Available response choices affect localization of sound

Successful replication of an experiment by Butler and Humanski (1992) showed that listeners are able to proficiently localize sources on a lateral vertical plane on the basis of interaural differences alone. When a lateral horizontal array was included in the test setup, that finding was replicated only for a broadband signal interacting with the pinna, not for ones (lowpass and pure tone) providing only interaural differences. Cross-plane errors conforming to “cones of confusion” were observed for those latter sounds. In a second experiment, response options were made more unconstrained, which clarified the nature of the cross-plane confusions. Lowpass signals from lateral vertical plane sources tend to be heard at or close to the horizon. Measurement of cue values needs to take account of the response options available to listeners, as well as signal properties.     

The role of posture in sound localization

Studies of auditory localization revealed that where a subject hears a sound is dependent on both his perceived head position and the auditory cues at his ears. If an error is induced between his true and registered head posture, then errors in his auditory localizations of corresponding size and time course result. The presence of visual information prevents the development of postural errors and, consequently, prevents the development of errors in auditory localization, too. These observations are related to the oculogravic illusion and are interpreted as one aspect of the functioning of a spatial reference system involved in the maintenance of the constancies of auditory and visual detection.     

Relation of sound intensity and accuracy of localization

Tests were carried out on 17 subjects to determine the accuracy of monaural sound localization when the head is not free to turn toward the sound source. Maximum accuracy of localization for a constant-volume sound source coincided with the position for maximum perceived intensity of the sound in the front quadrant. There was a tendency for sounds to be perceived more often as coming from a position directly toward the ear. That is, for sounds in the front quadrant, errors of localization tended to be predominantly clockwise (i.e., biased toward a line directly facing the ear). Errors for sounds occurring in the rear quadrant tended to be anticlockwise. The pinna's differential effect on sound intensity between front and rear quadrants would assist in identifying the direction of movement of objects, for example an insect, passing the ear.     

Binaural and monaural localization of sound in two-dimensional space

Two experiments were conducted. In experiment 1, part 1, binaural and monaural localization of sounds originating in the left hemifield was investigated. 104 loudspeakers were arranged in a 13 x 8 matrix with 15 degrees separating adjacent loudspeakers in each column and in each row. In the horizontal plane (HP), the loudspeakers extended from 0 degrees to 180 degrees; in the vertical plane (VP), they extended from -45 degrees to 60 degrees with respect to the interaural axis. Findings of special interest were: (i) binaural listeners identified the VP coordinate of the sound source more accurately than did monaural listeners, and (ii) monaural listeners identified the VP coordinate of the sound source more accurately than its HP coordinate. In part 2, it was found that foreknowledge of the HP coordinate of the sound source aided monaural listeners in identifying its VP coordinate, but the converse did not hold. In experiment 2, part 1, localization performances were evaluated when the sound originated from consecutive 45 degrees segments of the HP, with the VP segments extending from -22.5 degrees to 22.5 degrees. Part 2 consisted of measuring, on the same subjects, head-related transfer functions by means of a miniature microphone placed at the entrance of their external ear canal. From these data, the 'covert' peaks (defined and illustrated in text) of the sound spectrum were extracted. This spectral cue was advanced to explain why monaural listeners in this study as well as in other studies performed better when locating VP-positioned sounds than when locating HP-positioned sounds. It is not claimed that there is inherent advantage for localizing sound in the VP; rather, monaural localization proficiency, whether in the VP or HP, depends on the availability of covert peaks which, in turn, rests on the spatial arrangement of the sound sources.     

Development of a device to measure human sound localization

Measurements of human sound discrimination and localization are important for basic empirical and clinical applications. After a short survey of other methods such as evoked potentials, the development of a new device to measure human sound localization is described and its use illustrated with some examples. Built from a polyacrylic hemisphere or--in a later version--from an orbicular aluminum frame, the apparatus uses multiple speakers to emit auditory stimuli. The patient sits in the middle of the perimeter and has to press a button when a sound is perceived. In addition, the participant has to identify the correct speaker as the source of the sound. With this method it is possible to map the auditory field.     

