# Effects of Visually Induced Self-Motion on Sound Localization Accuracy
## 2 Materials and Methods  
Eleven young adults (eight males and three females; average age 22.5 years) with normal hearing were recruited for the experiments performed in a dark anechoic room. All participants were checked that their audiograms were without hearing loss above 20 dB from 125 Hz to 8 kHz. This study was approved by the Ethics Committee of the Research Institute of Electrical Communication at Tohoku University. Figure 1 shows a diagram of the experimental setup. All auditory stimuli were generated using a circular array (radius = 1.4 m; range = \(\pm\)9.0\({}^{\circ}\)) of ten 30 mm loudspeakers (AuraSound NSW1-205-8A) separated by angular spacing of 2.0\({}^{\circ}\). The loudspeakers were placed horizontally at a height almost level with the participant's eyes. The participants were seated on chairs facing at 0\({}^{\circ}\) to the horizontal plane and were asked not to move their head during the experiment. Two conditions--one with a visually induced perception of self-motion (vection condition) and the other without vection (control condition)--were considered during the experiments. Under both conditions, the participants were asked to judge the azimuthal direction (left or right) of the sound image relative to their subjective midline orientation, i.e., the two-alternative forced-choice (2 AFC) approach. The subjective midline was measured as the section point in front with the horizontal plane where the loud speaker array was installed. The acoustic stimulus was a short pink noise burst. Each stimulus lasted 30 ms, including rise and decay times of 5 ms each. The A-weighted sound pressure level of the pink noise when presented steadily was set to 65 dB. The location of the acoustic stimulus was selected using the randomized maximum-likelihood adaptation method [26]. The participants wore a VR head-mounted display (HMD) (Oculus Rift CV1) and held an Oculus Rift controller and a switch button (FS3-P USB Triple Foot Switch Pedal) with their hands. Head motion was sensed via the HMD during the experiment. All visual stimuli were presented via the HMD. Even if the participants moved their heads slightly during the experiment, the visual stimuli were corrected to present a stable virtual environment according to the sensed head motion. Figure 1: Diagram of the experimental setting. A black spherical space was simulated around the participants by populating the virtual environment (radius, 2 m). At the beginning of the experiment, a red sphere with a radius of 5 cm was presented in the spherical space (visual angle of the sphere, 1.43\({}^{\circ}\)). The red sphere was presented in front of the participant, i.e., 0\({}^{\circ}\) in the horizontal and vertical planes. The participants were instructed to keep their eyes and head pointed in the sphere's direction and remain still until each trial was complete. After 3 s, the red sphere disappeared, and 100,000 green spheres were presented to the participants by populating the virtual environment. The radius of each sphere was 5 cm (visual angle of the sphere, 1.43\({}^{\circ}\)). The positions of the green spheres were randomized in each trial. In the section condition, the green spheres revolved horizontally around the center of the participant's head at a speed of 5\({}^{\circ}\)/s. Either counterclockwise or clockwise motion was used in any given trial, and the direction of motion was randomly selected with equal probability. The participants were instructed to press the switch button if they perceived self-motion. Immediately after the participants pressed the button, the acoustic stimulus was deployed from one loudspeaker located within \(\pm\)9.0\({}^{\circ}\) of the participants' SSA orientation. The participants were asked to determine the direction of the sound image. Four sessions were conducted for each participant, resulting in 200 trials (2 motion directions \(\times\) 100 trials). In the control condition, the green spheres did not move. The participants were instructed to press the switch button if they perceived the spheres.