Age-related differences in binaural masking level differences: Behavioral and electrophysiological evidence.

The effects of aging and stimulus configuration on binaural masking level differences (BMLDs) were measured behaviorally and electrophysiologically, using the frequency-following response (FFR) to target brainstem/midbrain encoding. The tests were performed in 15 younger normal-hearing (YNH; <30 yrs) and 15 older normal-hearing (ONH; >60 yrs) participants. The stimuli consisted of a 500-Hz target tone embedded in a narrowband (50-Hz bandwidth) or wideband (1500-Hz bandwidth) noise masker. The interaural phase conditions included NoSo (tone and noise presented interaurally in-phase), NoSπ (noise presented interaurally in-phase and tone presented out-of-phase), and NπSo (noise presented interaurally out-of-phase and tone presented in-phase) configurations. In the behavioral experiment, aging reduced the magnitude of the BMLD. The magnitude of the BMLD was smaller for the NoSo-NπSo threshold difference compared to the NoSo-NoSπ threshold difference, and it was also smaller in narrowband compared to wideband conditions, consistent with previous measurements. In the electrophysiology experiment, older adults had reduced FFR magnitudes and smaller differences between configurations. The changes in FFR magnitude between the NoSo to NπSo configurations were not significant for either bandwidth. The age-related reduction in FFR magnitudes suggests a temporal processing deficit, but no correlation was found between FFR magnitudes and behavioral BMLDs. Therefore, independent mechanisms may be contributing to the behavioral and neural deficits. Specifically, older adults had higher behavioral thresholds than younger adults for the NoSπ and NπSo configurations, but had equivalent thresholds for the NoSo configuration. However, FFR magnitudes were reduced in older adults across all configurations.