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PubMed2001

Effect of gaze direction on sound localization in rear space.

The effect of eccentric eye position on the localization of sound in rear space was investigated, using a two-alternative forced-choice method in combination with a visual fixation task. The azimuthal position of the rear sound was perceived as shifted slightly (mean 1.2 degrees ) to the left of the subjects' median plane when fixation was 30 degrees to the right, or to the right when fixation was 30 degrees to the left. Combined with previous studies on localization in frontal space, this finding suggests that eye-position signals influence processing of binaural, but not monaural spectral, cues for directional hearing.

Acoustic Stimulation↗

PubMed1998

Distinct short-term memory systems for sound content and sound localization.

Short-term memory for sound content and sound localization was investigated in normal subjects using the same/different comparison of two sound stimuli separated by an interval. Auditory or visual interference tasks requiring recognition or spatial judgements were introduced in the interval. Auditory interference tasks reduced memory for sound content and sound location in a specific way. Memory for sound content was significantly more reduced by auditory recognition than by auditory spatial interference task. Visual interference tasks reduced significantly memory for sound location but not for sound content. These results suggest that (i) short-term memory for sound content and that for sound location involve partially distinct processing; and (ii) auditory spatial functions are more closely linked to visual functions than auditory recognition.

Acoustic Stimulation↗

PubMed1997

Acute sensitivity to interaural time differences in the inferior colliculus of a bat that relies on passive sound localization.

Gleaning bats rely on passive hearing to detect and localize terrestrial prey, and display remarkable accuracy in their passive sound localization. This study examines binaural processing in the inferior colliculus (IC) of the pallid bat (Antrozous pallidus), a gleaner that attends to prey-generated noise transients to locate prey. The primary focus is to determine whether neurons in its lateral IC, a region that appears dedicated to passive localization, possess a level of sensitivity to interaural time difference (ITD) sensitivity sufficient to indicate the use of ITDs in sound localization. Such a sensitivity was suspected because the pallid bat is capable of very accurate passive sound localization at the lower end of its audible range, where interaural intensity differences (IIDs) are small and may not provide sufficient spatial information. Because the pallid bat's audible range is too high for neurons to phase-lock to carrier frequencies, neurons were tested with square-wave, amplitude-modulated tones and noise to determine their sensitivity to ITDs in the sound envelope. Their sensitivity to the bat's behaviorally relevant ITD range of +/- 70 micros, and their low average interaural time/ intensity trading ratios (18 micros/dB) suggest that the pallid bat IC may have the greatest ITD sensitivity reported in a high-frequency mammalian auditory system.

Acoustic Stimulation↗

PubMed1984

Monaural occlusion alters sound localization during a sensitive period in the barn owl.

Sound localization was disrupted in young barn owls by chronically plugging one ear. Owls that were younger than 8 weeks of age at the time of ear plugging recovered normal localization accuracy while plugged, whereas those that were older than 8 weeks at the time of ear plugging did not. The end of the sensitive period for the adjustment of sound localization accuracy coincides with the maturation of the head and ears, suggesting that the exposure of the auditory system to stable, adult-like acoustic cues could play a role in bringing the sensitive period to a close. The results demonstrate that, early in development, associations between auditory cues and locations in space can be altered by experience.

Acoustics↗

PubMed2001

Sound-localization experiments with barn owls in virtual space: influence of broadband interaural level different on head-turning behavior.

Interaural level differences play an important role for elevational sound localization in barn owls. The changes of this cue with sound location are complex and frequency dependent. We exploited the opportunities offered by the virtual space technique to investigate the behavioral relevance of the overall interaural level difference by fixing this parameter in virtual stimuli to a constant value or introducing additional broadband level differences to normal virtual stimuli. Frequency-specific monaural cues in the stimuli were not manipulated. We observed an influence of the broadband interaural level differences on elevational, but not on azimuthal sound localization. Since results obtained with our manipulations explained only part of the variance in elevational turning angle, we conclude that frequency-specific cues are also important. The behavioral consequences of changes of the overall interaural level difference in a virtual sound depended on the combined interaural time difference contained in the stimulus, indicating an indirect influence of temporal cues on elevational sound localization as well. Thus, elevational sound localization is influenced by a combination of many spatial cues including frequency-dependent and temporal features.

Acoustics↗

PubMed1976

Role of neocortex in binaural hearing in the cat. II. The 'precedence effect' in sound localization.

