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Recent studies have shown that adult humans can learn to localize sounds relatively accurately when provided with altered localization cues. These experiments provide further evidence for experience-dependent plasticity in the mature brain.
This study examined virtual sound localization in three conditions that differed according to the directional transfer functions (DTFs) that were used to synthesize the virtual targets. The own-ear and other-ear conditions used DTFs measured from listeners' own ears and those measured from other subjects, respectively. The scaled-ear condition employed other-ear DTFs that were scaled in frequency to minimize the mismatch between spectral features in the listener's and the other subject's DTFs. All measures of localization error typically were lowest in the own-ear condition. In other-ear conditions, all error measures tended to increase in proportion to the inter-subject differences in DTFs. When spectral features in an other-ear set of DTFs fell systematically lower in frequency than in a listener's own DTFs, low frontal targets typically were reported as low in the rear, and high rear targets were reported as high in front. When spectral features in a set of DTFs fell systematically higher in frequency than in a listener's own DTFs, elevation judgements showed an upward bias. In the scaled-ear condition, all measures of performance tended to improve relative to the other-ear condition. In the majority of cases, frequency scaling more than halved the penalty for use of another subject's DTFs.
The vibration characteristics (amplitude and phase as a function of frequency) of the tympanic membrane in the grass frog were measured using a laser-doppler velocity meter. It was tested to what extent the frog's acoustic system behaves as a pressure gradient receiver. This might clarify how the frog localizes sound. Using a closed sound system the membrane was stimulated at three different entrances: in front of the membrane, at the contralateral ear and from inside the mouth. A combination of these can describe the motion of the membrane under free field conditions. It is found that the sound entrance from inside the mouth will give almost identical vibration characteristics as stimulation in front of the membrane. This can yield a perfect gradient receiver mechanism, when the frog opens its mouth. It is doubted however whether the frog in nature needs to open its mouth for localization of sound. With mouth closed the effectiveness of the gradient receiver will be determined by the transmission characteristics of sound across the tissues of the mouth. The entrance of sound via the contralateral ear is only effective at frequencies between 800 and 1600 Hz. At those frequencies crosstalk between the membranes is however not more than -4 to -8 dB. This is subject to changes in the acoustic properties of the mouth cavity and can possibly be altered by the frog in free nature.
The influence of sound localization behavior on unit activity in the frontal cortex of awake rhesus monkeys was examined by comparing responses under three behavioral conditions: auditory localization, during which a response was required to the location of a sound (broad-band noise) source; auditory detect, during which a response was required to indicate the occurrence of the sound regardless of location; visual localization, during which no sounds were presented and a response was required to the location of a visual stimulus; and nonperform, presentation of auditory stimuli as in the first two conditions, but with the animal sitting passively. Extracellular microelectrode recordings were made in the periarcuate region and dorsal and ventral prefrontal areas near the principal sulcus. Four monkeys were used with a total of 498 cells studied. Of the total population, only five cells were found to have characteristics similar to those of auditory units in the primary auditory cortex and the surrounding belt area. More typically, units were found that had strong short-latency responses specific to the auditory and/or visual localization tasks. These units had no or weak responses when the same sound stimuli were presented in the auditory detect task or when a monkey received the sound stimuli in a nonperforming condition. Two regions were identified, one medial and/or posterior to the arcuate sulcus, in Brodmann's area 6; the second included parts of areas 8 and 9 within the genu of the arcuate sulcus. Units from these regions are referred to, respectively, as the postarcuate and the prearcuate populations. Both populations responded predominantly during active localization behavior. Sixty-two percent of the postarcuate population responded during auditory localization, 32% responded during auditory detect, and only 18% responded to acoustic stimuli presented in the nonperforming condition. In the prearcuate population percentages in these three conditions were 35, 25, and 12%, respectively. For visual localization, 54% in the postarcuate population responded, whereas 42% in the prearcuate responded. Spatial tuning of units during auditory localization was similar to that seen in units of the primary auditory cortex, with the greatest percentages of units responding to stimuli contralateral to the recording site. Similar tuning was observed for the visual localization task as well. Similarities in spatial tuning between the auditory and visual localization conditions were examined to assess the "bimodal" nature of the units.(ABSTRACT TRUNCATED AT 400 WORDS)
The ability of horses to use binaural time and intensity difference cues to localize sound was assessed in free-field localization tests by using pure tones. The animals were required to discriminate the locus of a single tone pip ranging in frequency from 250 Hz to 25 kHz emitted by loudspeakers located 30 degrees to the left and right of the animals' midline (60 degrees total separation). Three animals were tested with a two-choice procedure; 2 additional animals were tested with a conditioned avoidance procedure. All 5 animals were able to localize 250 Hz, 500 Hz, and 1 kHz but were completely unable to localize 2 kHz and above. Because the frequency of ambiguity for the binaural phase cue delta phi for horses in this test was calculated to be 1.5 kHz, these results indicate that horses can use binaural time differences in the form of delta phi but are unable to use binaural intensity differences. This finding was supported by an unconditioned orientation test involving 4 additional horses, which showed that horses correctly orient to a 500-Hz tone pip but not to an 8-kHz tone pip. Analysis of the superior olivary complex, the brain stem nucleus at which binaural interactions first take place, reveals that the lateral superior olive (LSO) is relatively small in the horse and lacks the laminar arrangement of bipolar cells characteristic of the LSO of most mammals that can use binaural delta I.
