Auditory Spatial Resolution Research Articles

Static and moving minimum audible angle: Independent contributions of reverberation and position.

Two measures of auditory spatial resolution, the minimum audible angle and the minimum audible movement angle, have been obtained in a simulated acoustic environment using Ambisonics sound field reproduction. Trajectories were designed to provide no reliable cues for the spatial discrimination task. Larger threshold angles were found in reverberant compared to anechoic conditions, for stimuli on the side compared to the front, and for moving compared to static stimuli. The effect of reverberation appeared to be independent of the position of the sound source (same relative threshold increase) and was independently present for static and moving sound sources.

Neural distance amplification of lexical tone in human auditory cortex.

In tonal languages, like Chinese, lexical tone serves as a key feature to provide contrast in word meaning. Behavior studies suggest that Mandarin Chinese tone is categorically perceived. However, the neural mechanism underlying Mandarin tone perception is still poorly understood. In this study, an Oddball paradigm was designed by selecting two standard-deviant stimulus pairs with same physical distance but different category labels, among the synthesized tones with continuously varying pitch contours. Using electrocorticography (ECoG) recording over human auditory cortex, high temporal and spatial resolution cortical neural signals were used for the first time to investigate the cortical processing of lexical tone. Here, we found different neural responses to the two standard-deviant tone pairs, and the difference increased from low to high level along the hierarchy of human auditory cortex. In the two dimensional neural space, cross-category neural distance of lexical tones is selectively amplified on those high level electrodes. These findings support a hierarchical and categorical model of Mandarin tone perception, and favor the using of high-level electrodes for a better performance of lexical tone discrimination in speech brain computer interface.

Pre-attentive cortical processing of behaviorally perceptible spatial changes in older adults-a mismatch negativity study.

From behavioral studies it is known that auditory spatial resolution of azimuthal space declines over age. To date, it is not clear how age affects the respective sensory auditory processing at the pre-attentive level. Here we tested the hypothesis that pre-attentive processing of behaviorally perceptible spatial changes is preserved in older adults. An EEG-study was performed in older adults (65–82 years of age) and a mismatch negativity (MMN) paradigm employed. Sequences of frequent standard stimuli of defined azimuthal positions were presented together with rarely occurring deviants shifted by 10° or 20° to the left or to the right of the standard. Standard positions were at +5° (central condition) from the midsagittal plane and at 65° in both lateral hemifields (±65°; lateral condition). The results suggest an effect of laterality on the pre-attentive change processing of spatial deviations in older adults: While for the central conditions deviants close to MAA threshold (i.e., 10°) yielded discernable MMNs, for lateral positions the respective MMN responses were only elicited by spatial deviations of 20° toward the midline (i.e., ±45°). Furthermore, MMN amplitudes were found to be insensitive to the magnitude of deviation (10°, 20°), which is contrary to recent studies with young adults (Bennemann et al., 2013) and hints to a deteriorated pre-attentive encoding of sound sources in older adults. The discrepancy between behavioral MAA data and present results are discussed with respect to the possibility that under the condition of active stimulus processing older adults might benefit from recruiting additional attentional top-down processes to detect small magnitudes of spatial deviations even within the lateral acoustic field.

Directing Eye Gaze Enhances Auditory Spatial Cue Discrimination

The present study demonstrates, for the first time, a specific enhancement of auditory spatial cue discrimination due to eye gaze. Whereas the region of sharpest visual acuity, called the fovea, can be directed at will by moving one's eyes, auditory spatial information is derived primarily from head-related acoustic cues. Past auditory studies have found better discrimination in front of the head [1-3] but have not manipulated subjects' gaze, thus overlooking potential oculomotor influences. Electrophysiological studies have shown that the inferior colliculus, a critical auditory midbrain nucleus, shows visual and oculomotor responses [4-6] and modulations of auditory activity [7-9], and that auditory neurons in the superior colliculus show shifting receptive fields [10-13]. How the auditory system leverages this crossmodal information at the behavioral level remains unknown. Here we directed subjects' gaze (with an eccentric dot) or auditory attention (with lateralized noise) while they performed an auditory spatial cue discrimination task. We found that directing gaze toward a sound significantly enhances discrimination of both interaural level and time differences, whereas directing auditory spatial attention does not. These results show that oculomotor information variably enhances auditory spatial resolution even when the head remains stationary, revealing a distinct behavioral benefit possibly arising from auditory-oculomotor interactions at an earlier level of processing than previously demonstrated.

