BINAURAL MODULATION MASKING

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (MO-S-pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (f(m)) was fixed at 16 Hz, and signal modulation frequency (f(s)) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as f(s) diverged from f(m), although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as f(s) decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at f(s) = 2 Hz and 5.3 dB at f(s) = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (f(m) = f(s) = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (f(s) = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (f(s) = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at f(s). There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be used based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.