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The effect of head rotation on monaural sound-image localization in the horizontal plane
Abstract
Spectral cues (SCs) formed by the pinna are known to be essential for sound externalization, and accurate localization of sound-source azimuth and elevation in binaural listeners. SCs are also know to play a key role in monaural sound localization. The experiments described in this article intended to clarify how changes in SCs associated with head rotation affect monaural listeners’ localization of the sound-image, i.e. the apparent sound object, in the horizontal plane. First, a monaural localization experiment under head-still conditions confirmed previous findings that SCs contribute to sound-source localization and sound-images are localized only in the open-ear hemifield. Second, a monaural localization experiment under head-turning conditions showed that the perceived sound-image of a stationary sound-source moves around the head according to the head rotation, suggesting that changes in SCs do not contribute to sound image localization in monaural listeners. Thirdly, sound image tracking experiments under head-turning conditions showed that: when the stationary sound source remains on the open-ear side during head-rotation, the apparent sound image moves only a little; when the sound source remains on the occluded-ear side during head-rotation, the sound image moves more substantially; and the perceived sound image movement is largest, often including a jump, when the sound source either enters into or comes out from the head-shadow of the open ear. In contrast with binaural listeners, head rotation does not help sound localization in monaural listeners.
... Finally, it was also expected that dMSS cues do not affect localization performance. This hypothesis is supported by the findings that dMSS cues during monaural listening (Hirahara et al., 2021) and pitch rotations (i.e., rotations along the interaural axis) (Thurlow and Runge, 1967;Kato et al., 2003) have no effect on sound localization. ...
... Often, however, pitch rotation did not produce an improvement in localization performance (Thurlow and Runge, 1967;Kato et al., 2003). In a monaural listening experiment, normal-hearing listeners with one ear plugged showed no benefit of head rotations when localizing sounds located in the horizontal plane (Hirahara et al., 2021). However, single-sided deaf listeners do appear to utilize and even rely on changes in head position to induce changes in the monaural cues produced by the direction-dependent high-frequency attenuation resulting from acoustic head shadowing (Pastore et al., 2020). ...
Natural listening involves a constant deployment of small head movement. Spatial listening is facilitated by head movements, especially when resolving front-back confusions, an otherwise common issue during sound localization under head-still conditions. The present study investigated which acoustic cues are utilized by human listeners to localize sounds using small head movements (below ±10° around the center). Seven normal-hearing subjects participated in a sound localization experiment in a virtual reality environment. Four acoustic cue stimulus conditions were presented (full spectrum, flattened spectrum, frozen spectrum, free-field) under three movement conditions (no movement, head rotations over the yaw axis and over the pitch axis). Localization performance was assessed using three metrics: lateral and polar precision error and front-back confusion rate. Analysis through mixed-effects models showed that even small yaw rotations provide a remarkable decrease in front-back confusion rate, whereas pitch rotations did not show much of an effect. Furthermore, MSS cues improved localization performance even in the presence of dITD cues. However, performance was similar between stimuli with and without dMSS cues. This indicates that human listeners utilize the MSS cues before the head moves, but do not rely on dMSS cues to localize sounds when utilizing small head movements.
... The steepness of the slope may explain why elevation estimation based on dynamic ITD only improves for elevations greater than 30°above or below the horizontal plane [6]. Generally, the relation between the dynamic ITD rate and elevation perception seems to be quite complex, as it seems to further depend on the stimulus bandwidth [61] and might even be supported by dynamic spectral cues [38], but not in a monaural listening situation [66]. ...
Over the decades, Bayesian statistical inference has become a staple technique for modelling human multisensory perception. Many studies have successfully shown how sensory and prior information can be combined to optimally interpret our environment. Because of the multiple sound localisation cues available in the binaural signal, sound localisation models based on Bayesian inference are a promising way of explaining behavioural human data. An interesting aspect is the consideration of dynamic localisation cues obtained through self-motion. Here we provide a review of the recent developments in modelling dynamic sound localisation with a particular focus on Bayesian inference. Further, we describe a theoretical Bayesian framework capable to model dynamic and active listening situations in humans in a static auditory environment. In order to demonstrate its potential in future implementations, we provide results from two examples of simplified versions of that framework.