Spatial resolution and the binaural display.

An important feature of a Jeffress‐type binaural display is that it not only allows for localization of a single sound source but it also allows for resolution of multiple sound sources. Here an experiment is reported which directly addresses spatial resolution with a paradigm similar to the notched‐noise paradigm which quantifies spectral resolution. Listeners are required to detect a band‐pass signal at a frontal position in the presence of two independent band‐stop maskers symmetrically arranged around the signal in space and frequency. Data show that signal detection does not improve when the spatial separation between the signal and the maskers is increased from ± 10 to ± 90 deg. When listeners are turned by 90 deg, i.e., the signal is presented from the left, release from masking is only observed when the maskers are moved into the right hemisphere, an effect closely related to BMLDs observed under headphones. The current data show that auditory spatial resolution of simultaneous sound sources is very bad and in line with a “two channel model,” which allows for spatial acuity but not for spatial resolution. Follow‐up experiments quantify the extent to which this can be compensated for by the recently shown high binaural temporal resolution.

Effects of omni-directional noise-exposure during hearing onset and age on auditory spatial resolution in the Mongolian gerbil ( Meriones unguiculatus) — a behavioral approach

Inhibitory inputs to the binaural brainstem nuclei medial and lateral superior olives (MSO and LSO, respectively) are thought to be important for sound localization in mammals. Here, we investigate whether aged gerbils that typically exhibit degenerative changes in auditory nuclei providing inhibition to MSO and LSO show diminished localization ability. We also tested the localization ability in gerbils reared in omni-directional white noise during hearing onset, a treatment that affects the adjustment of inhibitory inputs to MSO neurons possibly resulting in weakened sensitivity to interaural time difference. Localization ability of both groups was compared to that of young gerbils raised under control conditions. Stimuli had a duration of 125 ms and were pure tones of 0.5, 1, 2, 4 and 8 kHz, 300-Hz-bands of noise centered at 0.5, 2 and 8 kHz or broad-band noise. Gerbils trained in a two-alternative-forced-choice procedure indicated if sounds were presented from the left or from the right by choosing the respective response compartment of a Y-shaped experimental setup. The minimum resolvable angle (MRA) was calculated as the minimum angle between two loudspeaker locations that a gerbil was able to discriminate. MRAs for aged gerbils were higher compared to controls, whereas MRAs of noise-reared gerbils did not differ from those of the control group. Results are discussed with respect to the progressive degeneration affecting the gerbil's auditory system, changes in the anatomical arrangement of inhibitory inputs on binaural neurons in the MSO, and hearing thresholds.

Auditory spatial resolution in horizontal, vertical, and diagonal planes.

Minimum audible angle (MAA) and minimum audible movement angle (MAMA) thresholds were measured for stimuli in horizontal, vertical, and diagonal (60 degrees) planes. A pseudovirtual technique was employed in which signals were recorded through KEMAR's ears and played back to subjects through insert earphones. Thresholds were obtained for wideband, high-pass, and low-pass noises. Only 6 of 20 subjects obtained wideband vertical-plane MAAs less than 10 degrees, and only these 6 subjects were retained for the complete study. For all three filter conditions thresholds were lowest in the horizontal plane, slightly (but significantly) higher in the diagonal plane, and highest for the vertical plane. These results were similar in magnitude and pattern to those reported by Perrott and Saberi [J. Acoust. Soc. Am. 87, 1728-1731 (1990)] and Saberi and Perrott [J. Acoust. Soc. Am. 88, 2639-2644 (1990)], except that these investigators generally found that thresholds for diagonal planes were as good as those for the horizontal plane. The present results are consistent with the hypothesis that diagonal-plane performance is based on independent contributions from a horizontal-plane system (sensitive to interaural differences) and a vertical-plane system (sensitive to pinna-based spectral changes). Measurements of the stimuli recorded through KEMAR indicated that sources presented from diagonal planes can produce larger interaural level differences (ILDs) in certain frequency regions than would be expected based on the horizontal projection of the trajectory. Such frequency-specific ILD cues may underlie the very good performance reported in previous studies for diagonal spatial resolution. Subjects in the present study could apparently not take advantage of these cues in the diagonal-plane condition, possibly because they did not externalize the images to their appropriate positions in space or possibly because of the absence of a patterned visual field.

