Detection of Sound Image Movement During Horizontal Head Rotation.

Movement detection for a virtual sound source was measured during the listener's horizontal head rotation. Listeners were instructed to do head rotation at a given speed. A trial consisted of two intervals. During an interval, a virtual sound source was presented 60° to the right or left of the listener, who was instructed to rotate the head to face the sound image position. Then in one of a pair of intervals, the sound position was moved slightly in the middle of the rotation. Listeners were asked to judge the interval in a trial during which the sound stimuli moved. Results suggest that detection thresholds are higher when listeners do head rotation. Moreover, this effect was found to be independent of the rotation velocity.

Relative localization of auditory and visual events presented in peripheral visual field.

The brain apparently remaps the perceived locations of simultaneous auditory and visual events into a unified audio-visual space to integrate and/or compare multisensory inputs. However, there is little qualitative or quantitative data on how simultaneous auditory and visual events are located in the peripheral visual field (i.e., outside a few degrees of the fovea). We presented a sound burst and a flashing light simultaneously not only in the central visual field but also in the peripheral visual field and measured the relative perceived locations of the sound and flash. The results revealed that the sound and flash were perceptually located at the same location when the sound was presented at a 5 degrees periphery of the flash, even when the participants' eyes were fixed. Measurements of the unisensory locations of each sound and flash in a pointing task demonstrated that the perceived location of the sound shifted toward the front, while the perceived location of the flash shifted toward the periphery. As a result, the discrepancy between the perceptual location of the sound and the flash was around 4 degrees. This suggests that the brain maps the unisensory locations of auditory and visual events into a unified audio-visual space, enabling it to generate unisensory spatial information about the events.

Effects of head movement and proprioceptive feedback in training of sound localization.

We investigated the effects of listeners' head movements and proprioceptive feedback during sound localization practice on the subsequent accuracy of sound localization performance. The effects were examined under both restricted and unrestricted head movement conditions in the practice stage. In both cases, the participants were divided into two groups: a feedback group performed a sound localization drill with accurate proprioceptive feedback; a control group conducted it without the feedback. Results showed that (1) sound localization practice, while allowing for free head movement, led to improvement in sound localization performance and decreased actual angular errors along the horizontal plane, and that (2) proprioceptive feedback during practice decreased actual angular errors in the vertical plane. Our findings suggest that unrestricted head movement and proprioceptive feedback during sound localization training enhance perceptual motor learning by enabling listeners to use variable auditory cues and proprioceptive information.

Sounds can alter the perceived direction of a moving visual object.

Auditory temporal or semantic information often modulates visual motion events. However, the effects of auditory spatial information on visual motion perception were reported to be absent or of smaller size at perceptual level. This could be caused by a superiority of vision over hearing in reliability of motion information. Here, we manipulated the retinal eccentricity of visual motion and challenged the previous findings. Visual apparent motion stimuli were presented in conjunction with a sound delivered alternately from two horizontally or vertically aligned loudspeakers; the direction of visual apparent motion was always perpendicular to the direction in which the sound alternated. We found that the perceived direction of visual motion could be consistent with the direction in which the sound alternated or lay between this direction and that of actual visual motion. The deviation of the perceived direction of motion from the actual direction was more likely to occur at larger retinal eccentricities. These findings suggest that the auditory and visual modalities can mutually influence one another in motion processing so that the brain obtains the best estimates of external events.

Visual motion perception induced by sounds in vertical plane.

The alternation of sounds in the left and right ears induces motion perception of a static visual stimulus (SIVM: Sound-Induced Visual Motion). In this case, binaural cues were of considerable benefit in perceiving locations and movements of the sounds. The present study investigated how a spectral cue - another important cue for sound localization and motion perception - contributed to the SIVM. In experiments, two alternating sound sources aligned in the vertical plane were presented, synchronized with a static visual stimulus. We found that the proportion of the SIVM and the magnitude of the perceived movements of the static visual stimulus increased with an increase of retinal eccentricity (1.875-30 degree), indicating the influence of the spectral cue on the SIVM. These findings suggest that the SIVM can be generalized to the whole two dimensional audio-visual space, and strongly imply that there are common neural substrates for auditory and visual motion perception in the brain.

Alternation of sound location induces visual motion perception of a static object.

BACKGROUND: Audition provides important cues with regard to stimulus motion although vision may provide the most salient information. It has been reported that a sound of fixed intensity tends to be judged as decreasing in intensity after adaptation to looming visual stimuli or as increasing in intensity after adaptation to receding visual stimuli. This audiovisual interaction in motion aftereffects indicates that there are multimodal contributions to motion perception at early levels of sensory processing. However, there has been no report that sounds can induce the perception of visual motion. METHODOLOGY/PRINCIPAL FINDINGS: A visual stimulus blinking at a fixed location was perceived to be moving laterally when the flash onset was synchronized to an alternating left-right sound source. This illusory visual motion was strengthened with an increasing retinal eccentricity (2.5 deg to 20 deg) and occurred more frequently when the onsets of the audio and visual stimuli were synchronized. CONCLUSIONS/SIGNIFICANCE: We clearly demonstrated that the alternation of sound location induces illusory visual motion when vision cannot provide accurate spatial information. The present findings strongly suggest that the neural representations of auditory and visual motion processing can bias each other, which yields the best estimates of external events in a complementary manner.

Estimation of interaural level difference based on anthropometry and its effect on sound localization.

Individualization of head-related transfer functions (HRTFs) is important for highly accurate sound localization systems such as virtual auditory displays. A method to estimate interaural level differences (ILDs) from a listener's anthropometry is presented in this paper to avoid the burden of direct measurement of HRTFs. The main result presented in this paper is that localization is improved with nonindividualized HRTF if ILD is fitted to the listener. First, the relationship between ILDs and the anthropometric parameters was analyzed using multiple regression analysis. The azimuthal variation of the ILDs in each 1/3-octave band was then estimated from the listener's anthropometric parameters. A psychoacoustical experiment was carried out to evaluate its effectiveness. The experimental results show that the adjustment of the frequency characteristics of ILDs for a listener with the proposed method is effective for localization accuracy.