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PubMed1984

Binaural interactions of single neurons in posterior field of cat auditory cortex.

In the auditory cortex of barbiturate-anesthetized cats, the posterior auditory field (area P) was identified by its tonotopic organization, and single neurons in that field were studied quantitatively with regard to their binaural interactions at their respective best frequencies, using calibrated, sealed stimulating systems. Almost 60% of the neurons studied displayed " summative " binaural interactions in that their responses to binaural, equally intense stimulation of the two ears were stronger than were their responses to monaural stimuli of the same intensity. For these neurons, latent periods were shorter for binaural stimuli than for monaural stimuli. Some field P neurons were sensitive to interaural intensity disparities and manifested that sensitivity in one of two forms. Cells that were excited by stimulation of one ear and inhibited by stimulation of the other typically displayed a sigmoidal relation of spike count to intensive disparity, with spike counts being larger when the disparity favored the contralateral ear. Cells that were unresponsive to monaural stimuli but responded securely to binaural stimuli usually displayed a peaked, nonmonotonic relation of spike count to interaural intensity disparity, with maximal responses being elicited by stimuli with zero or near-zero disparity. Some neurons of low best frequency were sensitive to variations in interaural phase delay. In all cases, this sensitivity was manifested as a cyclical relation of spike count to interaural delay, with the period of the cycle being that of the stimulating tone. The fact that the binaural interactions of field P neurons were similar to those of cells in the primary auditory cortex suggests that the previously described heightened spectral-amplitude selectivity of field P neurons has been achieved without cost to their sensitivity to a variety of parameters of binaural stimulation. The particular sensitivity of cortical neurons to variations in interaural disparities associated with midline or near-midline azimuths might constitute a neural mechanism for the behavioral finding that animals and humans show their greatest acuity in sound localization for stimulus locations in or near the midsagittal plane.

Animals↗
PubMed2004

A biologically motivated neural network for phase extraction from complex sounds.

We demonstrate that natural acoustic signals like speech or music contain synchronous phase information across multiple frequency bands and show how to extract this information using a spiking neural network. This network model is motivated by common neurophysiological findings in the auditory brainstem and midbrain of several species. A computer simulation of the model was tested by applying spoken vowels and organ pipe tones. As expected, spikes occurred synchronously in the activated frequency bands. This phase information may be used for sound separation with one microphone or sound localization with two microphones.

Acoustic Stimulation↗
PubMed2005

Contralateral effects and binaural interactions in dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) receives afferent input from the auditory nerve and is thus usually thought of as a monaural nucleus, but it also receives inputs from the contralateral cochlear nucleus as well as descending projections from binaural nuclei. Evidence suggests that some of these commissural and efferent projections are excitatory, whereas others are inhibitory. The goals of this study were to investigate the nature and effects of these inputs in the DCN by measuring DCN principal cell (type IV unit) responses to a variety of contralateral monaural and binaural stimuli. As expected, the results of contralateral stimulation demonstrate a mixture of excitatory and inhibitory influences, although inhibitory effects predominate. Most type IV units are weakly, if at all, inhibited by tones but are strongly inhibited by broadband noise (BBN). The inhibition evoked by BBN is also low threshold and short latency. This inhibition is abolished and excitation is revealed when strychnine, a glycine-receptor antagonist, is applied to the DCN; application of bicuculline, a GABAA-receptor antagonist, has similar effects but does not block the onset of inhibition. Manipulations of discrete fiber bundles suggest that the inhibitory, but not excitatory, inputs to DCN principal cells enter the DCN via its output pathway, and that the short latency inhibition is carried by commissural axons. Consistent with their respective monaural effects, responses to binaural tones as a function of interaural level difference are essentially the same as responses to ipsilateral tones, whereas binaural BBN responses decrease with increasing contralateral level. In comparison to monaural responses, binaural responses to virtual space stimuli show enhanced sensitivity to the elevation of a sound source in ipsilateral space but reduced sensitivity in contralateral space. These results show that the contralateral inputs to the DCN are functionally relevant in natural listening conditions, and that one role of these inputs is to enhance DCN processing of spectral sound localization cues produced by the pinna.

