[Role of various zones of the cerebral cortex in man in the evaluation of changes in localization of a sound source].
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Case-Control Studies↗
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A precedence effect in the perception of inter-aural cross correlation.
Does the precedence effect, well known in the field of sound localization or lateralization, also apply to other percepts based on binaural processing? We have compared, with one and the same experimental paradigm, a manifestation of the traditional precedence effect in lateralization with a possible similar effect in the perception of diffuseness or compactness of a sound image. With dichotic headphone stimulation, lateralization was controlled by the inter-aural time delay (IATD), and diffuseness/compactness by the inter-aural cross correlation (IACC). The experimental paradigm rests on the principle of estimating the over-all sensation of a 20-ms noise burst, which was subdivided in two parts, with the relevant dichotic information (IATD or IACC) in the leading part being opposite to that in the trailing part. When each part is 10 ms, it is found that the overall sensation is slightly dominated by the information in the leading part, both for lateralization and for compactness/diffuseness. This dominance of the leading part can be compensated by a certain decrease of its duration and/or amplitude relative to that of the trailing part. It is found that this quantitative measure for the 'strength' of the precedence effect for the present stimulus is essentially the same for IATD and IACC, suggesting that the precedence effect does not apply exclusively to sound localization or lateralization, but to at least one other percept based on binaural processing as well, namely the processing of inter-aural cross correlation.
Acoustic Stimulation↗
Cochlear microphonic measurements of interaural time differences in the chick.
The major cues for the sound localization are the interaural differences in the timing and intensity of acoustic information. This poses a difficult coding problem for animals with relatively small heads, such as birds, because interaural time differences (ITDs) would have a small range and magnitude and interaural intensity differences (IIDs) would be significant for only high frequency sounds. It has been suggested that this coding problem is mitigated in birds by an enhancement of ITDs and IIDs resulting from the acoustic coupling of the two middle ear cavities through an interaural canal. In this report, the functional ITDs for sounds at different azimuthal locations were recorded in young chicks, and the contribution of middle ear acoustic coupling was evaluated. ITDs were calculated from simultaneous cochlear microphonic (CM) recordings evoked by pure tone stimuli. These effective ITDs were larger than predicted by the physical separation of the two ears, and this enhancement was more pronounced at low (0.8 and 1 kHz) than at high (2 and 4 kHz) frequencies, reaching maximum values of approximately 180 and 100 microseconds, respectively. The amplitude of the CM also varied as a function of sound source location. This variation was as much as +/- 30%, even for the low frequency tones. This suggests that IID cues are also available to the chick. To determine the contribution of middle ear acoustic coupling to the timing and amplitude of the CM response, the CM in one ear was measured prior to, and following occlusion of the contralateral external auditory canal. The cochlear microphonic from the ear distal to the sound source advanced in time and increased in amplitude when the ear proximal to the sound source was sealed. These effects were more pronounced for low frequency sounds. These findings confirm that acoustic coupling of the middle ear cavities plays a role in enhancing sound localization cues in the chick.
Acoustic Stimulation↗
Spatial processing in the primate auditory cortex.
Spatial localization of auditory stimuli is dependent on the cerebral cortex, yet it remains unclear how cortical activity gives rise to spatial percepts. It has recently been proposed that spatial information is processed serially within the primate auditory cortex, initially in the primary auditory cortex (AI) through the auditory areas caudal to AI, particularly the caudomedial (CM) and caudolateral fields, and onward to the parietal lobe. The activity of single neurons in AI and CM supports this hypothesis, where a greater percentage of CM neurons are sensitive to the spatial location of acoustic stimuli than AI neurons, and the spatial sensitivity of CM neurons extends across a broader range of the stimulus spectrum compared to AI neurons. Further, populations of CM neurons are better able to predict sound localization ability than are populations of AI neurons. We have recently explored the effects of stimulus intensity on both sound localization performance and the spatial sensitivity of auditory cortical neurons. The preliminary results of these experiments again indicate that spatial information is serially processed between AI and the caudal fields. The effects of visual stimulation on auditory localization have also been investigated. Under the appropriate circumstances, visual stimuli can "capture" the spatial location of auditory stimuli in both humans and monkeys. This perceptual illusion suggests that there is a plastic shift in auditory spatial perception. Where the representation of this shift resides is unknown, although two likely candidates are the multimodal regions of the parietal lobe and the superior temporal sulcus.
