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PubMed1987

Infants' localization of sounds in the horizontal plane: effects of auditory and visual cues.

Infants 6, 9, 12, 15, and 18 months were seated in a dark room facing a semicircular array comprising 10 loudspeakers, 5 to their right and left at 18 degrees, 36 degrees, 54 degrees, 72 degrees, and 90 degrees from midline. Each infant received 2 types of trials: auditory-alone trials (only a sequence of 8 clicks played), and auditory-visual trials (following the first 4 clicks, a light display at the location of the loudspeaker was activated and remained on for the remaining 4 clicks). Calibration markers on the infant's head were used to measure azimuth error (i.e., the discrepancy between the angle of head turn and loudspeaker location). For auditory-visual trials, all infants turned their head within 4 degrees-6 degrees of the sound source; there was no effect of age or loudspeaker location. For auditory-alone trials, there were significant age differences in performance, with a systematic decrease in discrepancy angle with increasing age. By 18 months there was no difference in performance as a function of localization cue.

Attention↗

PubMed1998

Sensitive periods for visual calibration of the auditory space map in the barn owl optic tectum.

Previous studies have identified sensitive periods for the developing barn owl during which visual experience has a powerful influence on the calibration of sound localization behavior. Here we investigated neural correlates of these sensitive periods by assessing developmental changes in the capacity of visual experience to alter the map of auditory space in the optic tectum of the barn owl. We used two manipulations. (1) We equipped owls with prismatic spectacles that optically displaced the visual field by 23 degrees to the left or right, and (2) we restored normal vision to prism-reared owls that had been raised wearing prisms. In agreement with previous behavioral experiments, we found that the capacity of abnormal visual experience to shift the tectal auditory space map was restricted to an early sensitive period. However, this period extended until later in life (approximately 200 d) than described previously in behavioral studies (approximately 70 d). Furthermore, unlike the previous behavioral studies that found that the capacity to recover normal sound localization after restoration of normal vision was lost at approximately 200 d of age, we found that the capacity to recover a normal auditory space map was never lost. Finally, we were able to reconcile the behaviorally and neurophysiologically defined sensitive periods by taking into account differences in the richness of the environment in the two sets of experiments. We repeated the behavioral experiments and found that when owls were housed in a rich environment, the capacity to adjust sound localization away from normal extended to later in life, whereas the capacity to recover to normal was never lost. Conversely, when owls were housed in an impoverished environment, the capacity to recover a normal auditory space map was restricted to a period ending at approximately 200 d of age. The results demonstrate that the timing and even the existence of sensitive periods for plasticity of a neural circuit and associated behavior can depend on multiple factors, including (1) the nature of the adjustment demanded of the system and (2) the richness of the sensory and social environment in which the plasticity is studied.

Aging↗

PubMed1998

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

Perceived location of tonal stimuli d and 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 degrees through -45 degrees in the vertical plane; the horizontal plane coordinate was fixed at -90 degrees. On the basis of these measurements, we fabricated three different bandstop 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.

Dichotic Listening Tests↗

PubMed2005

Does the bone-anchored hearing aid have a complementary effect on audiological and subjective outcomes in patients with unilateral conductive hearing loss?

OBJECTIVES: To study the effect of a bone-anchored hearing aid (BAHA) in patients with unilateral conductive hearing loss. STUDY DESIGN: Prospective evaluation on 18 subjects. METHODS: Aided and unaided binaural hearing was assessed in the sound field using a sound localization test and a speech recognition in noise test with spatially separated sound and noise sources. The patients also filled out a disability-specific questionnaire. PATIENTS: 13 out of the 18 subjects had normal hearing on one side and acquired conductive hearing loss in the other ear. The remaining 5 patients had a unilateral air-bone gap and mild symmetrical sensorineural hearing loss. RESULTS: Sound localization with the BAHA improved significantly. Speech recognition in noise with spatially separated speech and noise sources also improved with the BAHA. Fitting a BAHA to patients with unilateral conductive hearing loss had a complementary effect on hearing. Questionnaire results showed that the BAHA was of obvious benefit in daily life. CONCLUSIONS: The BAHA proved to be a beneficial means to optimize binaural hearing in patients with severe (40-60 dB) unilateral conductive hearing loss according to audiometric data and patient outcome measures.

Adult↗

PubMed2002

The bone-anchored hearing aid in patients with a unilateral air-bone gap.