Acoustic Stimulation↗
PubMed1991

Auditory cortex: comparative aspects of maps and plasticity.

Much recent work in the field of auditory cortex analysis consists of an intensified search for complex sound representation and sound localization mechanisms using tonotopic maps as a frame of reference. Mammalian species rely on parallel processing in multiple tonotopic and non-tonotopic maps but show different degrees of unit complexity, and orderly representation of acoustic dimensions in such maps depending on the predictability of sounds in their environment. Birds appear to rely chiefly on one tonotopic map which harbours multidimensional complex representations. During development and after partial hearing loss, tonotopic organization changes in a predictable manner. Learning also modifies the spatial representation of sounds and even modifies tonotopic organization, but the spatial rules involved in this process have not yet emerged.

Animals↗
PubMed1999

Neural bases of an auditory illusion and its elimination in owls.

Humans and owls localize sounds by detecting the arrival time disparity between the ears. Both species determine the interaural time difference by finding the delay necessary to match the leading signal with the lagging one. This method produces ambiguity with periodic signals, because the two signals can be matched by delaying either one or the other. As predicted, owls localized periodic signals in illusory directions, whereas they always perceived the real source when signal bandwidth exceeded a certain value. This bandwidth also enabled higher-order auditory neurons to discriminate between real and illusory sources.

Acoustic Stimulation↗
PubMed2000

A performance adequate computational model for auditory localization.

A computational model of auditory localization resulting in performance similar to humans is reported. The model incorporates both the monaural and binaural cues available to a human for sound localization. Essential elements used in the simulation of the processes of auditory cue generation and encoding by the nervous system include measured head-related transfer functions (HRTFs), minimum audible field (MAF), and the Patterson-Holdsworth cochlear model. A two-layer feed-forward back-propagation artificial neural network (ANN) was trained to transform the localization cues to a two-dimensional map that gives the direction of the sound source. The model results were compared with (i) the localization performance of the human listener who provided the HRTFs for the model and (ii) the localization performance of a group of 19 other human listeners. The localization accuracy and front-back confusion error rates exhibited by the model were similar to both the single listener and the group results. This suggests that the simulation of the cue generation and extraction processes as well as the model parameters were reasonable approximations to the overall biological processes. The amplitude resolution of the monaural spectral cues was varied and the influence on the model's performance was determined. The model with 128 cochlear channels required an amplitude resolution of approximately 20 discrete levels for encoding the spectral cue to deliver similar localization performance to the group of human listeners.

Humans↗
PubMedDate not supplied

Contribution of click frequency bands to the human binaural interaction components.

The purpose of this study was to determine the contribution of click frequency bands (broad-band, >2000 Hz, <2000 Hz and <1000 Hz) to binaural interaction components (BICs) of the human auditory brainstem evoked potentials (ABEPs). The human BICs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to either ear. Effective frequency bands were derived using clicks alone or clicks with ipsilateral or binaural masking noise, high- or low-pass filtered at different cut-off frequencies. Analysis included single-channel vertex-cervical spinous process VII derivation of BIC and ABEP, as well as estimating the single, centrally located dipole equivalent of the surface activity from three orthogonally positioned electrode pairs, using the three-channel Lissajous' trajectory (3-CLT) analysis. All BIC 3-CLTs included three major components (labeled BdII, BeI, and BeII) approximately corresponding in latency to IIIn, V and VI ABEP peaks. All apex latencies of BIC 3-CLT, except BeI, were longer in response to <2000 Hz and <1000 Hz (low-frequency) effective clicks. Apex amplitude of components BeI and BeII of BIC 3-CLT were smaller with low-frequency effective clicks than with broad-band or high-frequency (>2000 Hz) clicks. We suggest that binaural interaction component BeI is mainly tuned to high frequencies, showing no frequency effect on latency, and decreasing in amplitude with decreasing click high frequency content. In contrast, BdII and BeII of the human BICs are evoked more synchronously by high-frequency binaural inputs, but are also sensitive to low frequencies, increasing in latency according to the cochleotopic activation pattern. These differences between BIC components may reflect their roles in sound localization.

