Individual differences in dynamic binaural-masking-level differences at 500 Hz in listeners older than 50 years

Abstract

One demonstration of binaural sluggishness is that the binaural masking level difference (BMLD) for a tone burst is strongly dependent upon the rate of modulation of the interaural correlation of a masking noise [D. W. Grantham and F. L. Wightman, ‘‘Detectibility of a pulsed tone in the presence of a masker with time-varying interaural correlation;’’ J. Acoust. Soc. Am. /bf 65, 1509–1517 (1979)]. This paradigm was used to measure such dynamic BMLDs for a group of 18 listeners, aged 50–78 and with low-frequency hearing losses of 7–43 dB. The signal was a 20-ms, 500-Hz tone placed in the temporal center of a 500-ms, 100–2000-Hz, 63-dB overall-level noise. The interaural correlation of the noise was either fixed at 1.0 (static conditions) or 100% modulated (dynamic conditions). As expected, the dynamic BMLD reduced as the modulation rate increased; the across-listener means were 11, 8, 6, 2, 0, and 0 dB at rates of 0.5, 1, 2, 4, 8, and 16 Hz. But the individual differences were substantial: e.g., for the 11 listeners who showed a static BMLD of 15 dB or more, their 2-Hz dynamic BMLDs varied between 2 and 10 dB. Those listeners with poor dynamic thresholds (and so more sluggish than the others) may have increased difficulties in detecting sounds in changing spatial environments.