Vocalizations often contain low-frequency modulations of the envelope of a high-frequency sound. The high-frequency portion of the cochlear nerve of mice (Mus musculus) generates a robust phase-locked response to these low-frequency modulations, and it can be easily recorded from the surface of the scalp. The cochlea is most sensitive to envelope modulation frequencies of approximately 500 to 2000 Hz. These responses have detection thresholds that are approximately 10 dB more sensitive than auditory brainstem responses, and they are very sharply tuned. These measurements may provide a nontraumatic means of repeatedly assessing cochlear functions involved in sound localization and perception of vocalizations.
Blind individuals who lost their sight as older children or adults were compared with normally sighted controls in their ability to focus auditory spatial attention and to localize sounds in a noisy acoustic environment. Event-related potentials (ERPs) were recorded while participants attended to sounds presented in free field from either central or peripheral arrays of speakers with the task of detecting infrequent targets at the attended location. When attending to the central array of speakers, the two groups detected targets equally well, and their spatial tuning curves for both ERPs and target detections were highly similar. By contrast, late blind participants were significantly more accurate than sighted participants at localizing sounds in the periphery. For both groups, the early N1 amplitude to peripheral standard stimuli displayed no significant spatial tuning. In contrast, the amplitude of the later P3 elicited by targets/deviants displayed a more sharply tuned spatial gradient during peripheral attention in the late blind than in the sighted group. These findings were compared with those of a previous study of congenitally blind individuals in the same task [Röder, B., Teder-Sälejärvi, W., Sterr, A., Rösler, F., Hillyard, S. A., & Neville, H. J. Improved auditory spatial tuning in blind humans. Nature, 400, 162-166, 1999]. It was concluded that both late blind and congenitally blind individuals demonstrate an enhanced capability for focusing auditory attention in the periphery, but they do so via different mechanisms: whereas congenitally blind persons demonstrate a more sharply tuned early attentional filtering, manifested in the N1, late blind individuals show superiority in a later stage of target discrimination and recognition, indexed by the P3.
We studied the case of a 48-year-old woman who had resection of a pineal body tumor in terms of postoperative audiological function. Postoperative magnetic resonance imaging disclosed partial inferior colliculi destruction and medial geniculate body degeneration. A pure tone audiogram revealed only moderate sensorineural hearing loss, but her speech perception was totally impaired. The binaural sound localization function was also impaired. The auditory brain stem response (ABR) showed waves I, III, and V to have normal latencies. The amplitude of wave III was larger than that of wave V. These results support the view that the waves of the ABR are elicited from multiple sources in the auditory brain stem nuclei and tracts. This case suggests a substantial role for the inferior colliculus and medial geniculate body in the processing of speech perception and sound localization.
A questionnaire on (1) difficulties in everyday sound localization (localization disabilities), (2) limitations and disadvantages related to such disabilities (localization handicaps), and (3) everyday speech hearing disabilities was completed by 104 people with symmetrical hearing losses. Respondents considered situations when listening with and without their usual hearing aid fittings. The hearing-impaired group rated their unaided localization ability as significantly poorer than a sample with no reported hearing loss, particularly discrimination of distance. Only slight limitations on everyday activity due to localization disability were found, but disadvantage was reported in the form of experiences of confusion of sounds, of resultant loss of concentration, and of a wish to escape settings where this occurred. There was a moderate correlation between localization and speech hearing, even when hearing level was controlled for. Aided listening, whether with one or with two hearing aids, was reported as enabling significant improvements in both functions.
BACKGROUND: auditory training in groups. AIM: to verify in a group of individuals with mental retardation the efficacy of auditory training in a workshop environment. METHOD a longitudinal prospective study with 13 mentally retarded individuals from the Associação de Pais e Amigos do Excepcional (APAE) of Congonhas divided in two groups: case (n=5) and control (n=8) and who were submitted to ten auditory training sessions after verifying the integrity of the peripheral auditory system through evoked otoacoustic emissions. Participants were evaluated using a specific protocol concerning the auditory abilities (sound localization, auditory identification, memory, sequencing, auditory discrimination and auditory comprehension) at the beginning and at the end of the project. Data (entering, processing and analyses) were analyzed by the Epi Info 6.04 software. RESULTS: the groups did not differ regarding aspects of age (mean = 23.6 years) and gender (40% male). In the first evaluation both groups presented similar performances. In the final evaluation an improvement in the auditory abilities was observed for the individuals in the case group. When comparing the mean number of correct answers obtained by both groups in the first and final evaluations, a statistically significant result was obtained for sound localization (p=0.02), auditory sequencing (p=0.006) and auditory discrimination (p=0.03). CONCLUSION: group auditory training demonstrated to be effective in individuals with mental retardation, observing an improvement in the auditory abilities. More studies, with a larger number of participants, are necessary in order to confirm the findings of the present research. These results will help public health professionals to reanalyze the theory models used for therapy, so that they can use specific methods according to individual needs, such as auditory training workshops.