Acoustic Stimulation↗
PubMed2001

Proprioceptive information from the pinna provides somatosensory input to cat dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) is a second-order auditory structure that also receives nonauditory information, including somatosensory inputs from the dorsal column and spinal trigeminal nuclei. Here we investigate the peripheral sources of the somatosensory inputs to DCN. Electrical stimulation was applied to cervical nerves C1-C8, branches of C2, branches of the trigeminal nerve, and hindlimb nerves. The largest evoked potentials in the DCN were produced by C2 stimulation and by stimulation of its branches that innervate the pinna. Electrical stimulation of C2 produced a pattern of inhibition and excitation of DCN principal cells comparable with that seen in previous studies with stimulation of the primary somatosensory nuclei, suggesting that the same pathway was activated. Because C2 contains both proprioceptive and cutaneous fibers, we applied peripheral somatosensory stimulation to identify the effective somatosensory modalities. Only stimuli that activate pinna muscle receptors, such as stretch or vibration of the muscles connected to the pinna, were effective in driving DCN units, whereas cutaneous stimuli such as light touch, brushing of hairs, and stretching of skin were ineffective. These results suggest that the largest somatosensory inputs to the DCN originate from muscle receptors associated with the pinna. They support the hypothesis that a role of the DCN in hearing is to coordinate pinna orientation to sounds or to support correction for the effects of pinna orientation on sound-localization cues.

Acoustic Stimulation↗
PubMed2004

Inhibitory control at a synaptic relay.

The mammalian medial nucleus of the trapezoid body (MNTB) harbors one of the most powerful terminals in the CNS, the calyx of Held. The mechanisms known to regulate this synaptic relay are relatively ineffective. Here, we report the presence of a remarkably robust and fast-acting glycinergic inhibitory system capable of suppressing calyceal transmission. Evoked glycinergic IPSCs were relatively small in 2-week-old rats, an age by which calyceal maturation has reportedly neared completion. However, by postnatal day 25 (P25), glycinergic transmission had undergone a vigorous transformation, resulting in peak synaptic conductances as high as 280 nS. These are comparable with glutamatergic conductances activated by calyceal inputs. Decay kinetics for IPSCs were severalfold faster than for glycinergic synaptic events reported previously. At physiological temperatures in P25 rats, IPSCs decayed in approximately 1 msec and could be elicited at frequencies up to 500 Hz. Moreover, EPSPs triggered by glutamatergic signals derived from the calyx or simulated by conductance clamp were suppressed when preceded by simulated glycinergic IPSPs. The matching of excitatory transmission in the calyx of Held by a powerful, precision inhibitory system suggests that the relay function of the MNTB may be rapidly modified during sound localization.

Animals↗
PubMed2002

Auditory brainstem responses and usefulness of hearing aids in hearing impaired children with Cornelia de Lange syndrome.

OBJECTIVE: To examine auditory brainstem evoked responses (ABRs) of children with Cornelia de Lange syndrome (CDLS) to evaluate hearing and the utility of hearing aids in hearing impaired cases. SUBJECTS AND METHODS: Thirteen Japanese infants and children with CDLS were studied. Behavioral observation audiometry and ABR were used to evaluate hearing. RESULT: Four different ABR patterns at 85 dB clicks were observed: no response in either ear (6 patients); clear ABRs in both ears (2 patients); no response in one ear but ABRs recorded in the other ear (3 patients); and no peaks after wave III in one ear and ABRs recorded in the other ear (2 patients). However, in 2 patients with no response in either ear at the first measurement, ABRs were recorded in one ear within 2 years. Three out of 13 patients exhibited better responses to sound through the use of hearing aids and auditory training. CONCLUSIONS: The fitting of hearing aids and early consistent training have a significant effect on auditory development in CDLS children in terms of making them aware of sound localization and the different types of environmental sound.

