Binaural Unmasking of Complex Signals

Abstract

Binaural masked thresholds were measured for periodic 100-μsec rectangular pulses of period T. Signal parameters were pulse rate (10, 50, 100, 250, and 1000 pps), interaural phase difference (0 and π rad), and interaural time difference (0–5 msec). Spectral content of the signal was controlled by addition or subtraction of phase-locked harmonic components and by high-pass and low pass filtering (cutoff frequencies 300, 600, 1200, and 2400 cps). Broad-band, interaurally in-phase, masking noise at 60 dB sensation level was used throughout. The signals were transduced by headphones, and the masked thresholds were determined by a modified Békésy technique. The difference between the threshold for a given test condition and threshold for the signal in-phase condition is defined as the masking-level difference (MLD), or the binaural release from masking. For all pulse rates, maximum release from masking obtains for an interaural time of 1.5 msec or T/2, whichever is smaller. The greatest binaural unmasking occurs for a pulse rate of 250 pps and is less at higher and lower rates. For all tonal cases (pulse rates of 250 pps and higher) the release from masking is found to relate primarily to the fundamental component. Elimination of the fundamental substantially reduces the MLD. At the lowest pulse rate, high pass filtering of the signal also reduces the MLD to a small value. The largest MLD's are found for signal conditions that give rise to basilar-membrane motion near the 300-cps place and that lead to interaural time differences of about 1.5 msec in the neural activity originating from these places. The same interaural disparity at higher-frequency places produces substantially smaller MLD's. The MLD therefore appears specific to membrane place as well as to interaural time difference.