OBJECTIVES: To study the benefit of the application of a bone-anchored hearing aid in patients with a unilateral air-bone gap. STUDY DESIGN: Prospective evaluation in eight patients. METHODS: Binaural hearing was assessed in the sound field by comparing aided and unaided scores obtained with a sound localization test and a speech recognition in noise test with spatially separated sound and noise sources. SETTING: Tertiary referral center. PATIENTS: The patients had subnormal hearing and unilateral conductive hearing loss. RESULTS: Sound localization improved significantly in the six patients with acquired hearing loss. The binaural advantage, studied with speech-in-noise tests with spatially separated speech and noise sources, proved to be comparable with that in a control group of subjects with normal hearing when they were listening monaurally versus binaurally. For one of the two patients with unilateral congenital conductive hearing loss, the results were ambiguous. This patient's age at the time of surgery was high: 40 years (the other patient was 19 years old at the time of surgery). This might have played a role. CONCLUSION: If reconstructive surgery is not possible (e.g., in a patient with a chronically draining ear or a severe congenital malformation), a bone-anchored hearing aid is an option to reestablish binaural hearing. The results reported herein suggest that, at least for patients with acquired hearing loss, the bone-anchored hearing aid is an effective treatment of unilateral conductive hearing loss.

Acoustic Stimulation↗

PubMed2000

Interdependence of spatial and temporal coding in the auditory midbrain.

To date, most physiological studies that investigated binaural auditory processing have addressed the topic rather exclusively in the context of sound localization. However, there is strong psychophysical evidence that binaural processing serves more than only sound localization. This raises the question of how binaural processing of spatial cues interacts with cues important for feature detection. The temporal structure of a sound is one such feature important for sound recognition. As a first approach, we investigated the influence of binaural cues on temporal processing in the mammalian auditory system. Here, we present evidence that binaural cues, namely interaural intensity differences (IIDs), have profound effects on filter properties for stimulus periodicity of auditory midbrain neurons in the echolocating big brown bat, Eptesicus fuscus. Our data indicate that these effects are partially due to changes in strength and timing of binaural inhibitory inputs. We measured filter characteristics for the periodicity (modulation frequency) of sinusoidally frequency modulated sounds (SFM) under different binaural conditions. As criteria, we used 50% filter cutoff frequencies of modulation transfer functions based on discharge rate as well as synchronicity of discharge to the sound envelope. The binaural conditions were contralateral stimulation only, equal stimulation at both ears (IID = 0 dB), and more intense at the ipsilateral ear (IID = -20, -30 dB). In 32% of neurons, the range of modulation frequencies the neurons responded to changed considerably comparing monaural and binaural (IID =0) stimulation. Moreover, in approximately 50% of neurons the range of modulation frequencies was narrower when the ipsilateral ear was favored (IID = -20) compared with equal stimulation at both ears (IID = 0). In approximately 10% of the neurons synchronization differed when comparing different binaural cues. Blockade of the GABAergic or glycinergic inputs to the cells recorded from revealed that inhibitory inputs were at least partially responsible for the observed changes in SFM filtering. In 25% of the neurons, drug application abolished those changes. Experiments using electronically introduced interaural time differences showed that the strength of ipsilaterally evoked inhibition increased with increasing modulation frequencies in one third of the cells tested. Thus glycinergic and GABAergic inhibition is at least one source responsible for the observed interdependence of temporal structure of a sound and spatial cues.

Acoustic Stimulation↗

PubMed2003

Sensitivity to spectral interaural intensity difference cues in space-specific neurons of the barn owl.

Barn owls use interaural intensity differences to localize sounds in the vertical plane. At a given elevation the magnitude of the interaural intensity difference cue varies with frequency, creating an interaural intensity difference spectrum of cues which is characteristic of that direction. To test whether space-specific cells are sensitive to spectral interaural intensity difference cues, pure-tone interaural intensity difference tuning curves were taken at multiple different frequencies for single neurons in the external nucleus of the inferior colliculus. For a given neuron, the interaural intensity differences eliciting the maximum response (the best interaural intensity differences) changed with the frequency of the stimulus by an average maximal difference of 9.4+/-6.2 dB. The resulting spectral patterns of these neurally preferred interaural intensity differences exhibited a high degree of similarity to the acoustic interaural intensity difference spectra characteristic of restricted regions in space. Compared to stimuli whose interaural intensity difference spectra matched the preferred spectra, stimuli with inverted spectra elicited a smaller response, showing that space-specific neurons are sensitive to the shape of the spectrum. The underlying mechanism is an inhibition for frequency-specific interaural intensity differences which differ from the preferred spectral pattern. Collectively, these data show that space-specific neurons are sensitive to spectral interaural intensity difference cues and support the idea that behaving barn owls use such cues to precisely localize sounds.