Audiometry↗
PubMed2005

Hierarchical processing of sound location and motion in the human brainstem and planum temporale.

Horizontal sound localization relies on the extraction of binaural acoustic cues by integration of the signals from the two ears at the level of the brainstem. The present experiment was aimed at detecting the sites of binaural integration in the human brainstem using functional magnetic resonance imaging and a binaural difference paradigm, in which the responses to binaural sounds were compared with the sum of the responses to the corresponding monaural sounds. The experiment also included a moving sound condition, which was contrasted against a spectrally and energetically matched stationary sound condition to assess which of the structures that are involved in general binaural processing are specifically specialized in motion processing. The binaural difference contrast revealed a substantial binaural response suppression in the inferior colliculus in the midbrain, the medial geniculate body in the thalamus and the primary auditory cortex. The effect appears to reflect an actual reduction of the underlying activity, probably brought about by binaural inhibition or refractoriness at the level of the superior olivary complex. Whereas all structures up to and including the primary auditory cortex were activated as strongly by the stationary as by the moving sounds, non-primary auditory fields in the planum temporale responded selectively to the moving sounds. These results suggest a hierarchical organization of auditory spatial processing in which the general analysis of binaural information begins as early as the brainstem, while the representation of dynamic binaural cues relies on non-primary auditory fields in the planum temporale.

Acoustic Stimulation↗
PubMed2003

Derivation of an optimal directivity pattern for sweet spot widening in stereo sound reproduction.

In this paper the correction of the degradation of the stereophonic illusion during sound reproduction due to off-center listening is investigated. The main idea is that the directivity pattern of a loudspeaker array should have a well-defined shape such that a good stereo reproduction is achieved in a large listening area. Therefore, a mathematical description to derive an optimal directivity pattern l(opt) that achieves sweet spot widening in a large listening area for stereophonic sound applications is described. This optimal directivity pattern is based on parametrized time/intensity trading data coming from psycho-acoustic experiments within a wide listening area. After the study, the required digital FIR filters are determined by means of a least-squares optimization method for a given stereo base setup (two pair of drivers for the loudspeaker arrays and 2.5-m distance between loudspeakers), which radiate sound in a broad range of listening positions in accordance with the derived l(opt). Informal listening tests have shown that the l(opt) worked as predicted by the theoretical simulations. They also demonstrated the correct central sound localization for speech and music for a number of listening positions. This application is referred to as "Position-Independent (PI) stereo."

Acoustics↗
PubMed1990

Direction-dependent spectral properties of cat external ear: new data and cross-species comparisons.

Free-field to eardrum transfer functions were measured in anesthetized cats inside an anechoic chamber. Direction-dependent transformations were determined by measurement of sound-pressure levels using a small probe tube microphone surgically implanted in a ventral position near the tympanic membrane. Loudspeaker and probe microphone characteristics were eliminated by subtraction of the signal recorded in the free field with no animal present. Complexities of the transfer function, which include the presence of prominent spectral notches in the 8- to 18-kHz frequency region, are due primarily to the acoustical properties of the pinna. Differential amplification of frequency components within the broadband stimulus occurs as a function of source direction. Spectral features vary systematically with changes in both elevation (EL) and azimuth (AZ). The contrast between a notch and its shoulders is enhanced in the interaural spectral records. Spectral data from single source locations and spatial data for single frequencies at many locations are presented and comparisons with other species are drawn. It is suggested that spectral features in the 8- to 18-kHz region provide some of the necessary spectral information for sound localization and that the contrast in spectral energy between the frequencies at the notch and its shoulders is a potential directional cue.

Acoustic Stimulation↗
PubMed2006

Comparison of midbrain and thalamic space-specific neurons in barn owls.

