase (identically) to both ears (N0) for 1,000 ms. One out of 10 different noise exemplars was drawn at random for each presentation (trial). The time delay between trials (noise onsets) was 0 ms. The target was a 500-Hz pure tone, presented for 100 ms with an onset at 450 ms after the onset of the masker. In N0S0 conditions, the target was presented in phase to both ears. Target levels were 61.2, 55.5, and 46.1 dB SPL in the high-, medium-, and low-level N0S0 conditions, respectively. The high- and medium-level N0S0 tones were audible to young adult listeners, while the low-level N0S0 tone was not. The low-level signal was presented in opposite phase (N0S), making it audible for
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