A study of sound localization performance was conducted using headphone-delivered virtual speech stimuli, rendered via HRTF-based acoustic auralization software and hardware, and blocked-meatus HRTF measurements. The independent variables were chosen to evaluate commonly held assumptions in the literature regarding improved localization: inclusion of head tracking, individualized HRTFs, and early and diffuse reflections. Significant effects were found-for azimuth and elevation error, reversal rates, and externalization.
This study compared the horizontal and median plane sound localization performances of binaural signals provided using a pinna-less dummy head or a stereo microphone that turns in synchronization with a listener's head yaw rotation in the head-still and head-movement tasks. Results show that the sound localization performances in the head-movement tasks are significantly higher than those in the head-still tasks in both the horizontal and median plane. The dynamic binaural cues synchronized with a listener's head yaw rotation dissolve the distance ambiguities, front-to-back ambiguities and elevation ambiguities, yielding better sound localization performances.
Localization of low-pass sounds was tested in relation to aspects of Wallach’s (1939, 1940) hypotheses about the role of head
movement in front/back and elevation discrimination. With a 3-sec signal, free movement of the head offered only small advantage
over a single rotation through 45° for detecting elevation differences. Very slight rotation, as observed using a 0.5-sec
signal, seemed sufficient to prevent front/back confusion. Cluster analysis showed that, in detecting elevation, some listeners
benefited from rotation, some benefited from natural movement, and some from both. Evidence was found indicating that a moving
auditory system generates information for the whereabouts of sounds, even when the movement does not result in the listener
facing the source. Results offer significant if partial support for Wallach’s hypotheses.
In keeping with our promise earlier in this review, we summarize here the process by which we believe spatial cues are used for localizing a sound source in a free-field listening situation. We believe it entails two parallel processes: 1. The azimuth of the source is determined using differences in interaural time or interaural intensity, whichever is present. Wightman and colleagues (1989) believe the low-frequency temporal information is dominant if both are present. 2. The elevation of the source is determined from spectral shape cues. The received sound spectrum, as modified by the pinna, is in effect compared with a stored set of directional transfer functions. These are actually the spectra of a nearly flat source heard at various elevations. The elevation that corresponds to the best-matching transfer function is selected as the locus of the sound. Pinnae are similar enough between people that certain general rules (e.g. Blauert's boosted bands or Butler's covert peaks) can describe this process. Head motion is probably not a critical part of the localization process, except in cases where time permits a very detailed assessment of location, in which case one tries to localize the source by turning the head toward the putative location. Sound localization is only moderately more precise when the listener points directly toward the source. The process is not analogous to localizing a visual source on the fovea of the retina. Thus, head motion provides only a moderate increase in localization accuracy. Finally, current evidence does not support the view that auditory motion perception is anything more than detection of changes in static location over time.
Four listeners (experimental group) received one trial a session for 60 sessions on a monaural localization task in which
six loudspeakers, positioned 15 deg apart, were placed on the side of the functioning ear. Four additional listeners (control
group) received 60 massed trials first and then one trial a session for 60 sessions. The stimulus consisted of a single train
of five high-frequency pulses. An ANOVA conducted on the 60 trials distributed over sessions showed that the experimental
group’s performance was significantly inferior to that of the control group. Members of the former continued to displace sounds
toward the open ear throughout the entire test. Those of the control group exhibited this tendency, but to a much lesser extent.
The experience of locating 60 massed trials clearly had a robust effect on subsequent performance. We contend that, by virtue
of presenting only one trial a session, the magnitude of the massed-trial effect was little attenuated by our tests to measure
it. We interpret our substantive finding to mean that during the 60 massed trials, the control group formed a perceptual representation
of the auditory space which served it well when placed on the one-trial-a-session regimen. Those in the experimental group
were unable to develop an adequate representation of the auditory space, presumably because their monaural localization experience
was restricted to only one trial a session.