Action Potentials↗

PubMedDate not supplied

Coincidence detection in the Hodgkin-Huxley equations.

Some of the cochlear nuclei in the auditory pathway are specialized for the sound localization. They compute the interaural time difference. The difference in sound timing is transduced by the dedicated neuronal circuit into a labeled line difference. The detector neurons along the delay line fire only when synaptic inputs reflecting signals from both cars arrive within a short time window. It was therefore called coincidence detection. We show, (1) what are the limits of coincidence detection in the leaky integrator model, which is a linear system, (2) how should the ideal coincidence detector based on the Hodkin-Huxley equations from real neurons look like, (3) what are the properties and physical limits in the real coincidence detection system. The conclusion is that the neuron with the Hodgkin Huxley dynamics has a fixed precision for the coincidence detection. The limits of the sound localization precision are set by the frequency of the sound and, therefore, by the vector strength of spike trains generated in the neuronal circuit in response to the sound.

Auditory Pathways↗

PubMed2002

A method to induce swapped binaural hearing.

This paper describes the application of a small hearing aid that precisely fits into a subject's ear canal (complete-in-canal, or CIC). The bandwidth of the device is about 7 kHz. The system allows for selective manipulation of the different acoustic cues used for sound localization. The potential of the system is illustrated by robustly interchanging the input of the left and right ear, and consequently changing the sign of the binaural difference cues (both interaural phase and intensity) that are used for horizontal sound localization. As a result, left-right perception is reversed, while high-frequency pinna cues are sufficiently preserved to maintain up-down localization. As the hearing condition is well-defined, the auditory system could in principle remap these cues into a new representation of sound azimuth by relating the modified cues to veridical sound locations. The hearing aids were applied in four human subjects. Swapped binaural hearing was tested in two of the subjects. Swapped localization experiments for an extended period indicated stable performance of both subjects. Interestingly, an adaptive response to the reversed interaural cues was not observed. The current system may prove useful for psychophysical studies that concern the independent processing of sound localization cues, as well as in long-term developmental and plasticity studies with animals.

Acoustic Stimulation↗

PubMed2002

Binaural cochlear implants placed during the same operation.

OBJECTIVE: To evaluate the binaural listening advantages for speech in quiet and in noise and to localize sound when independently programmed binaural cochlear implants are used, and to determine whether ears with different hearing ability and duration of profound deafness perform differently with cochlear implants as well as to what extent preimplant psychophysical and physiologic assessment could be predictive of performance. STUDY DESIGN: Prospective study in which patients were prospectively selected to undergo bilateral implantation during a single surgical procedure at a tertiary referral center. All testing was performed with patients using their right, left, or both cochlear implants. Preimplant and intraoperative measures used electrical stimulation at the round window and stimulation through the cochlear implant. RESULTS: Bilateral implantation during the same operation did not cause any postoperative problems such as severe vertigo or ataxia. At 1 year, results of speech testing in quiet demonstrated a binaural advantage for 2 of 10 subjects. Speech-in-noise testing demonstrated that two implants were beneficial for two individuals. All subjects benefited from a head shadow effect when an ear with a better signal-to-noise ratio was available. The ability to localize sound was improved with binaural implants in all subjects. Preimplant psychophysical or physiologic measures were not predictive of eventual speech perception performance. CONCLUSION: Binaural cochlear implants can assist in the localization of sounds and have the potential in some individuals to improve speech understanding in quiet and in noise.

Adult↗

PubMed1989

Directional sensitivity of sound-pressure levels in the human ear canal.