The effects of eye position and expectation on sound localization

Ss who are exposed to a sound coming from straight ahead, but who turn their eyes 20 deg to the side toward a visible speaker during the exposure period and expect to hear the sound coming from the visible source, show a shift in localization of the sound up to a maximum of about 9 deg. Ss who only turn their eyes 20 deg to the side during the exposure period show a smaller but significant shift in sound localization, while Ss who do not turn their eyes, but are led to expect that the sound will appear to come from a visible loudspeaker 20 deg to the side, show no significant shift. Comparison of test results before and after the exposure period, with eyes directed straight ahead and no visible speaker present, shows the presence of a localization aftereffect for those experimental groups that showed a significant localization shift during the exposure period. Sounds are localized a few degrees to the side of their physical location in the same direction as the shift in localization during the exposure period. Further experiments show that part, but not all, of the shift in localization during the exposure period can be understood in terms of a shift in perceived head direction. The localization aftereffects are shown not to be due to change in physical or perceived eye or head position.     

The influence of stimulus bandwidth on localization of sound in space

The ability of listeners, deprived of prominent interaural time and intensity cues, to locate noise bands differing in width was investigated. To minimize binaural cues, we placed the sound source at various positions in the median sagittal plane. To eliminate binaural cues, we occluded one ear. The stimuli consisted of broadband noise and bands of noise centered at 8.0 kHz. The width of the latter ranged from 1.0 to 6.0 kHz. The results from seven listeners showed that localization proficiency for sounds in the median sagittal plane decreased with decreases in bandwidth for both binaural and monaural listening conditions. This function was less orderly for monaural localization of horizontally positioned sounds. Another consequence of a reduction in bandwidth was an increasing tendency of listeners to select certain loudspeakers over others as the source of the sound. A previous finding showing that localization of sound in the median sagittal plane is more accurate when listening binaurally rather than monaurally was confirmed.     

The effects of attenuation of frequency segments on binaural localization of sound

Perceived location of tonal stimuli d narrow noise bands presented in two-dimensional space varies in an orderly manner with changes in stimulus frequency. Hence, frequency has a referent in space that is most apparent during monaural listening. The assumption underlying the present study is that maximum sound pressure level measured at the ear canal entrance for the various frequencies serves as a prominent spectral cue for their spatial referents. Even in binaural localization, location judgments in the vertical plane are strongly influenced by spatial referents. We measured sound pressure levels at the left ear canal entrance for 1.0-kHz-wide noise bands, centered from 4.0 kHz through 10.0 kHz, presented at locations from 60° through ?45° in the vertical plane; the horizontal plane coordinate was fixed at ?90°. On the basis of these measurements, we fabricated three different band-stop stimuli in which differently centered 2.0-kHz-wide frequency segments were filtered from a broadband noise. Unfiltered broadband noise served as the remaining stimulus. Localization accuracy differed significantly among stimulus conditions (p<.01). Where in the vertical plane most errors were made depended on which frequency segment was filtered from the broadband noise.     

A two-choice sound localization procedure for detecting lateralized tinnitus in animals

Rats were trained in a two-choice procedure to respond in the direction of left and right sounds. Silent trials, on which no sound was presented and for which the animals received no feedback, were interspersed among the sound trials to determine each animal’s natural side preference. Following training, the rats were exposed to a loud tone in the ear opposite their side preference. A shift in responding on the silent trials to the side of the exposed ear indicated that the animals were hearing a sound in that ear (i.e., tinnitus). Simulating lateralized tinnitus by presenting a low-level, continuous sound on one side also caused the rats to shift their responding on the silent trials to that side. Sham exposures indicated that halothane/nitrous oxide anesthesia could reinstate tinnitus in animals that had previously tested positive for it. Exposing rats to loud tones of various frequencies indicated that frequencies near the limits of the rat’s hearing range were less likely to cause tinnitus than tones in the midrange.     

Facial expressions modulate the gaze orienting effect on sound localization judgement

Recent studies have shown that cueing eye gaze can affect the processing of visual information, and this phenomenon is called the gaze-orienting effect (visual-GOE). Emerging evidence has shown that the cueing eye gaze also affects the processing of auditory information (auditory-GOE). However, it is unclear whether the auditory-GOE is modulated by emotion. We conducted three behavioural experiments to investigate whether cueing eye gaze influenced the orientation judgement to a sound, and whether the effect was modulated by facial expressions. The current study set four facial expressions (angry, fearful, happy, and neutral), manipulated the display type of facial expressions, and changed the sequence of gaze and emotional expressions. Participants were required to judge the sound orientation after facial expressions and gaze cues. The results showed that the orientation judgement of sound was influenced by gaze direction in all three experiments, and the orientation judgement of sound was faster when the face was oriented to the target location (congruent trials) than when the face was oriented away from the target location (incongruent trials). The modulation of emotion on auditory-GOE was observed only when gaze shifted followed by facial expression (Exp3); the auditory-GOE was significantly greater for angry faces than for neutral faces. These findings indicate that auditory-GOE as a social phenomenon exists widely, and the effect was modulated by facial expression. Gaze shift before the presentation of emotion was the key influencing factor for the emotional modulation in an auditory target gaze-orienting task. Our findings suggest that the integration of facial expressions and eye gaze was context-dependent.     

