Relationship between postural stability and spatial hearing.

BACKGROUND Maintaining balance is known to be a multisensory process that uses information from different sensory organs. Although it has been known for a long time that spatial hearing cues provide humans with moderately accurate abilities to localize sound sources, how the auditory system interacts with balance mediated by the vestibular system remains largely a mystery. PURPOSE The primary goal of the current study was to determine whether auditory spatial cues obtained from a fixed sound source can help human participants balance themselves as compared to conditions in which participants use vision. RESEARCH DESIGN The experiment uses modified versions of conventional clinical tests: the Tandem Romberg test and the Fukuda Stepping test. In the Tandem Romberg test, participants stand with their feet in a heel-to-toe position, and try to maintain balance for 40 sec. In the Fukuda Stepping test, a participant is asked to close his or her eyes and to march in place for 100 steps. The sway and angular deviation of each participant was measured with and without vision and spatial auditory cues. An auditory spatial reference was provided by presenting a broadband noise source from a loudspeaker directly in front of the participant located 1-2 m away. STUDY SAMPLE A total of 19 participants (11 women and 8 men; mean age = 27 yr; age range = 18∼52 yr), voluntarily participated in the experiment. All participants had normal vision, hearing, and vestibular function. INTERVENTION The primary intervention was the use of a broadband noise source to provide an auditory spatial referent for balance measurements in the Tandem Romberg test and Fukuda Stepping test. Conditions were also tested in which the participants had their eyes opened or closed. DATA COLLECTION AND ANALYSIS A head tracker recorded the position of the participant's head for the Tandem Romberg test. The angular deviation of the feet after 100 steps was measured in the Fukuda Stepping test. An average distance or angle moved by the head or feet was calculated relative to the head or feet resting position for each test. The average head sway or angular deviation was measured in an eyes-open condition (no sound), eyes-closed condition (no sound), and an eyes-closed condition with sound. Repeated-measures analysis of variance was used for both tests. RESULTS The results showed a significant benefit in postural stability in both experiments when spatial auditory cues were present (p < .01). For the Tandem Romberg test, the benefit from spatial auditory cues alone is a 9% reduction in mean sway, as compared to 44% from visual cues alone. For the Fukuda Stepping test, the benefit from spatial auditory cues alone is a 76% reduction in mean body sway, as compared to 98% from visual cues alone. CONCLUSIONS The current study demonstrated a connection between spatial hearing and balance. The experiments showed that a single fixed sound source can provide sufficient spatial cues for the central nervous system to better control postural stability. The compensation effect that the vestibular system receives from the auditory cues, however, is weaker than that received from visual cues.