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PubMedDate not supplied

The effects of aging on neuron number in the rat superior olivary complex.

The superior olivary complex (SOC) is an auditory center in the brainstem involved in the processing of binaural information. The ability to localize sounds is dependent on the efficient processing of binaural signals by the SOC and other auditory centers. It has been demonstrated that sound localization ability is impaired in aged rats, and morphological changes in the SOC may contribute to this deficit. In the present study, neuron counts were done on three key nuclei of the SOC: the medial nucleus of the trapezoid body (MNTB), the lateral superior olivary nucleus (LSO), and the medial superior olivary nucleus (MSO) in groups of Fischer 344 rats aged 3, 12, 24, and 30 months. Neuron number remains stable between 3 and 30 months of age in the LSO and MSO, however, in the MNTB, neuron number is significantly reduced at 24 and 30 months of age. Neuron loss in the MNTB of 24-month-old Fischer 344 rats is not as heavy as that reported earlier in 24-month-old Sprague-Dawley rats (8% loss versus 34% loss), indicating a strain difference with regard to aging in the SOC.

Aging↗
PubMed1989

Infants' monaural localization of sounds: effects of unilateral ear infection.

The aim in this study was to examine the impact of unilateral ear infection (i.e., otitis media with effusion) on infants' localization of sounds in the horizontal place. Twenty eight infants 6 to 18 months of age were tested at the time of an ear infection, as well as 2 weeks later. Sound localization was measured using a two-alternative forced-choice procedure to examine infants' abilities to discriminate a sound shift of 8 degrees, 12 degrees, 16 degrees, and 20 degrees off midline and along the horizontal axis, either ipsilateral or contralateral to the infected ear. A head and/or eye movement in the direction of the sound was designated as a correct response and was visually reinforced. Results revealed significant negative effects of unilateral ear infection on sound localization performance. All infants showed more correct localization responses for sounds shifted ipsilateral to the problem ear 2 weeks following their ear infection than at the time of the infection. Localization of sounds shifted contralateral to the infected ear did not vary with test date, and significantly exceeded ipsilateral performance when tested at the time of an ear infection. Results are consistent with adult data which indicates that, with unilateral hearing loss, a sound ipsilateral to the problem ear is displaced in location along the horizontal axis toward the well-functioning ear. These findings indicate the importance of balanced binaural functioning for horizontal localization and highlight the plasticity of the developing human auditory system.

Attention↗
PubMed1990

Infants' localization of sounds within hemifields: estimates of minimum audible angle.

Infants' acuity in localizing sounds within hemifields was examined by determining the smallest sound shift off 60 degrees and along the horizontal axis that infants could discriminate reliably, that is, minimum audible angle (MAA). Infants 6, 12, and 18 months of age were tested using a Go/No-Go conditioned head-turn procedure in which infants received an equal number of no-change (control) trials and sound-shift (experimental) trials. A correct response (i.e., a head turn toward the loud-speakers) on an experimental trial resulted in visual reinforcement. Localization acuity for sounds within hemifields was fairly poor and improved systematically with increasing age. MAA estimates at each age were significantly higher, indicating poorer localization acuity, in comparison to those obtained previously in research examining infants' resolution of auditory space near midline. These findings are consistent with adult data and demonstrate a finer partitioning of auditory space near midline than within hemifields. The implications of these findings for our understanding of the development of auditory processing mechanisms in sound localization are discussed.

Age Factors↗
PubMed1987

Infants' localization of sounds in the median vertical plane: estimates of minimum audible angle.

Infants 6, 9, 12, 15, and 18 months of age were seated in a dark room directly facing an array of nine loudspeakers positioned along the median vertical plane. One loudspeaker was positioned at ear level, 0 degree, and four others each were positioned above and below 0 degree. To examine infants' resolution of auditory space in the median vertical plane we sought to determine the smallest angular shift in the vertical location of a sound that infants could reliably detect (i.e., minimum audible angle). A two-alternative forced-choice procedure was used in which a sequence of white noise bursts was presented initially at 0 degree, and then shifted vertically (i.e., above or below 0 degree) and continued to be presented until the infant made a directional response; correct responses were visually reinforced. The smallest angular shift in vertical location that was reliably detected systematically decreased with increasing age between 6 months (15 degrees) and 18 months (4 degrees), suggesting a finer partitioning of auditory space along the vertical axis over this age range. By 18 months infants' performance matched that of a group of adults tested under the same circumstances.

Auditory Perception↗
PubMed1987

Development and preliminary evaluation of an earmold sound-to-tactile aid for the hearing-impaired.

