Xuan Zhong

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 participants 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.

Sound source localization identification accuracy: bandwidth dependencies.

Sound source localization accuracy using a sound source identification task was measured in the front, right quarter of the azimuth plane as rms (root-mean-square) error (degrees) for stimulus conditions in which the bandwidth (1/20 to 2 octaves wide) and center frequency (250, 2000, 4000 Hz) of 200-ms noise bursts were varied. Tones of different frequencies (250, 2000, 4000 Hz) were also used. As stimulus bandwidth increases, there is an increase in sound source localization identification accuracy (i.e., rms error decreases). Wideband stimuli (>1 octave wide) produce best sound source localization accuracy (~6°-7° rms error), and localization accuracy for these wideband noise stimuli does not depend on center frequency. For narrow bandwidths (<1 octave) and tonal stimuli, accuracy does depend on center frequency such that highest accuracy is obtained for low-frequency stimuli (centered on 250 Hz), worse accuracy for mid-frequency stimuli (centered on 2000 Hz), and intermediate accuracy for high-frequency stimuli (centered on 4000 Hz).

Judging sound rotation when listeners and sounds rotate: Sound source localization is a multisystem process

In four experiments listeners were rotated or were stationary. Sounds came from a stationary loudspeaker or rotated from loudspeaker to loudspeaker around an azimuth array. When either sounds or listeners rotate the auditory cues used for sound source localization change, but in the everyday world listeners perceive sound rotation only when sounds rotate not when listeners rotate. In the everyday world sound source locations are referenced to positions in the environment (a world-centric reference system). The auditory cues for sound source location indicate locations relative to the head (a head-centric reference system), not locations relative to the world. This paper deals with a general hypothesis that the world-centric location of sound sources requires the auditory system to have information about auditory cues used for sound source location and cues about head position. The use of visual and vestibular information in determining rotating head position in sound rotation perception was investigated. The experiments show that sound rotation perception when sources and listeners rotate was based on acoustic, visual, and, perhaps, vestibular information. The findings are consistent with the general hypotheses and suggest that sound source localization is not based just on acoustics. It is a multisystem process.

Active binaural localization of multiple sound sources

Sound source localization serves as a significant capability of autonomous robots that conduct missions such as search and rescue, and target tracking in challenging environments. However, localization of multiple sound sources and static sound source tracking in self-motion are both difficult tasks, especially when the number of sound sources or reflections increase. This study presents two robotic hearing approaches based on a human perception model (Wallach, 1939) that combines interaural time difference (ITD) and head turn motion data to locate sound sources. The first method uses a fitting-based approach to recognize the changing trends of the cross-correlation function of binaural inputs. The effectiveness of this method was validated using data collected from a two-microphone array rotating in a non-anechoic environment, and the experiments reveal its ability to separate and localize up to three sound sources of the same spectral content (white noise) at different azimuth and elevation angles. The second method uses an extended Kalman filter (EKF) that estimates the orientation of a sound source by fusing the robots self-motion and ITD data to reduce the localization errors recursively. This method requires limited memory resources and is able to keep tracking the relative position change of a number of static sources when the robot moves. In the experiments, up to three sources can be tracked simultaneously with a two-microphone array. Sound source localization methods based on robot head motion and ITD data are proposed.Multiple sources with overlapping spectra are localized with binaural inputs in non-anechoic spaces.The data fitting method separates the sources based on correlogram changing patterns.An EKF is established for each sound source to keep track of it during self-motion.

Dynamic binaural sound source localization with interaural time difference cues: Artificial listeners

When an array of two acoustic sensors is used to localize sound sources based on time differences alone, possible solutions form a cone of confusion. This study, together with a similar one for human listeners, demonstrates that azimuth/vertical planes localization of a sound source using only time difference information is feasible when self-motion measurement of the listener is available. In particular, the case of a static sound source playing that broadcasts low frequency pure tone signals was investigated. A dummy head is mounted on top of a rotating chair to mimic the head and body motion of human beings, as well as to collect audio signals. A gyroscope was mounted on top of the dummy head to collect self-motion data. A mathematical model was constructed to describe the interaural time difference (ITD) change over time, and an Extended Kalman Filter (EKF) was used to estimate the spatial angles of the sound sources with respect to the listener using the developed mathematical model and measured data. The effectiveness and robustness of the developed algorithm are shown by both the numerical and experimental results, which reveal the quick convergence of the estimated spatial angles toward their real values given noisy measured data. The possibilities of using other spatial hearing cues were also discussed.