Auditory spatial resolution in the barn owl under echoic and anechoic conditions

Experiments were designed to allow direct comparison of auditory spatial resolution, measured behaviorally, with that of single space‐specific neurons in the barn owl’s midbrain. Behavioral measurements of spatial discrimination were obtained using habituation and recovery of the pupillary dilation response (PDR). The acoustically evoked PDR habituates to repeated presentation of a sound, and recovers if the location changes. Thus, the difference in magnitudes of PDRs evoked by a sequence of test and habituating stimuli can be quantified using signal detection theory to provide a measure of discrimination [‘‘p(c),’’ computed from empirical ROC curves]. The minimum audible angles (MAAs) for single sound sources separated in azimuth and elevation were 3° and 9°, respectively. Under simulated echoic conditions, MAAs increased by a factor of 2 for direct sources, and 4 for simulated reflections. Neuronal discrimination was similarly quantified using ROC curves to calculate p(c) for spike discharges evoked by sources at different virtual locations. Preliminary analysis of neuronal data from anesthetized owls suggests that MAAs of the most sensitive neurons are similar to behavioral MAAs. Simultaneous measurement of neuronal and behavioral performance in awake owls is currently underway. [Work supported by the McKnight Foundation, NIH DC03925 and DC00448.]

The effects of nonconcurrent tonal and noise distractors upon auditory spatial resolution: The temporal, spatial, and spectral parameters underlying the ‘‘disruption effect.’’

The minimum audible angle (MAA) thresholds obtained by Mills [A. W. Mills, J. Acoust. Soc. Am. 30, 237–246 (1958)] involved two 1000-ms sinusoidal tone pulses presented sequentially with an interstimulus interval (ISI) of 1000 ms. In contrast to the generally good localization performance obtained by Mills, localization of two simultaneous events (concurrent localization or CMAA) results in relatively poor performance [D. R. Perrott, J. Acoust. Soc. Am. 76, 1704–1712 (1984)]. The current set of experiments attempts to bridge the gap between these two extreme paradigms. While the target tones (14-ms, 1000-Hz sinusoids) were presented sequentially (an MAA task), an attempt was made to systematically examine the consequences of the addition of a nonoverlapping, auditory event (tones or noise) during the interval between the target events. The results of this work indicate that localization performance is extremely sensitive to events that occur between the target events. The disruptive effect of a ‘‘nontarget’’ event during the ISI could even be observed when relatively long ISIs (814-ms) and very brief (14-ms) distractors events were used. Interesting interactions were encountered as a function of the spatial positioning of the nontarget (‘‘distractor’’) event. Implications of these results will be discussed.

Effects of uncertainty on auditory spatial resolution of tonal sequences

The horizontal-plane minimum audible angle (MAA) was measured for single components of a five-tone sequence, where each component was a 1000-Hz burst 100 ms in duration. The five components were presented in a pseudorandom order from the following five azimuths: −54°, −19°, 0°, +23°, and +56°. The target component whose azimuth change was to be detected was at either −54° or 0°, and could occur in temporal position 2 or 5 in the sequence. Stimulus uncertainty was varied from a minimal level to a high level by randomly varying target temporal position, target azimuth, or both from trial to trial. When the target position and azimuth within the sequence was unchanged from trial to trial (minimum stimulus uncertainty), MAAs for targets presented in the last temporal position at both target azimuths were as small as for tones presented in isolation. However, for targets presented earlier in the sequence, MAAs were elevated at both azimuths. The MAAs at 0° at both temporal positions were unaffected as uncertainty increased, but there were large effects of uncertainty at −54° azimuth. Results suggest that in a task requiring spatial resolution in the horizontal plane, when the time of arrival and location of an auditory signal are unknown, listeners focus their attention near midline, where resolution is typically most precise. [Work supported by U.S. Army and NIH DC00185.]

Mismatches between auditory demands and capacities in the industrial work environment.

Exposure limits to noise in the workplace have been set merely to prevent disabling hearing loss. Consequently, even in workplaces where a limit of 85 dBA-8 h is strictly enforced, the sound environment is very poorly matched to human auditory capabilities. In fact, ergonomists have been led to characterize this feature of the industrial work environment as an 'extreme condition' that is being imposed on a major segment of the workforce. This type of sound environment seriously restricts the possibilities of making use of auditory temporal, spatial, and frequency resolution, and often reduces the latter by damaging the inner ear. Sound warning signals are often too loud or too weak. The temporal features of such signals are frequently distorted by strong reverberation in the local itself. Spatial separation of sound sources is strongly limited by the masking effect of the background noise. The depth of the perceived sound field is generally reduced to very great degree. Workers experience isolation because verbal communication is severely restricted. Most discernable acoustic occurrences are devoid of meaning or are experienced as annoying or irritating. Noise has been reported as the most frequently mentioned factor of annoyance in industrial workplaces. Such environments are far from being matched to human capabilities and are scarcely conducive to work satisfaction. A general remedial framework for making the appropriate improvements is thereby proposed.