A binaural earmold sound-to-tactile aid was constructed by inserting a vibrating element into a Lucite earmold. The earmold could be vibrated at either 80 Hz (when incoming acoustic signals were below 2000 Hz), at 300 Hz (when incoming acoustic signals were above 2000 Hz), or both (when incoming acoustic signals were broadband). Subjects were fitted with one of these bimodal vibrating earmolds in each ear. Normal-hearing and hearing-impaired subjects were tested in three tasks: sound localization, environmental sound identification, and syllable rhythm and stress. The device provided some benefit to performance, although the amounts of improvement varied across tasks and subjects. Possible modifications in device design, and potential combinations of auditory and tactile input via earmold systems, are discussed.

Adult↗
PubMed1975

Auditory localization: role of auditory pathways in brain stem of the cat.

Cats were trained to localize sound in space. The animals' localization accuracy was determined before and after one of the following operations: 1) transection of the trapezoid body, 2) unilateral and 3) bilateral transection of the lateral lemniscus, 4) unilateral and 5) bilateral transection of the brachium of the inferior colliculus. The results after bilateral transections of the lateral lemniscus and the one deep bilateral transection of the brachium of the inferior colliculus indicate that some portion of the ascending auditory system must be intact above the medulla for an animal to be able to localize sound. A small loss in accuracy of localization was found after unilateral transection of the lateral lemniscus or brachium of the inferior colliculus. This loss, when compared with the much larger loss that monaural animals show, is an indication that binaural analysis, important for sound localization, occurs at the level of the medulla. Some transections of the trapezoid body resulted in a deficit in localization ability that appeared to be complete and permanent. The position of the lesions in the trapezoid body indicated that important encoding of the binaural cues to localization most likely occurs at the superior olivary complex, probably at the medial superior olive. But the trapezoid body or other commissures of the brain stem auditory system are probably also involved in transmission of information necessary for localization to higher centers.

Animals↗
PubMed1990

Binaural and monaural localization of sound in two-dimensional space.

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

Attention↗
PubMedDate not supplied

[Capacity for monaural localization of sound sources in the horizontal plane].

Using special procedures, we investigated monaural spatial sound as perceived in the horizontal plane in 30 otologically healthy subjects, aged 17 to 23 years. It was found that on the masked side (over 50 dB) the localization of sound sources in this plane was disturbed and the numbers of loudspeakers as indicated by the test subjects were shifted towards the open ear. On the open side they showed the ability to locate a loudspeaker positioned at an angle of 18 degrees and failure to locate loudspeakers positioned at angles of 9 degrees and 27 degrees. Also, 60 audiologically healthy people in the age group from 21 to 70 years were examined. Most subjects displayed a similar pattern of response which seems to be independent of their age, sex or the state of concha auriculae.

Adolescent↗
PubMed1986

The sensitive period for auditory localization in barn owls is limited by age, not by experience.

Early in life, the barn owl passes through a sensitive period during which it can interpret and make use of abnormal auditory cues for accurate sound localization. This capacity is lost at about 8 weeks of age, just after the head and ears reach adult size (knudsen et al. 1984a). The end of the sensitive period could be triggered either by an age-dependent process or by the exposure of the auditory system to stable or adult-like cues. To distinguish between these alternatives, we subjected baby owls to constant abnormal cues (chronic monaural occlusion) or to frequently changing abnormal cues (alternating monaural occlusion) throughout the sensitive period. In the first group of animals (n = 2), one ear was plugged continuously until 73 or 79 d of age, respectively, and then the earplug was switched to the opposite ear. Although these animals adjusted sound localization accuracy during the initial chronic monaural occlusion, they could not localize sounds at all after the earplug was switched to the opposite ear, and they remained unable to localize sounds as long as the opposite ear remained occluded (7 and 27 weeks, respectively). When the second monaural occlusion was finally removed, both birds localized sounds with errors that were similar to the errors they exhibited immediately after removal of the first monaural occlusion. One bird that was 127-d-old at the time the second earplug was removed corrected its localization error; the other bird, 250-d-old when the second earplug was removed, did not.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1989

Localization of sound in rooms. IV: The Franssen effect.

The Franssen effect is an illusion that causes human listeners to make large errors in localizing a sound source. This paper describes steps taken to convert the illusion into an experiment in order to study the localization precedence effect as it operates in rooms. The results of the experiment suggest that there are two components to the illusion: The first is the inability of listeners to localize a sine tone in a room in the absence of an onset; the second is the obscuring of modulation cues by the irregular transient response of a room. Experiments show that the Franssen effect fails completely in an anechoic environment, as expected if the effect depends upon the implausibility of steady-state cues in a room. The Franssen effect also fails when the spectrum of the sound is dense.