Changes in sound pressures measured in the ear canal are reported for broadband sound sources positioned at various locations about the subject. These location-dependent pressures are one source of acoustical cues for sound localization by human listeners. Sound source locations were tested with horizontal and vertical resolution of 10 degrees. Sound levels were measured with miniature microphones placed inside the two ear canals. Although the measured amplitude spectra varied with the position of the microphone in the ear canal, it is shown that the directional sensitivity at any particular frequency of the broadband stimulus is independent of microphone position anywhere within the ear canal. At any given frequency, the distribution of sound pressures as a function of sound source location formed a characteristic spatial pattern comprising one or two discrete areas from which sound sources produced maximum levels in the ear canal. The locations of these discrete areas varied in horizontal and vertical location according to sound frequency. For example, around 8 kHz, two areas of maximum sensitivity typically were found that were located laterally and were separated from each other vertically, whereas, around 12 kHz, two such areas were found located on the horizontal plane and separated horizontally. The spatial patterns of sound levels were remarkably similar among different subjects, although some frequency scaling was required to accommodate for differences in the subjects' physical sizes. Interaural differences in sound-pressure level (ILDs) at frequencies below about 8 kHz tended to increase monotonically with increasing distance of the sound source from the frontal midline and tended to be relatively constant as a function of vertical source location. At higher frequencies, however, ILDs varied both with the horizontal and with the vertical location of the sound source. At some frequencies, asymmetries between the left and right ears in a given subject resulted in substantial ILDs even for midline sound sources. These results indicate the types of horizontal and vertical spatial information that are available from sound level cues over various ranges of frequency and, within a small subject population, indicate the nature of intersubject variability.

Acoustic Stimulation↗

PubMed2005

Human brain activation during passive listening to sounds from different locations: an fMRI and MEG study.

Recent animal and human studies indicate the existence of a neural pathway for sound localization, which is similar to the "where" pathway of the visual system and distinct from the sound identification pathway. This study sought to highlight this pathway using a passive listening protocol. We employed fMRI to study cortical areas, activated during the processing of sounds coming from different locations, and MEG to disclose the temporal dynamics of these areas. In addition, the hypothesis of different activation levels in the right and in the left hemispheres, due to hemispheric specialization of the human brain, was investigated. The fMRI results indicate that the processing of sound, coming from different locations, activates a complex neuronal circuit, similar to the sound localization system described in monkeys known as the auditory "where" pathway. This system includes Heschl's gyrus, the superior temporal gyrus, the supramarginal gyrus, and the inferior and middle frontal lobe. The MEG analysis allowed assessment of the timing of this circuit: the activation of Heschl's gyrus was observed 139 ms after the auditory stimulus, the peak latency of the source located in the superior temporal gyrus was at 156 ms, and the inferior parietal lobule and the supramarginal gyrus peaked at 162 ms. Both hemispheres were found to be involved in the processing of sounds coming from different locations, but a stronger activation was observed in the right hemisphere.

Acoustic Stimulation↗

PubMed2002

Experience-dependent refinement of inhibitory inputs to auditory coincidence-detector neurons.

The spatial arrangement of inputs on to single neurons is assumed to be crucial in accurate signal processing. In mammals, the most precise temporal processing occurs in the context of sound localization. Medial superior olivary neurons can encode microsecond differences in the arrival time of low-frequency sounds at the two ears. Here we show that in mammals with well developed low-frequency hearing, a spatial refinement of ionotropic inhibitory inputs occurs on medial superior olivary neurons during development. This refinement is experience dependent and does not develop in mammals that do not use interaural time differences for sound localization.

Afferent Pathways↗

PubMed2005

Unilateral profound hearing loss and the effect on quality of life after cerebellopontine angle surgery.

OBJECTIVE: To assess patients' quality of life after cerebellopontine angle surgery, and in particular, quality of life related to unilateral profound hearing loss. STUDY DESIGN AND SETTING: Cross-sectional in a tertiary referral center. Quality of life of 51 postoperative patients was assessed by using the Glasgow Benefit Inventory (GBI). Thirty patients with unilateral profound hearing loss who had undergone the translabyrinthine approach completed a subsequent quality-of-life questionnaire on speech discrimination and sound localization. RESULTS: Ninety-four percent of respondents to the 2nd survey reported difficulties with speech discrimination, and 97%, with sound localization. The general health and overall GBI indices correlated significantly (P < 0.01) with a number of speech and localization difficulties. CONCLUSION: Unilateral profound hearing loss may be a significant factor in a change in quality of life after cerebellopontine angle surgery. SIGNIFICANCE: Rehabilitation devices that improve discrimination and localization, and hearing preservation surgery, if indicated, should be considered for these patients.

Adult↗

PubMed1995

Left-right discrimination of sound onset by the Mauthner system.

