Compression of Auditory Space during Forward Self-Motion

<div><h3>Background</h3><p>Spatial inputs from the auditory periphery can be changed with movements of the head or whole body relative to the sound source. Nevertheless, humans can perceive a stable auditory environment and appropriately react to a sound source. This suggests that the inputs are reinterpreted in the brain, while being integrated with information on the movements. Little is known, however, about how these movements modulate auditory perceptual processing. Here, we investigate the effect of the linear acceleration on auditory space representation.</p> <h3>Methodology/Principal Findings</h3><p>Participants were passively transported forward/backward at constant accelerations using a robotic wheelchair. An array of loudspeakers was aligned parallel to the motion direction along a wall to the right of the listener. A short noise burst was presented during the self-motion from one of the loudspeakers when the listener’s physical coronal plane reached the location of one of the speakers (null point). In Experiments 1 and 2, the participants indicated which direction the sound was presented, forward or backward relative to their subjective coronal plane. The results showed that the sound position aligned with the subjective coronal plane was displaced ahead of the null point only during forward self-motion and that the magnitude of the displacement increased with increasing the acceleration. Experiment 3 investigated the structure of the auditory space in the traveling direction during forward self-motion. The sounds were presented at various distances from the null point. The participants indicated the perceived sound location by pointing a rod. All the sounds that were actually located in the traveling direction were perceived as being biased towards the null point.</p> <h3>Conclusions/Significance</h3><p>These results suggest a distortion of the auditory space in the direction of movement during forward self-motion. The underlying mechanism might involve anticipatory spatial shifts in the auditory receptive field locations driven by afferent signals from vestibular system.</p> </div

Distortion of Auditory Space during Linear Self-Motion

This study investigated how auditory space was represented during linear self-motion. Participants were passively transported forward or backward at constant accelerations by a robotic wheelchair. A short noise burst (30 ms) was presented during self-motion via a loudspeaker aligned parallel with the traveling direction. In Experiment 1, the participants judged in which direction (incoming or outgoing) the noise burst was presented (ie,, two alternative forced choice task). The auditory stimulus perceived to be aligned with the subjective coronal plane shifted in the traveling direction only during forward self-motion. The amount of shift increased with increasing acceleration. In Experiment 2, we examined the accuracy of sound localization during forward self-motion by employing a pointing method. Whereas the auditory stimulus located on the physical coronal plane was almost accurately perceived, those well located in the incoming space were perceived closer to the participants than the physical positions during forward self-motion. These results suggest that the representation of auditory space in the traveling direction was compressed during forward accelerations

Effects of acceleration on auditory space representation observed in Experiment 1.

<p>The mean sound positions aligned with the participants’ subjective coronal planes are shown as a function of acceleration. The black squares and a white circle represent forward self-motion and no-motion, respectively, whereas the white triangles represent backward self-motion. The null point indicates the physical coronal plane. Error bars denote standard errors.</p

Schematic diagram of experimental setup used in Experiment 3 (a) and Effect of acceleration on auditory egocentric localization observed in Experiment 3 (b).

<p>The rod-pointing device was set very close to the participant’s body in the mid-sagittal plane. The participants were transported forward at 0.4 m/s² of acceleration. The black squares represent forward self-motion, whereas the white circles represent no self-motion. The dashed line indicates the ideal performance. Error bars denote standard errors.</p

Effect of acceleration on auditory egocentric localization observed in the follow-up experiment included in Experiment 3 (<i>N</i> = 8).

<p>The participants were transported forward at 0.2 m/s² of acceleration while the velocity of the chair was identical with that of the main experiment of Experiment 3 (0.9 m/s). The black triangles represent forward self-motion at 0.2 m/s² of acceleration, whereas the gray squares and open circles represent the results of the main experiment of Experiment 3 (forward self-motion at 0.2 m/s² and no self-motion conditions, respectively). The dashed line indicates the ideal performance. Error bars denote standard errors.</p

Effect of velocity on auditory space representation observed in Experiment 2.

<p>Participants were transported forward at 0.4 m/s² of acceleration, irrespective of velocity. Error bars denote standard errors.</p