Listeners perceive the sounds of the real world to be externalized. The sound images are compact and correctly located in space. The experiments reported in this article attempted to determine the characteristics of signals appearing in the ear canals that are responsible for the perception of externalization. The experiments used headphones to gain experimental control, and they employed a psychophysical method whereby the measurement of externalization was reduced to discrimination. When the headphone signals were synthesized to best resemble real-world signals (the baseline synthesis) listeners could not distinguish between the virtual image created by the headphones and the real source. Externalization was then studied, using both discrimination and listener rating, by systematically modifying the baseline synthesis. It was found that externalization depends on the interaural phases of low-frequency components but not high-frequency components, as defined by a boundary near 1 kHz. By contrast, interaural level differences in all frequency ranges appear to be about equally important. Other experiments showed that externalization requires realistic spectral profiles in both ears; maintaining only the interaural difference spectrum is inadequate. It was also found that externalization does not depend on dispersion around the head; an optimum interaural time difference proved to be an adequate phase relationship.
A study of sound localization performance was conducted using headphone-delivered virtual speech stimuli, rendered via HRTF-based acoustic auralization software and hardware, and blocked-meatus HRTF measurements. The independent variables were chosen to evaluate commonly held assumptions in the literature regarding improved localization: inclusion of head tracking, individualized HRTFs, and early and diffuse reflections. Significant effects were found for azimuth and elevation error, reversal rates, and externalization.
Monaurally deaf people lack the binaural acoustic difference cues in sound level and timing that are needed to encode sound location in the horizontal plane (azimuth). It has been proposed that these people therefore rely on spectral pinna cues of their normal ear to localize sounds. However, the acoustic head-shadow effect (HSE) might also serve as an azimuth cue, despite its ambiguity when absolute sound levels are unknown. Here, we assess the contribution of either cue in the monaural deaf to two-dimensional (2D) sound localization. In a localization test with randomly interleaved sound levels, we show that all monaurally deaf listeners relied heavily on the HSE, whereas binaural control listeners ignore this cue. However, some monaural listeners responded partly to actual sound-source azimuth, regardless of sound level. We show that these listeners extracted azimuth information from their pinna cues. The better monaural listeners were able to localize azimuth on the basis of spectral cues, the better their ability to also localize sound-source elevation. In a subsequent localization experiment with one fixed sound level, monaural listeners rapidly adopted a strategy on the basis of the HSE. We conclude that monaural spectral cues are not sufficient for adequate 2D sound localization under unfamiliar acoustic conditions. Thus, monaural listeners strongly rely on the ambiguous HSE, which may help them to cope with familiar acoustic environments.
It is well known that the interaural time difference, interaural level difference and spectral cues are used to determine three-dimensional sound localization in binaural hearing. In the case of monaural hearing, the interaural time difference and interaural level difference are not used. Therefore, it is assumed that there is a different perception of sound localization between binaural and monaural hearing. In this study, we investigate the difference in the horizontal localization of sound images and sources in monaural hearing. An experiment involving horizontal sound localization was performed with one female participant suffering from congenital complete hearing loss in the left ear. The experimental system consisted of 12 loudspeakers placed horizontally on the circumference of a circle having a radius of 1 m at 30° intervals. Four experimental sessions were performed (including 60 white-noise stimuli per session). Excluding the instances with no localization (12%), all sound images were localized on the right side (0-180°). It appeared that sound images were localized on the side with the normal-hearing ear but not on the side with the deaf ear. Sound source localization was possible generally over 360° (with ± 30° allowance, 90.8%). As a result, we confirmed that the localization of sound images and sources was different in congenital monaural hearing.
Partial hearing loss was simulated by insertion of V51‐R plastic earplugs. Subjects wore plugs continuously for periods ranging from 6 h to 3 days. Predictable shifts in localization errors were observed when the stimulus was a broad‐band and noise made up of frequencies above 3000 cps. Reorientation in azimuth localizazation with earplugs inserted required three days or more unless accelerated by specific training.
