Different audio spatial metric representation around the body

Abstract

Vision seems to have a pivotal role in developing spatial cognition. A recent approach, based on sensory calibration, has highlighted the role of vision in calibrating hearing in spatial tasks. It was shown that blind individuals have specific impairments during audio spatial bisection tasks. Vision is available only in the frontal space, leading to a "natural" blindness in the back. If vision is important for audio space calibration, then the auditory frontal space should be better represented than the back auditory space. In this study, we investigated this point by comparing frontal and back audio spatial metric representations. We measured precision in the spatial bisection task, for which vision seems to be fundamental to calibrate audition, in twenty-three sighted subjects. Two control tasks, a minimum audible angle and a temporal bisection were employed in order to evaluate auditory precision in the different regions considered. While no differences were observed between frontal and back space in the minimum audible angle (MAA) and temporal bisection task, a significant difference was found in the spatial bisection task, where subjects performed better in the frontal space. Our results are in agreement with the idea that vision is important in developing auditory spatial metric representation in sighted individuals.

Figure 1

Set up and tasks. The spatial bisection task was performed at ear and foot level in both frontal and rear space. Three sounds were delivered from the three different speakers. Subjects had to judge whether the second sound was closer to the first or to the last sound. The spatial order of the three sounds was fixed in the space (they always started from the same speaker). Minimum audible angle task: the task was performed at ear and foot level in both frontal and rear space. Subjects had to judge the position of two sounds. In the frontal condition, subjects had to report which of the two sounds was further right, while in the rear condition, they had to state which of the two sounds was further left. Temporal bisection was performed at ear and foot level in both frontal and rear space. Three sounds were delivered from the same speaker with different delays between each other, and subjects had to judge which was the shortest interval between sounds.

Figure 2

Performance of a typical subject in the spatial bisection. On the y axis is represented the probability that the second sound was closer to the third sound, while on the x axis are represented the degrees. Degrees are calculated on the base of ear distance from the sound source. As shown in the figure, from the psychometric curves, we estimated not only the perceptive threshold (PSE) but also the variability around the threshold (precision), represented by the two dashed lines represent. Specifically, we evaluated precision as the steepness of the psychometric curve fitted to the data, which is the SD of the corresponding Gaussian curve.

Figure 3

Thresholds in the spatial bisection and MAA at ear level. red bar reports threshold (deg) for frontal condition, while blue bar refers to back condition. Dots represent the performance of each subject. As can be seen, subjects were more precise in the frontal space than in the back, suggesting that vision plays an important role in calibrating spatial hearing. No difference between frontal and back was found in the MAA. ^*Indicated p < 0.05.

Figure 4

Thresholds in the spatial bisection and MAA at foot level. red bar reports threshold (deg) for frontal condition, while blue bar refers to back condition. Dots represent the performance of each subject. As can be seen, subjects were more precise in the frontal space than in the back. No difference between frontal and back was found in the MAA. ^*Indicated p < 0.05.

Figure 5

Thresholds in the temporal bisection. No difference between front (red bar) and back (blue bar) conditions were found at either ear or foot level. Dots represent the performance of each subject.

Figure 6

Normalized thresholds in the spatial bisection and MAA. red bar reports threshold (deg) for frontal condition, while blue bar refers to back condition. Dots represent the performance of each subject. As can be seen, in the bisection task, subjects were more precise in the frontal space than in the back, suggesting that vision plays an important role in calibrating spatial hearing, while no difference were found between ear (yellow) and foot (green) level. An opposite pattern was found in the MAA: no difference between frontal and back, while a difference was found between ear and foot level. ^*Indicated p < 0.05.

Figure 7

Thresholds in the temporal bisection. No difference between front and back conditions was found at either ear or foot level. Dots represent the performance of each subject.