Abstract
For binocular visual space, it is well-known that horizontal lines which appear straight on a subjective frontoparallel plane are not always straight in the physical sense, and that the form of the lines depends on the distance from the observer. This phenomenon is known as Helmholtz's horopter. A similar phenomenon is also known for tactile space. Since sensory modalities are often analogous, it has been speculated that a similar phenomenon exists for auditory space, although evidence has not yet been provided of its existence. This may be due to the small number of studies on sound localization with distance which are necessary to measure the horopter for auditory space. We performed fundamental experiments on sound localization with distance and confirmed the existence of horopter phenomenon for perceived binaural auditory space. Namely:
(1) Horizontal lines which appear straight on a subjective frontoparallel plane in auditory space are not always physically straight, and their form depends on the distance between the subject and the sound sources.
(2) The dependency of the form is more similar to that for visual horopter than for tactile horopter.
(3) Although the structure of auditory space is similar to that of visual space, there exist some differences. First, the metric for auditory space and that for visual space of the same subject are not the same. Second, auditory space has more allowance than visual space for the synthesis of the information of the subjective position.