Some researchers have reported on how sound localization is affected by the temporal variation of the ITD, ILD and SCs. For horizontal sound localization, Perrett and Noble showed that head rotation facilitated sound localization with 2-kHz low-pass noise. They also showed that head rotation facilitated sound localization with the 2- and 4-kHz high-pass noise, but not when short tubes were inserted in listeners' ear canals. The fact that SCs diminished when short tubes were applied suggests that the temporal variation of SCs facilitates median sound localization but that of ILD does not. Under the head-still condition, subjects were instructed to keep their eyes closed and also to hold their head still while the stimulus was being reproduced. Under the head-rotation condition, subjects were instructed to keep their eyes closed but were free to move their head in a horizontal direction while the stimulus was being reproduced.
Experiments simulating sound fields radiated from a loudspeaker by using headphones are described. Comparing the differences in characteristics between the sound reproduced through a loudspeaker and that through headphone, speaker listening conditions were simulated by a headphone listening using digital signal processing techniques on the following two aspects: the static state transfer function from the loudspeaker to the external-ear entrance of a subject (referred to as “head-related transfer function” here); and the dynamic change in interaural time difference (ITD) caused by head movement of the subject. When only the static transfer function was simulated, perceived sounds were mostly localized at the intended direction outside subject's head, but a front-rear confusion was often observed when the intended direction was near the median plane. Furthermore, perceived distances of sounds are generally shorter than intended ones. When the dynamic changes in interaural time difference are simulated in addition to the static head-related transfer functions, however, the front-rear confusion remarkably decreased and natural sound localization was achieved.
Humans, and other mammals, make use of three cues to localise sound sources. Two of these are binaural, involving a comparison of the level and/or timing of the sound at each ear. For high frequencies, level differences result from shadowing by the head. For low-frequencies, localisation relies on the time differences between the signals at the ears that result from different sound paths to the ears. The third cue depends on sensitivity to the elevation-dependent pattern of spectral peaks and troughs that result from multiple sound waves interfering at the tympanic membrane. Different physiological mechanisms process these different localisation cues. Neurons in the dorsal cochlear nucleus are selectively sensitive to the spectral notches that result from interference between sound waves at the ear. Interaural level differences are initially processed in the lateral superior olive by neurons receiving inhibition from one ear and excitation from the other. Interaural time differences are converted into discharge rate by neurons in the medial superior olive with excitatory inputs from both ears and that only fire when their inputs are coincident. The contribution of such coincidence detectors to sound-source localisation is discussed in the light of recent observations.
In this paper, we examine how covering one or both external ears affects sound localization on the horizontal plane. In our experiments, we covered subjects' pinnae and external auditory canals with headphones, earphones, and earplugs, and conducted sound localization tests. Stimuli were presented from 12 different directions, and 12 subjects participated in the sound localization tests. The results indicate that covering one or both ears decreased their sound localization performance. Front-back confusion rates increased, particularly when covering both outer ears with open-air headphones or covering one ear with an intraconcha-type earphone or an earplug. Furthermore, incorrect answer rates were high when the sound source and the occluded ear that had an intraconcha-type earphone or an earplug were on the same side. We consider that the factors that cause poor performance can be clarified by comparing these results with characteristics of head-related transfer function.
Experiments of synthetic production of sound directions are reported which show that either vestibular cues or visual cues can replace head movements as such. In one group of experiments the blindfolded subject localized the sound while he was passively turned on a revolving chair, and in the other group the subject observed the direction of sound while seated inside a revolving screen. The results indicate that (1) fairly accurate representation of the actual displacement of the head is furnished by vestibular stimulation and that (2) visual stimulation, equivalent to that which actual displacement of the head would give, suffices to determine the direction of sound. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
With one ear occluded, 17 listeners were asked to locate tone bursts, .25, 4, .6, .9. l.4, 2.0, 3.2, 4.8, and 7.2 kHz, generated
by a loudspeaker concealed from view. The S’s response was to callout that number, from a series of numbers arranged horizontally,
behind which he thought the tone bursts originated. The listeners perceived the sounds as emanating from the side of the unoccluded
ear, but their judgments bore no consistent relation to the actual location of the sound source. Rather, the listeners showed
a strong tendency to locate a tone burst, within the range of .9 through 7.2 kHz, in a fixed spatial relation to the next
higher- and lower-pitched tone burst. Distorting the pinna of the unoccluded ear failed to modify the perceptual pattern.
