Measurement of Horopter and Alleys in Auditory Space

For binocular visual space, it is a well-known phenomenon that the horopter curves are not always physically straight, and that the form of the curves depends on the distance. A similar phenomenon is also known for tactile space. We conducted fundamental experiments on sound localization with distance, and confirmed that the horopter curves in auditory space are not always physically straight either, and that the form of the curves depends on the distance between the subject and the sound sources. We also clarified that the form of the horopter for auditory space is not the same but relatively similar to that for visual space, and is quite different from that for tactile space. The use of a virtual environmental display system enabled the construction of a psychophysical experimental system to measure subjectively straight lines running phenomenally parallel to the median plane, the parallel alley, and subjectively straight lines running phenomenally equidistant from the median plane, the equidistance alley, in binaural auditory space. Experiments were conducted using the constructed experimental system, to measure the parallel alley and the equidistance alley in binaural auditory space. The following results were confirmed: (1) The parallel alley and the equidistance alley in auditory space in a virtual environment are not always straight in the physical sense, and their forms depend on the distance from the median plane. (2) The parallel alley lies inside the equidistance alley. These tendencies are the same as those in visual space and tactile space in the real world. Moreover, employing sound intensity and binaural time differences as parameters, space perception models were constructed to explain the dependency of the curves of the horopter, the parallel alley, and the equidistance alley on distance in auditory space. Based on the results of the simulation performed using the models, it was confirmed that the models successfully explain the results obtained in our psychophysical experiments.