We present a neural model for how the Mauthner system could compute the direction of a transient sound stimulus originating on either the left or right side of a fish. This computation results in an initial orientation of an escape response away from the side of the stimulus. Our idea is based on the phase model of underwater sound localization by fishes. If the phase model is applicable to the Mauthner system, then the problem of sound localization can be reduced to a logical operator, the EXCLUSIVE-NOR (or XNOR). We show how this can be solved by the Mauthner system using afferents that convey separate inputs of sound pressure transduced by the swimbladder (rarefaction and compression) and particle displacement (left and right) from the inner ear. In our model, both pressure components are responsible for bringing the Mauthner cell to threshold. Mauthner firing is gated by the inhibitory PHP neurons receiving specific combinations of pressure and displacement that implement the XNOR logic. We refer to this as the XNOR model. This model is experimentally verifiable and makes specific predictions about the expected acoustic response characteristics of the Mauthner and PHP neurons. Our model places a component of PHP function into a new neuroethological context and may provide insights into the central neurophysiological mechanisms of directional hearing in fishes. In particular, we show how the XNOR model can be applied to predict the activity of diverse neural elements involved in acoustic localization by fishes.

Animals↗

PubMed1991

Directional hearing in the gray tree frog Hyla versicolor: eardrum vibrations and phonotaxis.

1. We used laser vibrometry to study the vibrational frequency response of the eardrum of female gray tree frogs for different positions of the sound source in three-dimensional space. Furthermore, we studied the accuracy of 3-D phonotaxis in the same species for sounds with different frequency contents. 2. The directionality of the eardrum was most pronounced in a narrow frequency range between 1.3 and 1.8 kHz. 3. The average 3-D, horizontal and vertical jump error angles for phonotactic approaches with a sound similar to the natural advertisement call (1.1 and 2.2 kHz frequency components) were 23 degrees, 19 degrees and 12 degrees, respectively. 4. 3-D jump error angle distributions for the 1.4 + 2.2 kHz, 1.0 kHz and 2.0 kHz sounds were not significantly different from that for the 1.1 + 2.2 kHz sound. 5. The average 3-D jump error angle for the 1.4 kHz sound was 36 degrees, and the distribution was significantly different from that for the 1.1 + 2.2 kHz sound. Hence, phonotactic accuracy was poorer in the frequency range of maximum eardrum directionality. 6. Head scanning was not observed and is apparently unnecessary for accurate sound localization in three-dimensional space. 7. Changes in overall sound pressure level experienced by the frog during phonotactic approaches are not an important cue for sound localization.

Acoustic Stimulation↗

PubMed2000

Ventriloquism in patients with unilateral visual neglect.

Can visual stimuli that go undetected, because they are presented in the extinguished region of neglect patients' visual field, nevertheless shift in their direction the apparent location of simultaneous sounds (the well-known 'ventriloquist effect')? This issue was examined using a situation in which each trial involved the simultaneous presentation of a tone over loudspeakers, together with a bright square area on either the left, the right or both sides of fixation. Participants were required to report the presence of squares, and indicate by hand pointing the apparent location of the tone. Five patients with left hemineglect consistently failed to detect the left square, either presented alone or together with another square on the right. Nevertheless, on bimodal trials with a single undetected square to the left, their sound localization was significantly shifted in the direction of that undetected square. By contrast, in bimodal trials with either a single square on the right or a square on each side, their sound localization showed only small and non-significant shifts. This particular result might be due to a combination of low discrimination of lateral sound deviations with variable individual strategies triggered by conscious detection of the right square. The important finding is the crossmodal bias produced by the undetected left visual distractors. It provides a new example of implicit processing of inputs affected by unilateral visual neglect, and on the other hand is consistent with earlier demonstrations of the automaticity of crossmodal bias.

Aged↗

PubMed2003

Utility of monaural spectral cues is enhanced in the presence of cues to sound-source lateral angle.

The contention that normally binaural listeners can localize sound under monaural conditions has been challenged by Wightman and Kistler (J. Acoust. Soc. Am. 101:1050-1063, 1997), who found that listeners are almost completely unable to localize virtual sources of sound when sound is presented to only one ear. Wightman and Kistler's results raise the question of whether monaural spectral cues are used by listeners to localize sound under binaural conditions. We have examined the possibility that monaural spectral cues provide useful information regarding sound-source elevation and front-back hemifield when interaural time differences are available to specify sound-source lateral angle. The accuracy with which elevation and front-back hemifield could be determined was compared between a monaural condition and a binaural condition in which a wide-band signal was presented to the near ear and a version of the signal that had been lowpass-filtered at 2.5 kHz was presented to the far ear. It was found that accuracy was substantially greater in the latter condition, suggesting that information regarding sound-source lateral angle is required for monaural spectral cues to elevation and front-back hemifield to be correctly interpreted.

Acoustic Stimulation↗