Acoustic Stimulation↗
Azimuthal tuning of human perceptual channels for sound location.
Human sound localization is acute for frontal locations, but relatively poor in the lateral hemifields. Previous studies in man have not, however, provided evidence on the tuning of the perceptual channels for auditory space that subserve this pattern of acuity. The spatial tuning of perceptual channels used in human azimuthal sound localization was determined using a between-channel auditory temporal gap detection paradigm. In this paradigm, gap thresholds are low when the markers bounding the silent period (gap) activate the same perceptual channel but are elevated when the two markers activate different channels. To determine the tuning of spatial channels, gap thresholds were obtained in an anechoic room with white noise markers coming from each combination of 12 leading marker locations and 18 trailing marker locations throughout the full 360 degrees of azimuth in the horizontal plane through the interaural axis. Gap thresholds remained low (2-4 ms) for all combinations of leading and trailing markers between 30 degrees and 150 degrees in both lateral hemifields. When the leading marker was located deep in one hemifield, and the trailing marker was in the opposite hemifield, gap thresholds rose to 8-16 ms. For leading marker locations at 30 degrees from the midline, gap thresholds were low for all trailing marker locations in the ipsilateral hemifield and locations near the midline in the contralateral hemifield, and were elevated (6-8 ms) only near the contralateral pole. Finally, for leading marker locations at 0 degree or 180 degrees, gap thresholds were low for any trailing location within 30 degrees of the midline at the front or back, and thresholds were elevated for trailing locations at the lateral poles. These data are accountable in terms of two broadly tuned perceptual channels, occupying the left and right auditory hemifields, respectively, each extending 30 degrees across the midline. These channels have widths and locations similar to the spatial receptive fields previously described for central auditory neurons in animals. The data suggest a model of spatial acuity based on the rates of activation of two spatially overlapping channels, rather than the selective activation of members of a large population of finely tuned channels.
Adult↗
Dynamic theory of sound-source localization.
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Acoustic Stimulation↗
Bone-anchored hearing aids in unilateral inner ear deafness.
In nine patients with unilateral deafness and normal hearing in the contralateral ear, measurements of sound localization and speech perception were obtained before intervention, with a conventional contralateral routing of sound (CROS) hearing aid and later with a bone-anchored hearing aid (BAHA) implanted in the deaf ear. Sound localization did not show any differences between the three conditions. Speech perception using short, everyday sentences showed a reduction in the head-shadow effect of 2 dB for both the conventional CROS hearing aid and the BAHA in comparison to the unaided condition. Patients' real-life experiences of the three conditions were evaluated using the Abbreviated Profile of Hearing Aid Benefit (APHAB) questionnaire. The results showed a significant benefit with the BAHA in situations involving background noise and reverberation and a reduced aversion to loud sounds in comparison to the unaided and conventional CROS conditions.
Audiometry, Speech↗
Head-related transfer functions of the Rhesus monkey.