Sound localization in humans depends largely on interaural time delay (ITD). The ability to discriminate differences in ITD is highly accurate. ITD discrimination (Delta ITD) thresholds, under some circumstances, are as low as 10-20 micros. It has been assumed that thresholds this low could only be obtained if the outputs from many neurons were combined. Here we use Receiver Operating Characteristic analysis to compute neuronal Delta ITD thresholds from 53 cells in the inferior colliculus in guinea pigs. The Delta ITD thresholds of single neurons range from several hundreds of micros down to 20-30 micros. The lowest single-cell thresholds are comparable to human thresholds determined with similar stimuli. This finding suggests that the highly accurate sound localization of human observers is consistent with the resolution of single cells and need not reflect the combined activity of many neurons.
This study examines the extent to which the auditory middle latency response (MLR) of the guinea pig is sensitive to sound localization cues such as interaural level and time differences (ILD and ITD, respectively). The MLR was recorded with an epidural electrode placed over the auditory cortex of an anesthetized guinea pig. Click stimuli were presented monaurally or binaurally with various ILDs and ITDs. The MLR was much larger for contralateral stimulation than for ipsilateral stimulation, and its amplitude was intermediate for diotic stimulation. The MLR amplitude was sensitive to both ILD and ITD: it decreased as the ipsilateral stimulus increased in level or advanced in time relative to the contralateral stimulus. The steep slope of the amplitude-versus-ITD function fell within an ITD range of +/-330 micros, namely the guinea pig's physiological ITD range. The response reduction that resulted from increasing the relative level of the ipsilateral level could be cancelled out by advancing the contralateral onset time relative to the ipsilateral onset time. This parallels the "time-intensity trading" in sound lateralization exhibited in human psychophysics. The results imply that the binaural interaction in the guinea pig MLR reflects aspects of neural processes that are involved in sound localization.
A microelectrode survey was made of the midbrain auditory nuclei of the brushtailed possum (Trichosurus vulpecula), a common Australian marsupial. Information was sought on the tuning characteristics of individual neurones, tonotopic organization and mechanisms of sound localization. It was felt that such information would be of use in future studies of the development and evolution of mammalian hearing. Twelve possums were anaesthetized with ketamine and chloralose-urethane, and recordings were made of extracellular unit discharges in the inferior colliculus during monaural and binaural tonal stimulation. The inferior colliculus of the possum consists of a central nucleus - a darkly stained, densely packed group of cells - flanked laterally by an external nucleus with a lower density of paler cells. Tonotopic organization was demonstrated by discretelytuned elements in the central nucleus, but was not observed in the external nucleus. In the latter region broad and irregular tuning was commonly seen. Most units in both divisions were influenced by binaural stimuli, with patterns of binaural interaction similar to those observed in the cat inferior colliculus. Cells influenced by changes in the interaural time and intensity difference were commonly observed, but only a subclass of these were suited in sensitivity for sound localization. In general, the midbrain auditory system of the possum was similar in unit discharge characteristics and organization to those of the eutherian mammals commonly studied.
The purpose of this study was to determine how well humans localize sound sources in the horizontal plane while wearing protective headgear with and without hearing protection. In a source identification task, a stimulus was presented from 1 of 20 loudspeakers arrayed in a semicircular arc, and participants stated which loudspeaker emitted the sound. Each participant was tested in 8 conditions involving various combinations of wearing a Kevlar army helmet and two types of earplugs. Testing was conducted at each of 2 orientations (frontal and lateral). In the frontal orientation, overall error was slightly greater in all protected conditions than in the bare-head control condition. In the lateral orientation, overall error score in the protected conditions was substantially and significantly greater than in the bare-head control conditions. Most errors in the lateral orientation were accounted for by front-back confusions, indicating that the protective devices disrupted high-frequency spectral cues that are the basis for discriminating front from back sound sources. The results have practical implications for the use of protective headgear and earplugs in industrial or military environments where localization of critical sounds is important.
The role of spectral cues in the sound source to ear transfer function in median plane sound localization is investigated in this paper. At first, transfer functions were measured and analyzed. Then, these transfer functions were used in experiments where sounds from a source on the median plane were simulated and presented to subjects through headphones. In these simulation experiments, the transfer functions were smoothed by ARMA models with different degrees of simplification to investigate the role of microscopic and macroscopic patterns in the transfer functions for median plane localization. The results of the study are summarized as follows: (1) For front-rear judgment, information derived from microscopic peaks and dips in the low-frequency region (below 2 kHz) and the macroscopic patterns in the high-frequency region seems to be utilized; (2) for judgment of elevation angle, major cues exist in the high-frequency region above 5 kHz. The information in macroscopic patterns is utilized instead of that in small peaks and dips.
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