The 'precedence effect' in sound localization refers to the situation in which two speakers, separated in space, emit identical sounds but one speaker leads by a few milliseconds. Normal observers perceive all the sound as originating from the leading speaker. In the present experiment cats were tested before and after unilateral ablation of auditory cortex on how well they could transfer a learned sound loclization response from situations involving unpaired sound sources to those with paired sources. Cats were tested with tone pairs separated by 5 msec and with clicks separated by delays of 3, 5, 7, 9, 12 and 16 msec. Preoperatively, the cats averaged 98% correct at a delay of 3 msec and 58% correct at 16 msec. Before surgery the probability of an error was independent of which speaker was leading; after surgery cats made significantly more errors when the leading speaker was located opposite the side of the lesion. Considerable individual variability was found with the 3 and 5 msec delay tests; some cats showed no evidence of a cortical deficit while others exhibited initial severe deficits which disappeared with training. At delays of 7-16 msec all cats showed severe directional deficits which were still present after 9 months of retraining. Results are interpreted as supporting the hypothesis that unilateral lesions disrupt binaural loudness rather than temporal relations.

Animals↗

PubMed2002

Sound localization in patients with cochlear implant--preliminary results.

OBJECTIVES: To evaluate sound localization ability in patients with unilateral cochlear implant, who do not wear a hearing aid on the contralateral ear, and to try to improve this ability by training. SETTING: Tertiary academic referral center. METHODS: In the initial test, patients were exposed to sound stimuli from different directions and were asked to localize them. Following a training period the patients were re-evaluated by the same test. For each test, the percentage of correct answers and the final test score were calculated. RESULTS: In the initial test, the mean score of the study group of four cochlear implant users was 42.8 (out of a maximal score of 100), the mean rate of correct responses was 27.5%. Following a training period (6.5 sessions on the average), on the final test the mean score of the group was 74.3, while the mean rate of correct responses was 66.5%. CONCLUSIONS: The results demonstrated that patients with unilateral cochlear implant have some ability to localize sound, and that this ability may be improved by regular training.

Adolescent↗

PubMedDate not supplied

Spike encoding mechanisms in the sound localization pathway.

Two distinct neural circuits calculate the direction of incoming sound in mammals. Lower frequency sounds are processed in the medial superior olive (MSO) and higher frequencies are processed in the lateral superior olive (LSO); together they constitute the superior olivary complex. We show that the spike generation mechanisms of coincidence detection (CD) are employed in both these branches of sound localization pathway. Our description uses the concepts of probabilistic spike generation and spike timing jitter. We explain the notch in sound localization sensitivity described in human psychophysics. We estimate the processing time in the superior olivary complex and discuss possible spike processing mechanisms. Among them, we distinguish between the excitatory coincidence detection (ECD) and the inhibitory coincidence detection (ICD). We compare the latter to the mechanism of firing rate subtraction traditionally attributed to the lateral superior olive.

Action Potentials↗

PubMed1998

Improvement in aided sound localization with open earmolds: observations in people with high-frequency hearing loss.

Sound localization ability was tested in nine people with marked, bilateral, high-frequency hearing loss and little or no low-frequency loss. They had previously been fitted bilaterally with behind-the-ear hearing aids and closed earmolds and showed poorer aided than unaided localization performance. Further testing was conducted in unaided and aided conditions, with the aids coupled to closed, open, and "sleeve" (extra-open) earmolds. Closed earmolds affected localization, particularly in the frontal horizontal plane, but performance was restored to unaided levels in both of the open earmold conditions. In the lateral vertical plane, localization was found to correlate with the extent of difference between low- and high-frequency hearing levels. That result is discussed in terms of a dynamic, low-frequency localization cue. Open earmolds are argued to improve aided sound localization for the present sample by permitting undistorted access to low-frequency interaural time/phase differences.

Aged↗

PubMed2001

Sound-localization experiments with barn owls in virtual space: influence of interaural time difference on head-turning behavior.

