A study published Monday in the Proceedings of the National Academy of Sciences shed light on a novel mechanism that is employed by humans to estimate their relative distance from sound sources.
The study reveals that humans can perform this task more efficiently when they are allowed to move. “When we move, the apparent position of an object that is closer to us moves more within our visual field than an object located further away. This relative motion provides information about the relative distance of the two objects,” said Lutz Wiegrebe at Ludwig-Maximilians-Universitaet, the paper’s senior author. But localization of sounds is particularly challenging when the nature of the sound source is not clearly defined, he said.
Wiegrebe and his team carried out an experiment in a non-reverberant chamber to ensure that the participants with blindfolding goggles could not assess relative distances from the locations of sounds on the basis of echoes or reverberation.
They were seated facing two sound sources that could be positioned at different distances from the subject. One of the sources, chosen at random, emitted high-pitched and the other low-pitched sounds and the subjects’ task was to determine which of the sound sources was closer to them. They found that participants who moved their upper bodies sideways so that the sound sources were further to the right and then further to the left, were better able to estimate the distance between the sound sources.
“This result demonstrates that humans can use auditory motion parallax to estimate relative distances from sound sources,” said Wiegrebe.
The study identified that those participants were able to do so even when the distance difference between the two sound sources was only 16 cm. In another two experiments, the subjects were moved left-and-right on a motion platform, and in the other the loudspeakers were moved. The results showed that the participants performed best when they were allowed to move actively.
The interaction between self-motion and the auditory system has facilitated the processing of the expected change in the relative positions of the sound sources in the brain. This mechanism is also an advantage in situations in which different sounds impinge on the ears from different directions.
“For example, moving around at a party helps us to discriminate between sounds in the hubbub. Movement alters the spatial sound properties in our ears and thus enables us to determine which sound sources are closer to us,” said Wiegrebe. As the study has shown, humans make readily use of auditory motion parallax associated with self-motion to localize sounds. Whether or not other mammals exploit auditory motion parallax is not yet known.