Influence of acoustic context on sound localization: An auditory Roelofs effect

The Roelofs effect is a visual direction illusion: if a large rectangular frame is seen offset from the straight-ahead direction, a small target presented simultaneously is mislocalized in the opposite direction. To investigate whether a similar context illusion might affect auditory localization, we presented a frame of 6 speakers driven with a 300-Hz square wave, 30° left or right of center. The target was a speaker driven with the same waveform, with the two sources in random phase relationship. The target was mislocalized in a direction opposite the frame, an auditory Roelofs effect. A second experiment, using dissimilar sounds for frame and target, yielded no frame-dependent mislocalizations. The effect appeared both in verbal position estimation, a measure of cognitive localization, and in open-loop pointing, a measure of localization in a sensorimotor system. We conclude that audition possesses only one representation of space, in contrast to the two (cognitive and sensorimotor) of vision. The auditory representation corresponds most closely to vision's cognitive system.     

Congruent auditory display and confusion in sound localization: Case of elderly drivers

Elderly people may suffer from age-related hearing loss. They often report difficulties to perceive and localize sound sources in noisy environment. How this can be a driving safety issue? This study investigates the effect of hearing impairment, on the driver behavior to localize external sound sources such as emergency vehicles sirens. Subjective tests show that localization confusion appears to be a common problem. Thus, we focus on how to assist the driver, taking into consideration the age-related hearing impairment, to better localize emergency vehicle siren. The hypothesis was based on the stimulus-response compatibility using an effective congruent auditory display, for attentional guidance toward the direction of arrival (DOA) of the external alarm. The proposed approach aims to reduce front-back confusion and enhance the sound localization accuracy of the driver, which is very important for elderly driver, subject to older age cognitive decline. A localization test was performed in lab and in vivo on a test track, where a group of drivers was asked to identify the DOA of an emergency vehicle siren, with and without a dedicated embedded system set up to assist them on sound localization task.     

The influence of the acoustic context on vertical sound localization in the median plane

The influence of background sounds (frames) on vertical localization of single sound sources (targets) was examined in four experiments. Loudspeakers (five targets and four frames) were positioned in the median plane, ranging from +30 degrees to -30 degrees above and below the subject's ear level. The subjects determined the vertical position of the targets by either verbal judgments or manual pointing. Frame and target sounds were presented concurrently or successively with a 1-sec interval; both consisted of (1) 300-Hz square waves, (2) noise, or (3) targets of noise and frames of 300-Hz square waves. Particularly in the second condition, the subjects consistently shifted the apparent target positions away from the frame locations. This contrast effect persisted even 1 sec after the offset of the frames. No effect was found with different waveforms for the frame and the target. Results are related to recent findings indicating a similar effect in the azimuthal dimension. Possibly the effect is based on a mechanism in which the auditory system adapts to recently heard sound source positions.     

Neuroanatomical basis of binaural phase-difference analysis for sound localization: a comparative study.

Four varieties of mammals whose medial superior olives range from large to none at all were tested for their ability to localize single, brief tone pips at various frequencies. Although each animal could localize high-frequency tone pips, their ability to localize middle- and low-frequency tone pips corresponded to the size of their medial superior olive (MSO). Since this latter range of frequencies is the one in which binaural phase-difference cues predominate, this anatomical-behavioral correspondence supports the idea that MSO is the chief binaural time-analyzing center for sound localization.     

Infants' perception of illusions in sound localization: reaching to sounds in the dark.

Sixteen infants each at 4, 6, and 8 months of age were tested for reaching to sounding toys in the dark under two auditory illusion conditions: the Haas-effect, which creates the illusion of a single lateralized sound based on an interaural intensity difference (the toy was visible and invisible under some test conditions); and the midline illusion, which creates the illusion of a single sound at midline due to an absence of any interaural time or intensity differences (invisible toy condition only). No-sound control trials indicated the level of spontaneous reaching in the dark. Results indicate that by 4 months infants perceive both the Haas-effect and midline illusions. The ability to reach both for invisible and visible sounding objects in the dark was well developed by 4 months of age, although developmental changes in aspects of reaching behavior were observed and, at all ages, object contact was most frequent when visual localization cues accompanied sound localization cues. The incidence of spontaneous reaching in the dark was low and did not vary with age. Theoretical and methodological implications of this research are discussed.