Head-related transfer functions (HRTFs) are direction-specific acoustic filters formed by the head, the pinnae and the ear canals. They can be used to assess acoustical cues available for sound localization and to construct virtual auditory environments. We measured the HRTFs of three anesthetized Rhesus monkeys (Macaca mulatta) from 591 locations in the frontal hemisphere ranging from -90 degrees (left) to 90 degrees (right) in azimuth and -60 degrees (down) to 90 degrees (up) in elevation for frequencies between 0.5 and 15 kHz. Acoustic validation of the HRTFs shows good agreement between free field and virtual sound sources. Monaural spectra exhibit deep notches at frequencies above 9 kHz, providing putative cues for elevation discrimination. Interaural level differences (ILDs) and interaural time differences (ITDs) generally vary monotonically with azimuth between 0.5 and 8 kHz, suggesting that these two cues can be used to discriminate azimuthal position. Comparison with published subsets of HRTFs from squirrel monkeys (Saimiri sciureus) shows good agreement. Comparison with published human HRTFs from the frontal hemisphere demonstrates overall similarity in the patterns of ILD and ITD, suggesting that the Rhesus monkey is a good acoustic model for these two sound localization cues in humans. Finally, the measured ITDs in the horizontal plane agree well between -40 degrees and 40 degrees in azimuth with those calculated from a spherical head model with a radius of 52 mm, one-half the interaural distance of the monkey.
Animals↗
Auditory function in Alzheimer's disease.
The pattern of cerebral degeneration in Alzheimer's disease (AD) patients suggests that basic auditory capacities should be normal in AD, whereas progressively higher levels of auditory function should be increasingly impaired. To test this hypothesis, we administered tests of auditory capacities associated with primary auditory cortex (sound localization and perception of complex tones) and auditory association cortex (phoneme discrimination, timbre discrimination, and tonal memory) to 19 mildly to moderately demented AD patients, 21 elderly control subjects (ECS), and 14 young control subjects (YCS). The results showed significant differences between YCS and ECS on phoneme discrimination with synthetic speech and on tonal memory. The AD group differed from the ECS group on sound localization, one measure of synthetic speech discrimination, and timbre discrimination. Performance did not correlate with age, dementia severity, or duration of illness on any test condition. These findings indicate that although AD is accompanied by specific auditory deficits, the increase in neuropathologic change between primary auditory and auditory association cortices is not reflected in an increased impairment of functions that are mediated by these areas. Degraded aural language comprehension, which is characteristic of AD, likely reflects disruption of language processes, rather than dysfunction specific to auditory circuits.
Acoustic Stimulation↗
Azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats.
Previous studies have indicated that the auditory midbrain, the inferior colliculus, is important for both sound localization and mediation of prepulse inhibition of the startle reflex. The present study investigated the azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats. The pinna startle reflex was measured by recording multi-unit action potentials from the cervicoauricular muscles. The startling noise burst (94 dB SPL) was produced by a stationary speaker at 0 degrees azimuth, and the non-startling prepulse noise burst (46 dB SPL) was produced by a movable speaker whose direction was changed in the frontal azimuthal plane. The interval between the onset of the prepulse sound and the onset of the startling sound was 100 ms. The pinna reflex to the startling sound was strongly inhibited by the prepulse sound, and the inhibited startle response exhibited a flat azimuthal directional curve. In addition to further confirming that the neural pathways mediating prepulse inhibition are located in the brainstem, the present results indicate that interaural disparities of binaural inputs used for sound localization are not capable of modulating prepulse inhibition of the startle reflex.
Acoustic Stimulation↗
Effects of pinna position on head-related transfer functions in the cat.
To measure the effects of the pinna position on spectral sound localization cues, the head-related transfer function (HRTF) from the free-field to a point in the ear canal was measured for anesthetized cats with their pinnae in three positions: the relaxed, anesthetized position; pulled forward into an approximation of the "alert cat" posture; and pulled back. The general features of HRTFs are not changed by moving the pinna, although the mapping of particular HRTF features onto directions in space is changed. As an approximation, the pinna behaves like a fixed-shaped sound collector, so that HRTFs shift with the pinna when it moves; however, pinna movement changes some quantitative details of HRTFs beyond what is predicted by this approximation. When viewed as directional gain, pinna movements serve to optimize listening conditions. However, when considering sound localization, pinna movements lead to ambiguities regarding source location. If pinna position is not incorporated into the computation, and spectral cues alone are used for localization, the ambiguity is about 60 degrees in azimuth and 30 degrees in elevation. Pinna movements produce similar azimuthal ambiguity in interaural level differences. Interaural time difference cues could be used to reduce the ambiguity in azimuth, but a knowledge of pinna position seems to be necessary to resolve ambiguities in elevation.