Dynamic binaural sound source localization with ITD cues: Human listeners

In real life, human listeners rarely experience cone of confusion errors in localization of sound sources due to the limitation of interaural time difference (ITD) as a spatial hearing cue. Previous work on robotics suggested that the confusion is disambiguated as successive observation of ITD is made over time, when self-motion of the microphone system was allowed. In this behavioral study, we investigated whether horizontal/vertical planes localization with time difference is possible when self-motion is allowed for human listeners. In particular, the case of a static sound source playing low frequency pure tone signal was studied. Human listeners were seated in a rotating chair in the middle of a loudspeaker array, and a low frequency audio signal was played at an elevation. The task was to identify the elevation spatial angle under conditions when vision was allowed and denied. The hypothesis was, with vision, a much more robust observation of self-motion is available, and the report of elevation woul...

Using tactile aids to provide low frequency information for cochlear implant users

Cochlear implant (CI) users have shown benefit from residual low-frequency hearing in the contra-lateral ear (Dorman and Gifford, 2010). One source of this benefit is the enhancement of cues important for identifying word boundaries (Spitzer et al., 2009). However, there are a large number of CI users who do not have residual hearing, but who could presumably benefit from cues available in low-frequency information. Because the frequency sensitivity of human haptic sensation is similar to that of human acoustic hearing in low frequencies, we examined the ability of tactile aids to convey low-frequency cues. Using experimental phrases designed to have low inter-word predictability, and balanced for syllabic stress (trochaic/iambic), 5 CI users and 10 normal hearing participants (simulation) provided transcriptions that were scored for percent words-correct and for errors in word segmentation (lexical boundary errors, LBE). A 350 Hz sinusoid carrier modulated with overall envelope of corresponding acoustic ...

Sound source localization from tactile aids for unilateral cochlear implant users

The present research asks whether two tactile aids with directional microphones, by providing additional inter-channel level information and etc., could help unilateral cochlear implant (CI) localize sound sources. For normal hearing subjects, sound source localization based on tactile vibration cues alone can be as accurate as auditory localization in the frontal horizontal plane (Gescheider, 1970). CI users may as well benefit from additional tactile aids just as normal hearing people do. The current study uses two bone-anchored hearing aids (BAHA) as sources of tactile vibration. The two BAHAs, bonded together by a special gadget to maintain a particular distance and angle, both have directional microphones, and are programed so that one point to the front-left side and the other to the front-right side. Unilateral CI users voluntarily participated in the experimental study. Wide band noise stimuli were presented at 65 dB SPL. The subjects hold one BAHA in each hand and do localization tasks with (1) C...

How many images are in an auditory scene

If an auditory scene consists of many spatially separated sound sources, how many sound sources can be processed by the auditory system? Experiment I determined how many speech sources could be localized simultaneously on the azimuth plane. Different words were played from multiple loudspeakers, and listeners reported the total number of sound sources and their individual locations. In experiment II the accuracy of localizing one speech source in a mixture of multiple speech sources was determined. An extra sound source was added to an existing set of sound sources, and the task was to localize that extra source. In experiment III the setup and task were the same as in experiment I, except that the sounds were tones. The results showed that the maximum number of sound sources that listeners could perceive was limited to approximately four spatially separated speech signals and three for tonal signals. The localization errors increased along with the increase of total number of sound sources. When four or more speech sources already existed, the accuracy in localizing an additional source was near chance.

Rotating sound sources and listeners: Sound source localization is a multisensory/cognitive process

When sound sources or listeners rotate, the acoustic cues used for sound source localization change, but in the everyday world listeners perceive sound rotation only when the sound source rotates not when the listener rotates. That is, in the everyday world, sound source locations are referenced to positions in the environment (a world-centric reference system). The acoustic cues for sound source location indicate a sound source’s location relative to the head (a head-centric reference system), not locations relative to the world. To compute world-centric locations of sound sources, the auditory spatial system must have information about the acoustic cues used for sound source location and cues about the position of the head. The use of visual and vestibular information in determining head position in sound rotation perception was investigated in four experiments. The experiments clearly show, for the first time, that sound source localization when sound sources and listeners rotate is based on acoustic a...