It was suggested that the perceived spatial relations among the various frequencies was a by-product of the tonotopic organization
of the auditory nervous system.
TeleHead I is an acoustical telepresence robot that we built on the basis of the concept that remote sound localization could be best achieved by using a user-like dummy head whose movement synchronizes with the user's head movement in real time. We clarified the characteristics of the latest version of TeleHead I, TeleHead II, and verified the validity of this concept by sound localization experiments. TeleHead II can synchronize stably with the user's head movement with a 120-ms delay. The driving noise level measured through headphones is below 24 dB SPL from 1 to 4 kHz. The shape difference between the dummy head and the user is about 3% in head width and 5% in head length. An overall measurement metric indicated that the difference between the head-related transfer functions (HRTFs) of the dummy head and the modeled listener is about 5 dB. The results of the sound localization experiments using TeleHead II clarified that head movement improves horizontal-plane sound localization performance even when the dummy head shape differs from the user's head shape. In contrast, the results for head movement when the dummy head shape and user head shape are different were inconsistent in the median plane. The accuracy of sound localization when using the same-shape dummy head with movement tethered to the user's head movement was always good. These results show that the TeleHead concept is acceptable for building an acoustical telepresence robot. They also show that the physical characteristics of TeleHead II are sufficient for conducting sound localization experiments.
Monaural localization of low-pass noise positioned in the horizontal plane on the side of the unoccluded ear was investigated. Listeners showed no localizing ability until the upper cutoff frequency of the noise was advanced to 5.0 kHz. Performance proficiency increased progressively with the inclusion of still higher audio frequencies. Consonant with the behavioral data were the results of sound spectrum measurements performed on a head–pinna model. The spectral composition of a broad-band noise positioned at various azimuthal positions became clearly distinguishable, one from another, only at the higher frequencies.
Subject Classification: 65.62, 65.75, 65.54.
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.
The role of spectral cues in the sound source to ear transfer function in median plane sound localization is investigated in this paper. At first, transfer functions were measured and analyzed. Then, these transfer functions were used in experiments where sounds from a source on the median plane were simulated and presented to subjects through headphones. In these simulation experiments, the transfer functions were smoothed by ARMA models with different degrees of simplification to investigate the role of microscopic and macroscopic patterns in the transfer functions for median plane localization. The results of the study are summarized as follows: (1) For front-rear judgment, information derived from microscopic peaks and dips in the low-frequency region (below 2 kHz) and the macroscopic patterns in the high-frequency region seems to be utilized; (2) for judgment of elevation angle, major cues exist in the high-frequency region above 5 kHz. The information in macroscopic patterns is utilized instead of that in small peaks and dips.
A study is reported in which the acuity of azimuth and elevation discrimination under monaural listening conditions was measured. Six subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degrees to 180 degrees), at 10 degrees intervals, on the left side of the head. Monaural listening conditions were established by the fitting of an ear defender and one earmuff to the right ear. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The results indicate that all subjects suffered a marked reduction of azimuth acuity under monaural conditions, although a coarse capacity to discriminate azimuth still remained. Considerable between-subject variability was observed. Front/back discrimination was retained, although it was slightly impaired compared to that observed under normal listening conditions. Elevation discrimination was, on the whole, quite good under monaural conditions. However, a comparison of the performance of these subjects under monaural conditions with that observed under normal listening conditions indicated that some reduction in elevation localisation acuity occurred in the frontal quadrants in the median plane and in the upper quadrants of more lateral source positions. The reduction in acuity seen in these regions is attributed to the loss of information from the pinna of the occluded ear rather than to the observed reduction in azimuth error. The results provide partial support for the binaural pinna disparity model.