Animals↗
Spatial selectivity and binaural responses in the inferior colliculus of the great horned owl.
In this study we have investigated the processing of auditory cues for sound localization in the great horned owl (Bubo virginianus). Previous studies have shown that the barn owl, whose ears are asymmetrically oriented in the vertical plane, has a 2-dimensional, topographic representation of auditory space in the external division of the inferior colliculus (ICx). As in the barn owl, the great horned owl's ICx is anatomically distinct and projects to the optic tectum. Neurons in ICx respond over only a small range of azimuths (mean = 32 degrees), and azimuth is topographically mapped. In contrast to the barn owl, the great horned owl has bilaterally symmetrical ears and its receptive fields are not restricted in elevation. The binaural cues available for sound localization were measured both with cochlear microphonic recordings and with a microphone attached to a probe tube in the auditory canal. Interaural time disparity (ITD) varied monotonically with azimuth. Interaural intensity differences (IID) also changed with azimuth, but the largest IIDs were less than 15 dB, and the variation was not monotonic. Neither ITD nor IID varied systematically with changes in the vertical position of a sound source. We used dichotic stimulation to determine the sensitivity of ICx neurons to these binaural cues. Best ITD of ICx units was topographically mapped and strongly correlated with receptive-field azimuth. The width of ITD tuning curves, measured at 50% of the maximum response, averaged 72 microseconds. All ICx neurons responded only to binaural stimulation and had nonmonotonic IID tuning curves. Best IID was weakly, but significantly, correlated with best ITD (r = 0.39, p less than 0.05). The IID tuning curves, however, were broad (mean 50% width = 24 dB), and 67% of the units had best IIDs within 5 dB of 0 dB IID. ITD tuning was sensitive to variations in IID in the direction opposite to that expected for time-intensity trading, but the magnitude of this effect was only 1.5 microseconds/dB IID. We conclude that, in the great horned owl, the spatial selectivity of ICx neurons arises primarily from their ITD tuning. Except for the absence of elevation selectivity and the narrow range of best IIDs, ICx in the great horned owl appears to be organized much the same as in the barn owl.
Acoustic Stimulation↗
Projections to the cochlear nuclei from principal cells in the medial nucleus of the trapezoid body in guinea pigs.
Spherical and globular cells in the cochlear nucleus provide input to the cell groups in the superior olivary complex devoted to the analysis of binaural cues. Descending projections from the superior olivary complex appear to inhibit the spherical and globular cells. It is not known which of the numerous cell types in the superior olive provide this descending input, but recent studies have shown that some of the cells are located in the medial nucleus of the trapezoid body (MTB). The present experiments were designed to determine whether the MTB projections arise from principal cells, which are known to play a role in sound localization, and to determine whether their projections terminate on spherical or globular cells. Principal cells in the MTB are characterized by their contacts with synaptic specializations called calyces, which arise from the axons of cells in the contralateral cochlear nucleus. In the first experiment, a fluorescent tracer was injected into one cochlear nucleus to label the calyces anterogradely. A different tracer was injected into the opposite cochlear nucleus to label cells retrogradely in the MTB. In every case, some of the labeled cells were enveloped by a labeled calyx, demonstrating that principal cells do project to the cochlear nucleus. In the second experiment, fluorescent tracers were injected into different parts of the cochlear nucleus. Analysis of the distribution of labeled cells suggested that MTB projections selectively target the globular cell region of the cochlear nucleus. In a third experiment, the axonal arborizations arising from this projection were labeled with biocytin or wheat germ agglutinin conjugated to horseradish peroxidase. Labeled boutons appeared to contact globular cells but not spherical cells. Multipolar cells in the ventral cochlear nucleus and cells in the dorsal cochlear nucleus were also contacted. The results suggest that MTB projections to the cochlear nucleus arise largely from principal cells and contact, at least in part, cells in the cochlear nucleus that give rise to ascending pathways involved in sound localization.
