Binaural Lateralization of Cophasic and Antiphasic Clicks

Four subjects listened binaurally to equal-amplitude, 100-μsec rectangular pulses. The pulses were delivered at a sensation level of 40 db and at a rate of 10 sec−1. For wide-band reproduction and good transient response, the stimuli were transduced by condenser microphones fitted with ear insert plugs. The relative times of arrival of the pulses at the two ears (both lead and lag) were under continuous control of the subjects. The latter adjusted the interaural time difference to produce fused sound images. The polarities of the pulses were connected for cophasic (rarefaction-rarefaction) and antiphasic (condensation-rarefaction) conditions. Besides the unmasked conditions, lateralizations were made when the two ears were masked by additive, uncorrelated noise, high-pass filtered at 600, 1200 and 2400 cps, respectively. The results indicate that: (a) the interaural time difference for principal fusion of the cophasic condition, both masked and unmasked, is nominally zero; (b) the interaural time difference for principal fusions of the unmasked, antiphasic condition is on the order of 200 to 300 μsec; (c) masking by uncorrelated noise, high-pass filtered at frequencies less than about 1–2 kc/sec, produces antiphasic lateralizations at time differences of approximately ±1/2fc, where fc is the cutoff frequency of the noise; (d) masking by noise high-passed at frequencies greater than 1–2 kc yields lateralizations essentially the same as the unmasked case. Secondary fusions, generally lying off the medial plane, are frequently heard. Earlier calculations on the mathematical models of basilar membrane displacement [J. Acoust. Soc. Am. 32, 1494(A) (1960)] suggest the present results and, in fact, prompted the experiment.