Auditory Perception↗
PubMed1987

Speculations about noise and the evolution of vertebrate hearing.

Two of the most conspicuous and celebrated properties of human hearing--the abilities to discriminate pitch and to localize sound sources, may have been secondary derivations to which the ear was preadapted by having evolved in the inevitable presence of noise.

Adaptation, Physiological↗
PubMed1991

A blind mobility aid modeled after echolocation of bats.

A new model of a mobility aid for the blind was designed using microprocessor and ultrasonic devices. This mobility aid was evaluated based on psychophysical experiments. In this model, a downswept FM ultrasound signal is emitted from a transmitting array with broad directional characteristics in order to detect obstacles. The ultrasound reflections from the obstacles are picked up by a two-channel receiver. The frequency of the emitted ultrasound is swept from 70 to 40 kHz within 1 ms, so it has almost the same characteristics as the ultrasound a bat produces for echolocation. The frequency of the reflected ultrasound wave is down converted by about 50:1 by using a microcomputer with A/D and D/A converters. These audible waves are then presented binaurally through earphones. In this method obstacles may be perceived as localized sound images corresponding to the direction and the size of the obstacles. From the results of psychophysical experiments, it was found that downswept FM ultrasound was superior for the recognition of small obstacles compared to other ultrasonic schemes. With it a blind person can recognize a 1-mm-diameter wire. It was also proved that the blind could discriminate between several obstacles at the same time without any virtual images. This mobility aid, modeled after the bat's echolocation system, is very effective at detecting small obstacles placed in front of the head.

Animals↗
PubMed1992

Localization of sound in the vertical plane with and without high-frequency spectral cues.

Binaural localization of 3.0-kHz high- and lowpass noise presented in the median vertical plane (MVP) and lateral vertical plane (LVP) was investigated. We anticipated superior performance when localizing the highpass noise by virtue of the availability of pinna cues. The viability of this supposition was strengthened by monaural localization tests in which performance proficiency for the highpass noise exceeded that for the lowpass noise (p less than .01). The main result showed that binaural localization of proficiency for highpass noise surpassed that for lowpass noise for all listening conditions (p less than .01). However, the importance of binaural temporal and level differences in vertical-plane localization was demonstrated by the highly respectable performances when the lowpass noise was presented in the LVP. Data from binaural localization in the MVP and monaural localization in the LVP suggested that the influence of pinna cues diminishes for source elevations above 45 degrees.

Adult↗
PubMed1987

Infants' localization of sounds in the median sagittal plane: effects of signal frequency.

The purpose of this research was to determine if infants, like adults, show differential localization performance in the median sagittal plane (MSP) as a function of the spectrum of the signal. Infants 6-18 months of age were seated in a dark room facing an array of nine loudspeakers, with one loudspeaker positioned at ear level, 0 degrees, and four each positioned above and below ear level at 4 degrees, 8 degrees, 12 degrees, and 16 degrees. A two-alternative, forced-choice procedure was used in which a sequence of noise bursts was presented at 0 degrees and then shifted vertically, above or below 0 degrees, and continued to be presented until the infant made a directional head and/or eye movement; correct responses were visually reinforced. For each of three bandpass noise conditions (less than 4 kHz, 4-8 kHz, 8-12 kHz), minimum audible angle (MAA) for each listener, i.e., the smallest of the four angular shifts in vertical sound location that the listener could reliably detect, was estimated. Results indicated that MAA systematically decreased with increasing age, revealing an increasingly finer partitioning of auditory space. Moreover, performance at each age revealed the importance of high frequencies for localization in the MSP. Infants did not reliably localize the low-pass signal (less than 4 kHz) and showed the best performance to the signal comprising the highest frequencies (8-12 kHz). These findings reveal systematic age-related improvements in sound localization abilities during infancy, and suggest that spectral cues similar to those for adults operate for infants in vertical localization.

Auditory Perception↗
PubMed1991

Stretched and upside-down maps of auditory space in the optic tectum of blind-reared owls; acoustic basis and behavioral correlates.