Specific cues in a sound signal are naturally linked to certain parameters in acoustic space. In the barn owl, interaural time difference (ITD) varies mainly with azimuth, while interaural level difference (ILD) varies mainly with elevation. Previous data suggested that ITD is indeed the main cue for azimuthal sound localization in this species, while ILD is an important cue for elevational sound localization. The exact contributions of these parameters could be tested only indirectly because it was not possible to generate a stimulus that contained all relevant spatial information on the one hand, and allowed for a clean separation of these parameters on the other hand. Virtual auditory worlds offer this opportunity. Here we show that barn owls responded to azimuthal variations in virtual space in the same way as to variations in free-field stimuli. We interpret the increase of turning angle with sound-source azimuths (up to +/- 140 degrees) such that the owls did not experience front/back confusions with virtual stimuli. We then separated the influence of ITD from the influence of all other stimulus parameters by fixing the overall ITD in virtual stimuli to a constant value (+100 micros or +100 micros) while leaving all other sound characteristics unchanged. This manipulation influenced both azimuthal and elevational components of head arms. Since the owls' azimuthal head-turn amplitude always resembled the value signified by the ITD, these data demonstrated that azimuthal sound localization is influenced only by ITD both in the frontal hemisphere and in large parts of the rear hemisphere. ILDs did not have an influence on azimuthal components of head turns. While response latency to normal virtual stimuli was found to be largely independent of stimulus position, response delays of the head turns became longer if the ITD information pointed into a different hemisphere as the other cues of the sounds.

Animals↗

PubMed1998

Relearning sound localization with new ears.

Because the inner ear is not organized spatially, sound localization relies on the neural processing of implicit acoustic cues. To determine a sound's position, the brain must learn and calibrate these cues, using accurate spatial feedback from other sensorimotor systems. Experimental evidence for such a system has been demonstrated in barn owls, but not in humans. Here, we demonstrate the existence of ongoing spatial calibration in the adult human auditory system. The spectral elevation cues of human subjects were disrupted by modifying their outer ears (pinnae) with molds. Although localization of sound elevation was dramatically degraded immediately after the modification, accurate performance was steadily reacquired. Interestingly, learning the new spectral cues did not interfere with the neural representation of the original cues, as subjects could localize sounds with both normal and modified pinnae.

Adaptation, Physiological↗

PubMed2000

The effect of aging on horizontal plane sound localization.

An experiment was conducted to determine the effect of aging on sound localization. Seven groups of 16 subjects, aged 10-81 years, were tested. Sound localization was assessed using six different arrays of four or eight loudspeakers that surrounded the subject in the horizontal plane, at a distance of 1 m. For two 4-speaker arrays, one loudspeaker was positioned in each spatial quadrant, on either side of the midline or the interaural axis, respectively. For four 8-speaker arrays, two loudspeakers were positioned in each quadrant, one close to the midline and the second separated from the first by 15 degrees, 30 degrees, 45 degrees, or 60 degrees. Three different 300-ms stimuli were localized: two one-third-octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. The stimulus level (75 dB SPL) was well above hearing threshold for all subjects tested. Over the age range studied, percent-correct sound-source identification judgments decreased by 12%-15%. Performance decrements were apparent as early as the third decade of life. Broadband noise was easiest to localize (both binaural and spectral cues were available), and the 0.5-kHz noise band, the most difficult to localize (primarily interaural temporal difference cue available). Accuracy was relatively higher in front of than behind the head, and errors were largely front/back mirror image reversals. A left-sided superiority was evident until the fifth decade of life. The results support the conclusions that the processing of spectral information becomes progressively less efficient with aging, and is generally worse for sources on the right side of space.

Adolescent↗

PubMed2000

Sensory habituation of auditory receptor neurons: implications for sound localization.

Auditory receptor neurons exhibit sensory habituation; their responses decline with repeated stimulation. We studied the effects of sensory habituation on the neural encoding of sound localization cues using crickets as a model system. In crickets, Teleogryllus oceanicus, sound localization is based on binaural comparison of stimulus intensity. There are two potential codes at the receptor-neuron level for interaural intensity difference: interaural difference in response strength, i.e. spike rate and/or count, and interaural difference in response latency. These are affected differently by sensory habituation. When crickets are stimulated with cricket-song-like trains of sound pulses, response strength declines for successive pulses in the train, and the decrease becomes more pronounced as the stimulus intensity increases. Response decrement is thus greater for receptors serving the ear ipsilateral to the sound source, where intensity is higher, resulting in a decrease in the interaural difference in response strength. Sensory habituation also affects response latency, which increases for responses to successive sound pulses in the stimulus train. The change in latency is independent of intensity, and thus is similar for receptors serving both ears. As a result, interaural latency difference is unaffected by sensory habituation and may be a more reliable cue for sound localization.

Acoustic Stimulation↗

PubMed2003

Sound localization and delay lines--do mammals fit the model?