The spectral cues used for median plane localization are described by 3 experiments. First, the frequency spectrum necessary for localization is measured by noting the accuracy of subjects localizing low and high pass filtered white noise. Second, several high pass, low pass, bandpass, and bandstop filters are associated with the subjective impression of direction by observing what directions are most frequently perceived by subjects localizing white noise colored by each filter. Third, the frequency responses of several artificial ears are measured for different angles of median plane sound incidence. Results show that sound spectra from 4 to 16 kHz are necessary for localization. Frontal cues are a 1 octave notch, with a lower frequency cutoff between 4 and 10 kHz and increased energy above 13 kHz. The 'above' cue is a 1/4 octave peak between 7 and 9 kHz, with a high frequency cutoff at 10 kHz. The 'behind' cue is a small peak from 10 to 12 kHz. Increases in frontal elevation are signaled by an increase in the lower cutoff frequency of the 1 octave notch. This notch appears to be generated by time delayed reflections off the posterior concha wall interfering with sound directly entering the external auditory canal.
We tested the ability of human listeners to localize broadband noise bursts in the absence of binaural localization cues. The subject population consisted of five patients, who had normal hearing in one ear and congenital deafness in the other, and seven normal controls, who were tested with both ears open and with one ear plugged. Consistent with previous reports, the introduction of an ear plug unilaterally into control subjects resulted in a prominent lateral displacement in their localization judgements by an average of 30.9 degrees toward the side of the open ear. Vertical localization was less strongly impaired. The five monaural patients showed a considerable range of ability to localize sounds. Two of the patients were essentially indistinguishable from the plugged control subjects in that they showed a prominent displacement of responses toward the side of the hearing ear. The other three subjects localized significantly better than the plugged controls, in that they demonstrated little or no lateral displacement toward the hearing side and that they localized targets on the hearing and on the impaired sides about equally well. The performance of these latter patients demonstrates that monaural cues can provide useful localization information in the horizontal as well as in the vertical dimension.
Research reported during the past few decades has revealed the importance for human sound localization of the so-called "monaural spectral cues." These cues are the result of the direction-dependent filtering of incoming sound waves accomplished by the pinnae. One point of view about how these cues are extracted places great emphasis on the spectrum of the received sound at each ear individually. This leads to the suggestion that an effective way of studying the influence of these cues is to measure the ability of listeners to localize sounds when one of their ears is plugged. Numerous studies have appeared using this monaural localization paradigm. Three experiments are described here which are intended to clarify the results of the previous monaural localization studies and provide new data on how monaural spectral cues might be processed. Virtual sound sources are used in the experiments in order to manipulate and control the stimuli independently at the two ears. Two of the experiments deal with the consequences of the incomplete monauralization that may have contaminated previous work. The results suggest that even very low sound levels in the occluded ear provide access to interaural localization cues. The presence of these cues complicates the interpretation of the results of nominally monaural localization studies. The third experiment concerns the role of prior knowledge of the source spectrum, which is required if monaural cues are to be useful. The results of this last experiment demonstrate that extraction of monaural spectral cues can be severely disrupted by trial-to-trial fluctuations in the source spectrum. The general conclusion of the experiments is that, while monaural spectral cues are important, the monaural localization paradigm may not be the most appropriate way to study their role.
Normally, the apparent position of a sound source corresponds closely to its actual position. However, in some experimental situations listeners make large errors, such as indicating that a source in the frontal hemifield appears to be in the rear hemifield, or vice versa. These front-back confusions are thought to be a result of the inherent ambiguity of the primary interaural difference cues, interaural time difference (ITD) in particular. A given ITD could have been produced by a sound source anywhere on the so-called "cone of confusion." More than 50 years ago Wallach [J. Exp. Psychol. 27, 339-368 (1940)] argued that small head movements could provide the information necessary to resolve the ambiguity. The direction of the change in ITD that accompanies a head rotation is an unambiguous indicator of the proper hemifield. The experiments reported here are a modern test of Wallach's hypothesis. Listeners indicated the apparent positions of real and virtual sound sources in conditions in which head movements were either restricted or encouraged. The front-back confusions made in the restricted condition nearly disappeared in the condition in which head movements were encouraged. In a second experiment head movements were restricted, but the sound source was moved, either by the experimenter or by the listener. Only when the listener moved the sound source did front-back confusions disappear. The results clearly support Wallach's hypothesis and suggest further that head movements are not required to produce the dynamic cues needed to resolve front-back ambiguity.
Some particulars of directional hearing
- de Boer