Auditory spatial resolution in the horizontal plane as a function of reference angle: Microstructure of the azimuth function

Auditory spatial resolution in the horizontal plane was measured in five normal-hearing subjects by determining the minimum audible angle (MAA) for brief 1.0-kHz tones. Using a left–right discrimination paradigm, MAAs were determined for 22 reference azimuths ranging from −62° to +63°. The gross pattern of the mean data was similar to that reported previously for more sparsely sampled azimuth functions: spatial resolution tended to be best when the reference sound was near midline, becoming worse as the reference moved toward the periphery. However, there were remarkable asymmetries and nonmonotonicities in individual subject’s MAA versus azimuth functions, and these nonmonotonic features were highly subject-dependent. Although such individual differences may be due partly to differences among subjects in head and pinna shape, it is possible that the highly individual and idiosyncratic MAA functions also reflect subject-dependent nonmonotonicities in the underlying interaural time difference (ITD) discrimination functions as reference ITD is varied in fine increments. [Work supported by NIH and by the U.S. Army.] a)Current address: ENT/Audiology (HSHJ-SET-A), Madigan Army Med. Ctr., Tacoma, WA 98431-5374.

Effects of age on auditory spatial resolution in the horizontal plane

The purpose of this research was to determine if auditory spatial resolution in the horizontal plane changes with age. Spatial resolution was measured in static conditions, represented by the minimum audible angle (MAA), and in dynamic conditions, represented by the minimum audible movement angle (MAMA). MAAs and MAMAs were measured for adult subjects 60 to 80 years of age with normal (or near normal) hearing thresholds. All measurements were obtained in a darkened anechoic chamber. Thresholds were measured as a function of stimulus bandwidth, center frequency, and listening azimuth, for both MAA and MAMA. Additionally, MAMAs were measured as a function of the velocity of the moving stimulus source. MAAs and MAMAs of the elderly will be compared with previous measurements for young normal adults that were obtained under the same stimulus conditions [D. W. Chandler and D. W. Grantham, J. Acoust. Soc. Am. Suppl. 1 87, S64 (1990)]. It is hypothesized that auditory spatial resolution in the horizontal plane diminishes with age because of generalized deficits of central nervous system processing. This diminished resolution should be reflected by poorer MAAs and poorer MAMAs among the elderly subjects than among the young, normal-hearing subjects. [Work supported by NIH.]

Monaural spatial resolution in the horizontal plane under dynamic and static listening conditions

Auditory spatial resolution was measured in the horizontal plane for three normal‐hearing adult subjects in a darkened anechoic chamber. In all conditions, subjects' right ears were occluded by an EAR foam insert plug plus an external sound‐attenuating muff, providing a total of about 40 dB of attenuation. In the dynamic conditions, the minimum audible movement angle (MAMA) was measured—that is, the angular extent a moving target had to traverse to be just discriminable from a stationary target. In the static conditions, the minimum audible angle (MAA) was measured—that is, the minimum angular separation between two sequentially presented stationary targets that was just discriminable from a single stationary target presented twice in succession. In general, MAMAs and MAAs decreased as stimulus bandwidth increased from 0 Hz (pure tone at 3000 Hz) to wideband. MAAs were extremely variable across and within subjects (varying from 10° to 40° of arc). MAMAs for slow‐velocity targets (10°/s) were usually lower...

Minimum audible movement angle in the horizontal plane as a function of stimulus frequency and bandwidth, source azimuth and velocity, and number of ears

Minimum audible movement angles (MAMAs) were measured in the horizontal plane for four adult subjects in a darkened anechoic chamber. On each trial, a single stimulus was presented, and the subject had to say whether it came from a stationary loudspeaker or from a loudspeaker that was moving at a constant angular velocity around him. Thresholds were established by adaptively varying stimulus duration. MAMAs were measured under both monaural and binaural listening conditions as a function of center frequency (500–5000 Hz), bandwidth (pure tone to 1 oct), velocity (10°–180°/s), azimuth (0°–90°), and direction of motion (left versus right). Preliminary data are consistent with previous observations for frequency and source velocity effects [D. R. Perrott and J. Tucker, J. Acoust. Soc. Am. 83, 1522–1526 (1988)]. Data will be compared to static measures of auditory spatial resolution (the minimum audible angle) obtained under the same stimulus conditions from the same subjects. Discussion will focus on whether specialized mechanisms in the auditory system are required to account for dynamic spatial resolution. [Work supported by NIH.]

