Effects of Masking upon Lateralization of Cophasic and Antiphasic Clicks

At the last meeting of the Society [J. Acoust. Soc. Am. 33, 840(A) (1961)], we described an experiment to measure the interaural time difference for binaurally lateralizing cophasic (rarefaction-rarefaction) pulses, and antiphasic (rarefaction-condensation) pulses. High-passed masking moise, with cutoff frequency fc, was used to cause the coherent neural information to originate in the apical half of the basilar membrane. The disparity in interaural times for lateralizing cophasic and antiphasic conditions was explicable in terms of (in fact, predicted by) the calculated displacements of the basilar membrane. Principal cophasic fusions of the sound image were made for zero interaural time. Principal antiphasic fusions, on the other hand, were made for a lead of the condensation pulse by a time approximately equal to 12fc sec. Using the procedure previously outlined, we have extended the experiment to the complementary situation where the significant neural information is constrained to originate from the basal portion of the membrane. To this end, low-passed masking noise is used with cutoff frequencies of 600, 1200, and 2400 cps. Again, the principal cophasic fusions are found for interaural times which are nominally zero. The principal antiphasic fusions, however, are now made for interaural times which are nominally constant at about 200–400 μsec, and with condensation lead. As before, computations from a mathematical model [J. Acoust. Soc. Am. 32, 1494(A) (1960)] show that this result is also explicable in terms of middle-ear transmission and basilar membrane displacement.