A discrepancy between experimental observation and theoretical construct is pointed out with reference to the lateralizing effects of interaural phase differences. In essence, the observation that a phase difference of 90 ø produces maximal sound lateralization appears less amenable to physiological explanation than a theoretical relationship in which lateralizing effect is proportional to the magnitude of interaural phase difference, at least up to an area of uncertainty near 180% N experiment by Garner and Wertheimer has suggested that sound lateralization resulting from interaural phase difference is maximal when the difference is 90 ø . This observation is generally accepted without question, apparently due to the assumption that the maximally effective phase difference must lie midway between two minimally effective differences, i.e., 0 ø and 180 ø. However, a problem arises when we attempt to find an adequate physiological model to explain, among other things, why a phase difference of 90 ø should produce a greater lateralization effect than, for example, one of 120 ø . On theoretical grounds, it would seem that the relationship between interaural phase difference and lateralization effect would resemble that shown in Fig. 1. 0 90 180 INTERAURAL PHASE DIFFERENCE (IN DEGREES) FIG. 1. Theoretical conception of the relationship between lateralizing effect and interaural phase difference. The effect is proportional to the magnitude of the phase difference, with the exception of a narrow range of phase differences near 180 ø . In this range, lateralization judgments become uncertain, presumably due to a breakdown in coincidence detection in the auditory nervous system. This Figure illustrates a simple positive correlation, except for phase-difference values in the region of 180 ø at which lateralization judgments become uncertain. In the course of some related work, we recently reexamined the lateralization effect of various interaural phase differences, using a RIGHT tEFT• EARPHO. E coNTIroL' i s• FIG, 2. Block diagram of equipment tlsed itl this sttldy, 250 HZ. ß •or; o 90 700 HZ. 5 0 0 N=9 400 HZ. N:9 500 HZ. ß ß õ ß ß ß ß ß ß ,* , , o. ,8o o •'o ,•o o 9o N = 8 4000 HZ. N = 8 N=8 180 ß ß ß ß ' •o ' IJ•O o i ß , , o ;o ,•o INTERAURAL PHASE DIFFERENCE (IN DEGREES) Fro. 3. The lateralizing effects of interaural phase differences at 20 dB SL. Each experimental point represents the mean of N observations. Ordinate values denote the relative intensity necessary to compensate for phase leads of the magnitude indicated on the abscissa. phase/intensity trading procedure. As shown in the block diagram in Fig. 2, the output of an audio/Sscillator (Hewlett-Packard 200 AB) was fed through an electronic switch (Grason-Stadler 829S)to a phase shifter (Grason-Stadler E 3520AA). The two outputs of the phase shifter were channeled through separate amplifiers (RCA BA-24A) and attenuators (Hewlett-Packard 350B), the Channel 2 signal being fed additionally to a motor-driven recording attenuator (Grason-Stadler E 3262A). Impedance...