Vision during early life plays an important role in calibrating sound localization behavior. This study investigates the effects of visual deprivation on sound localization and on the neural representation of auditory space. Nine barn owls were raised with eyelids sutured closed; one owl was congenitally anophthalmic. Data from these birds were compared with data from owls raised with normal visual experience. Sound localization behavior was significantly less precise in blind-reared owls than in normal owls. The scatter of localization errors was particularly large in elevation, though it was abnormally large in both dimensions. However, there was no systematic bias to the localization errors measured over a range of source locations. This indicates that the representation of auditory space is degraded in some way for blind-reared owls, but on average is properly calibrated. The spatial tuning of auditory neurons in the optic tectum was studied in seven of the blind-reared owls to assess the effects of early visual deprivation on the neural representation of auditory space. In normal owls, units in the optic tectum are sharply tuned for sound source location and are organized systematically according to the locations of their receptive fields to form a map of auditory space. In blind-reared owls, the following auditory properties were abnormal: (1) auditory tuning for source elevation was abnormally broad, (2) the progression of the azimuths and elevations of auditory receptive fields across the tectum was erratic, and (3) in five of the seven owls, the auditory representation of elevation was systematically stretched, and in the two others large portions of the representation of elevation were flipped upside down. The following unit properties were apparently unaffected by blind rearing: (1) the sharpness of tuning for sound source azimuth, (2) the orientation of the auditory representation of azimuth, and (3) the mutual alignment of the auditory and visual receptive fields in the region of the tectum representing the area of space directly in front of the animal. The data demonstrate that the brain is capable of generating an auditory map of space without vision, but that the normal precision and topography of the map depend on visual experience. The space map results from the tuning of tectal units for interaural intensity differences (IIDs) and interaural time differences (ITDs; Olsen et al., 1989).(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1990

Tracking of a "moving" fused auditory image under conditions that elicit the precedence effect.

Pursuit auditory tracking of a fused auditory image (FAI), based on stimulus conditions known to elicit the precedence effect phenomenon in sound localization, was investigated in 36 normal subjects and in a small group of subjects with known neuropathology. Movement of the FAI was simulated by incrementally varying the delay between two clicks presented, one each, from two loudspeakers placed on opposite sides of the listener. The group of normal listeners tracked the movement of the FAI without difficulty and with great accuracy; the perceived location of the FAI varied linearly with the interspeaker delay. The sensitivity of the task in detecting neural timing or integration deficits was investigated in 5 subjects with neuropathology, including subjects with unilateral temporal lobe lesions, multiple sclerosis, or dyslexia. These disorders, previously shown to disrupt neural timing, yielded characteristic patterns of tracking inaccuracy for this task. These subjects had no difficulty localizing either a moving unitary click source or sounds in daily life. These data support the suggestion that sound localization using stimulus conditions known to elicit the precedence effect places greater demands on neural timing and integration than conventional tests of localization, and may provide a more sensitive index of neural function.

Acoustics↗
PubMed1988

The contribution of the near and far ear toward localization of sound in the sagittal plane.

Eight listeners were required to locate a train of 4.5-kHz high-pass noise bursts emanating from loudspeakers positioned +/- 30, +/- 20, +/- 10, and 0 deg re: interaural axis. The vertical array of loudspeakers was placed at 45, 90, and 135 deg left of midline. The various experimental conditions incorporated binaural and monaural listening with the latter utilizing the ear nearest or ear farthest from the sound source. While performance excelled when listening with only the near ear, the contribution of the far ear was statistically significant when compared to localization performance when both ears were occluded. Based on head related transfer functions for stimuli whose bandwidth was 1.0 kHz, four spectral cues were selected as candidates for influencing location judgments. Two of them associated relative changes in energy across center frequencies (CFs) with vertical source positions. The other two associated an absolute minimum (maximum) energy for specific CFs with a vertical source position. All but one cue when measured for the near ear could account for localization proficiency. On the other hand, when listening with the far ear, maximum energy at a specific CF outperformed the remaining cues in accounting for localization proficiency.

Acoustic Stimulation↗
PubMedDate not supplied

[Hearing function in patients with disseminated sclerosis].

The hearing function in 32 patients with various forms of disseminated sclerosis was measured. Their hearing sensitivity, loudness and discrimination as well as spatial hearing and ability to localize sound sources were determined. In the absence of changes in hearing sensitivity, loudness and discrimination all the patients displayed marked disorders or complete loss of their capability to localize sound sources in the vertical plane but retained their capability to localize sound sources in the horizontal plane. Therefore, in contrast to the widely recognized concept that the hearing function in patients with disseminated sclerosis remains intact, spatial hearing impairment should be viewed as a pathognomonic symptom which is very important for diagnosing it at all stages of its development, including the earliest stages.

Adolescent↗