Animals↗
Structure and function of the bat superior olivary complex.
The superior olivary complex (SOC) is a mammalian auditory brainstem structure that contains several nuclei. Some of them are part of the ascending system projecting to higher auditory centers, others belong to the descending system projecting to the cochlear nuclei or the cochlea itself. The main nuclei of the ascending system, the lateral and medial superior olive (LSO, MSO), as well as the lateral and medial nuclei of the trapezoid body (LNTB, MNTB), have been traditionally associated with sound localization. Here we review the results of recent studies on the main SOC nuclei in echolocating bats. These studies suggest that some SOC structures and functions are highly conserved across mammals (e.g., the LSO, which is associated with interaural intensity difference processing), while others are phylogenetically highly variable in both form and function (e.g., the MSO, traditionally associated with interaural time difference processing). For the MSO, these variations indicate that we should broaden our view regarding what functions the MSO might participate in, since its function in echolocation seems to lie in the context of pattern recognition rather than sound localization. Furthermore, across bat species, variations in the form and physiology of the MSO can be linked to specific behavioral adaptations associated with different echolocation strategies. Finally, the comparative approach, including auditory specialists such as bats, helps us to reach a more comprehensive view of the functional anatomy of auditory structures that are still poorly understood, like the nucleus of the central acoustic tract (NCAT).
Animals↗
Effect of bilateral auditory cortex lesions on absolute thresholds in Japanese macaques.
1. The behavioral audiograms of four Japanese macaques (Macaca fuscata) were assessed before and after receiving two-stage bilateral lesions of auditory cortex. Thresholds were assessed for each ear with the use of insertion earphones. 2. The bilateral lesions resulted in a large initial hearing loss followed by partial recovery that left the animals with a permanent hearing loss in both ears. 3. The initial hearing loss consisted of a total insensitivity to sound in the ear contralateral to the second lesion with limited hearing in the other ear. However, the animal with the most complete lesion was initially unable to hear sound in either ear. Broadband noise was often more effective in eliciting a behavioral response than tones. 4. Partial recovery occurred in all animals and was observed as early as the first week after surgery. Most of this recovery occurred during the first 3-7 wk after surgery. This rapid phase of recovery was sometimes followed by a more gradual phase although thresholds were still elevated after 94 wk. 5. The permanent hearing loss, which averaged from 30 to 44 dB, was not constant across frequency. Threshold shifts were smallest at 63 Hz and progressively increased with frequency to a maximum loss from 8 to 25 kHz with slightly less loss at 32 kHz. 6. Analysis of the psychophysical functions and threshold stability gave no indication of any nonsensory deficits in attention or vigilance. 7. These results, taken with those of previous experiments, indicate that each hemisphere is primarily involved in the detection of sound in the contralateral ear and secondarily involved in detection in the ipsilateral ear. This arrangement differs from that seen in sound localization where each hemisphere is involved with the contralateral hemifield as opposed to the contralateral ear. Thus it appears that the functional organization of auditory cortex for sound localization is different from that for the detection and identification of sound itself.
Animals↗
Influence of head position on the spatial representation of acoustic targets.