The current dominant model of binaural sound localization proposes that the lateral position of a sound source is determined by the position of maximal activation within an array of binaural coincidence-detector neurons that are tuned to different interaural time differences (ITDs). The tuning of a neuron for an ITD is determined by the difference in axonal conduction delay from each ear--the so-called "delay line" hypothesis. Although studies in birds appear to support this model, recent evidence from mammals suggests that the model does not provide accurate descriptions of how ITDs are encoded in the mammalian auditory brainstem or of how ITD-sensitive neurons contribute to mammalian sound localization.

Animals↗

PubMed1996

Sound localization in noise: the effect of signal-to-noise ratio.

The sound localization ability of human observers has been frequently examined in quiet environments, but there have been relatively few studies that have considered the effect of noise on sound localization. In this study, three subjects judged the perceived direction of broadband click-train signal in the quiet and in the presence of a broadband noise at nine signal-to-noise ratios, which varied over a 23 dB range. The signal could originate from any of 239 spatial locations that completely surrounded the subjects in azimuth 360 degrees) and ranged from -45 degrees to (+)90 degrees in elevation; the masker (when present) was always located directly in front of the subjects at 0 degrees azimuth and 0 degree elevation. The subjects indicated the perceived direction of the signal by pointing at a 20-cm-diam spherical model of auditory space. As the signal-to-noise ratio was lowered, the accuracy of localization judgments decreased nearly monotonically. However, the accuracy of judgments relative to the median plane (i.e., the left/right dimension) was less strongly influenced by the presence of noise than was the accuracy of judgments relative to the horizontal plane (i.e., the up/down dimension). The accuracy of judgments relative to the frontal plane (i.e., the front/back dimension) was most strongly influenced by noise.

Adult↗

PubMed2005

The impact of hearing protection on sound localization and orienting behavior.

The effect of hearing protection devices (HPDs) on sound localization was examined in the context of an auditory-cued visual search task. Participants were required to locate and identify a visual target in a field of 5, 20, or 50 visual distractors randomly distributed on the interior surface of a sphere. Four HPD conditions were examined: earplugs, earmuffs, both earplugs and earmuffs simultaneously (double hearing protection), and no hearing protection. In addition, there was a control condition in which no auditory cue was provided. A repeated measures analysis of variance revealed significant main effects of HPD for both search time and head motion data (p < .05), indicating that the degree to which localization is disrupted by HPDs varies with the type of device worn. When both earplugs and earmuffs are worn simultaneously, search times and head motion are more similar to those found when no auditory cue is provided than when either earplugs or earmuffs alone are worn, suggesting that sound localization cues are so severely disrupted by double hearing protection the listener can recover little or no information regarding the direction of sound source origin. Potential applications of this research include high-noise military, aerospace, and industrial settings in which HPDs are necessary but wearing double protection may compromise safety and/or performance.

Adolescent↗

PubMed1986

Speculations on the role of frequency in sound localization.

This presentation reviews behavioral and physiological evidence suggesting that the perceived power spectrum of a sound, as modified by the external ears, provides important cues for horizontal and vertical sound localization. This spatial information is mostly likely encoded by the relative levels of excitation among the tonotopically organized hair cells of the cochlea. This suggests that the preservation of the cochlear frequency map within the central auditory system contributes an anatomical substrate for the localization of sound.

Animals↗

PubMed2006

Resolution in azimuth sound localization in the Mongolian gerbil (Meriones unguiculatus).

Minimum resolvable angles (MRAs) for sound localization in azimuth in the gerbil were determined in a behavioral study using tones, 300-Hz bands of noise centered at frequencies between 500 Hz and 8 kHz and broad-band noise of on average 60 dB SPL overall level. Using the method of constant stimuli, seven gerbils were trained in a two-alternative-forced-choice procedure to indicate if sounds were presented to them from the left or from the right by choosing the left or right arm of a Y-shaped cage. The MRA is the minimum angle between two loudspeaker locations that the gerbils discriminated. Animals were either stimulated from the front (N=4) or from the back (N=3). The MRA for broad-band noise randomly varying in level by +/- 6 dB was 23 degrees and 45 degrees for gerbils stimulated from the front or back, respectively. Generally a gerbil's MRA for tones declined up to 2 kHz reaching 20 degrees and 31 degrees for gerbils stimulated from the front or back, respectively, and the MRA was generally increased above this frequency. Results for narrow-band noise stimuli were similar. Results are discussed with respect to the available interaural cues and physiological mechanisms of sound localization in the gerbil.

Acoustic Stimulation↗