Minimum audible angle thresholds for broadband noise as a function of the delay between the onset of the lead and lag signals.

Minimum audible angle (MAA) thresholds were determined for six experienced subjects using a two-alternative, forced-choice adaptive paradigm. Broadband pink noise from a single generator was led to two identical speakers. The two sources were activated sequentially, each for a period of 10 ms. The subject's task was to indicate whether the second (lag) sound came from a source to the right or left of the first (lead) sound. The delay between the onset of the lead and the onset of the lag signal [interstimulus onset interval (ISOI)] was systematically varied from 1 ms (both 10-ms signals were concurrently active for 9 ms) to 200 ms. For a given ISOI, the spatial separation was varied adaptively to determine the MAA. A 450% improvement in auditory spatial resolution was evident as the ISOI increased from 1 to 150 ms. A further increase in the ISOI had no systematic effect on spatial resolution. These results indicate that there is a minimum integration period between 100-150 ms for the resolution of spatial information in the auditory modality.

Detection and discrimination of simulated motion of auditory targets in the horizontal plane.

Three experiments investigated subjects' ability to detect and discriminate the simulated horizontal motion of auditory targets in an anechoic environment. "Moving" stimuli were produced by dynamic application of stereophonic balancing algorithms to a two-loudspeaker system with a 30 degree separation. All stimuli were 500-Hz tones. In experiment 1, subjects had to discriminate a left-to-right moving stimulus from a stationary stimulus pulsed for the same duration (300 or 600 ms). For both durations, minimum audible "movement" angles ("MAMA's") were on the order of 5 degrees for stimuli presented at 0 degrees azimuth (straight ahead), and increased to greater than 30 degrees for stimuli presented at +/- 90 degrees azimuth. Experiment 2 further investigated MAMA's at 0 degrees azimuth, employing two different procedures to track threshold: holding stimulus duration constant (at 100-600 ms) while varying velocity; or holding the velocity constant (at 22 degrees-360 degrees/s) while varying duration. Results from the two procedures agreed with each other and with the MAMA's determined by Perrott and Musicant for actually moving sound sources [J. Acoust. Soc. Am. 62, 1463-1466 (1977b)]: As stimulus duration decreased below 100-150 ms, the MAMA's increased sharply from 5 degrees-20 degrees or more, indicating that there is some minimum integration time required for subjects to perform optimally in an auditory spatial resolution task. Experiment 3 determined differential "velocity" thresholds employing simulated reference velocities of 0 degrees-150 degrees/s and stimulus durations of 150-600 ms. As with experiments 1 and 2, the data are more easily summarized by considering angular distance than velocity: For a given "extent of movement" of a reference target, about 4 degrees-10 degrees additional extent is required for threshold discrimination between two "moving" targets, more or less independently of stimulus duration or reference velocity. These data suggest that for the range of simulated velocities employed in these experiments, subjects respond to spatial changes--not velocity per se--when presented with a "motion" detection or discrimination task.

Auditory spatial resolution under static and dynamic conditions

The minimum audible movement angle (MAMA) is the minimum angle a horizontally moving sound source must traverse to be just discriminable from a stationary source [Perrott and Musicant, J. Acoust. Soc. Am. 62. 1463–1466 (1977)]. To further explore the relationship between the MAMA and the traditional minimum audible angle determined with stationary sources (MAA), we examined auditory spatial resolution of 500‐Hz tones under both dynamic and static conditions: (1) In the static (MAA) condition subjects had to respond whether the second of a pair of stationary 20‐ms tone bursts was to the left or the right of the first; (2) In the dynamic (MAMA) condition subjects had to respond whether a single moving stimulus moved left or right. The independent variable was the duration of the interstimulus interval (ISI) in condition 1, or of the moving stimulus in condition 2. While the MAA increased from 1°–2° as ISI decreased from 500 to 50 ms, the MAMA increased from 3°–10° as stimulus duration decreased over this range. These results suggest that the severe degradation of the MAMA as duration decreases, originally reported by Perrott and Musicant, is primarily a consequence of the dynamic nature of the stimulus and not of the brief duration per se. [Work supported by NSF.]