Sound localization in humans relies on binaural differences (azimuth cues) and monaural spectral shape information (elevation cues) and is therefore the result of a neural computational process. Despite the fact that these acoustic cues are referenced with respect to the head, accurate eye movements can be generated to sounds in complete darkness. This ability necessitates the use of eye position information. So far, however, sound localization has been investigated mainly with a fixed head position, usually straight ahead. Yet the auditory system may rely on head motor information to maintain a stable and spatially accurate representation of acoustic targets in the presence of head movements. We therefore studied the influence of changes in eye-head position on auditory-guided orienting behavior of human subjects. In the first experiment, we used a visual-auditory double-step paradigm. Subjects made saccadic gaze shifts in total darkness toward brief broadband sounds presented before an intervening eye-head movement that was evoked by an earlier visual target. The data show that the preceding displacements of both eye and head are fully accounted for, resulting in spatially accurate responses. This suggests that auditory target information may be transformed into a spatial (or body-centered) frame of reference. To further investigate this possibility, we exploited the unique property of the auditory system that sound elevation is extracted independently from pinna-related spectral cues. In the absence of such cues, accurate elevation detection is not possible, even when head movements are made. This is shown in a second experiment where pure tones were localized at a fixed elevation that depended on the tone frequency rather than on the actual target elevation, both under head-fixed and -free conditions. To test, in a third experiment, whether the perceived elevation of tones relies on a head- or space-fixed target representation, eye movements were elicited toward pure tones while subjects kept their head in different vertical positions. It appeared that each tone was localized at a fixed, frequency-dependent elevation in space that shifted to a limited extent with changes in head elevation. Hence information about head position is used under static conditions too. Interestingly, the influence of head position also depended on the tone frequency. Thus tone-evoked ocular saccades typically showed a partial compensation for changes in static head position, whereas noise-evoked eye-head saccades fully compensated for intervening changes in eye-head position. We propose that the auditory localization system combines the acoustic input with head-position information to encode targets in a spatial (or body-centered) frame of reference. In this way, accurate orienting responses may be programmed despite intervening eye-head movements. A conceptual model, based on the tonotopic organization of the auditory system, is presented that may account for our findings.
Acoustic Stimulation↗
Neural organization and responses to complex stimuli in the dorsal cochlear nucleus.
The dorsal division of the cochlear nucleus (DCN) is the most complex of its subdivisions in terms of both anatomical organization and physiological response types. Hypotheses about the functional role of the DCN in hearing are as yet primitive, in part because the organizational complexity of the DCN has made development of a comprehensive and predictive model of its input-output processing difficult. The responses of DCN cells to complex stimuli, especially filtered noise, are interesting because they demonstrate properties that cannot be predicted, without further assumptions, from responses to narrow band stimuli, such as tones. In this paper, we discuss the functional organization of the DCN, i.e. the morphological organization of synaptic connections within the nucleus and the nature of synaptic interactions between its cells. We then discuss the responses of DCN principal cells to filtered noise stimuli that model the spectral sound localization cues produced by the pinna. These data imply that the DCN plays a role in interpreting sound localization cues; supporting evidence for such a role is discussed.
Acoustic Stimulation↗
Gain adjustment of inhibitory synapses in the auditory system.
A group of central auditory neurons residing in the lateral superior olivary nucleus (LSO) responds selectively to interaural level differences and may contribute to sound localization. In this simple circuit, ipsilateral sound increases firing of LSO neurons, whereas contralateral sound inhibits the firing rate via activation of the medial nucleus of the trapezoid body (MNTB). During development, individual MNTB fibers arborize within the LSO, but they undergo a restriction of their boutons that ultimately leads to mature topography. A critical issue is whether a distinct form of inhibitory synaptic plasticity contributes to MNTB synapse elimination within LSO. Whole-cell recording from LSO neurons in brain slices from developing gerbils show robust long-term depression (LTD) of the MNTB-evoked IPSP/Cs when the MNTB was activated at a low frequency (1 Hz). These inhibitory synapses also display mixed GABA/glycinergic transmission during development, as assessed physiologically and immunohistochemically (Kotak et al. 1998). While either glycine or GABA(A) receptors could independently display inhibitory LTD, focal delivery of GABA, but not glycine, at the postsynaptic-locus induces depression. Furthermore, the GABA(B) receptor antagonist, SCH-50911, prevents GABA or synaptically induced depression. Preliminary evidence also indicated strengthening of inhibitory transmission (LTP) by a distinct pattern of inhibitory activity. These data support the idea that GABA is crucial for the expression inhibitory LTD and that this plasticity may underlie the early refinement of inhibitory synaptic connections in the LSO.
Acoustic Stimulation↗