Spatial receptive fields of neurons in the auditory pathway of the barn owl result from the sensitivity to combinations of interaural time (ITD) and level differences across stimulus frequency. Both the forebrain and tectum of the owl contain such neurons. The neural pathways, which lead to the forebrain and tectal representations of auditory space, separate before the midbrain map of auditory space is synthesized. The first nuclei that belong exclusively to either the forebrain or the tectal pathways are the nucleus ovoidalis (Ov) and the external nucleus of the inferior colliculus (ICx), respectively. Both receive projections from the lateral shell subdivision of the inferior colliculus but are not interconnected. Previous studies indicate that the owl's tectal representation of auditory space is different from those found in the owl's forebrain and the mammalian brain. We addressed the question of whether the computation of spatial cues in both pathways is the same by comparing the ITD tuning of Ov and ICx neurons. Unlike in ICx, the relationship between frequency and ITD tuning had not been studied in single Ov units. In contrast to the conspicuous frequency independent ITD tuning of space-specific neurons of ICx, ITD selectivity varied with frequency in Ov. We also observed that the spatially tuned neurons of Ov respond to lower frequencies and are more broadly tuned to ITD than in ICx. Thus there are differences in the integration of frequency and ITD in the two sound-localization pathways. Thalamic neurons integrate spatial information not only within a broader frequency band but also across ITD channels.

Acoustic Stimulation↗
PubMed2003

[Dependence of the mouse inferior colliculus neurons activity on position and direction of movement of the spectral contrast].

Response variability of the single neurons of the inferior colliculus of mouse (Mus musculus) to series of noise bands and of notch noises with regular 1/12 octave steps of the band/notch center frequency and width of noise band/notch 1/3 octave, was studied. Neurons with strong inhibitory influence in excitatory response area (inhibitory-dominated) show low impulse activity when noise band exceeded excitatory response area. Spectral contrasts crossing the center of excitatory response area (at CF or nearly CF) were found to be the most efficient stimuli for such neurons. Neuron responses to spectral contrasts derived both from noise band and noise notch were identical. Approaching of inhibitory and excitatory inputs is expected to sharpen the auditory neurons frequency tuning to position of spectral contrasts, similar to neuronal processing in visual system. Neuron selectivity to the direction of spectral contrasts movement was determined in neuron response differences when the noise band or notch shifted from excitatory area to inhibitory areas as compared with shift in the opposite direction. Functional role of contrast mechanism for sound localization on the base spectral cues related to external ear transfer characteristics is discussed.

Acoustic Stimulation↗
PubMed2005

Reduced order modeling of head related impulse responses for virtual acoustic displays.

This study investigated the use of reduced order head related impulse response (HRIR) models to improve the computational efficiency in acoustic virtual displays. State space models of varying order were generated from zero-elevation HRIRs using a singular value decomposition technique. A source identification experiment was conducted under anechoic conditions in which three subjects were required to localize sounds in the front horizontal plane. The sounds were either (1) real sources (emitted by individual loudspeakers in a semi-circular array), (2) virtual sources generated from the original HRIRs, or (3) virtual sources generated using reduced order state space models. All virtual sources were created by simultaneous activation of two loudspeakers at +/- 30 degrees using a virtual source imaging technique based on either the measured or modeled HRIRs. The errors in the perceived direction of the virtual sources generated from the reduced order models were compared to errors in localization using the original HRIRs. The results demonstrate that a very significant reduction in model size can be achieved without significantly affecting the fidelity of the virtual display of horizontally placed sources.

Acoustics↗
PubMed2003

Acoustic response properties of lagenar nerve fibers in the sleeper goby, Dormitator latifrons.

Auditory and vestibular functions of otolithic organs vary among vertebrate taxa. The saccule has been considered a major hearing organ in many fishes. However, little is known about the auditory role of the lagena in fishes. In this study we analyzed directional and frequency responses from single lagenar fibers of Dormitator latifrons to linear accelerations that simulate underwater acoustic particle motion. Characteristic frequencies of the lagenar fibers fell into two groups: </=50 Hz and 80-125 Hz. We observed various temporal response patterns: strong phase-locking, double phase-locking, phase-locked bursting, and non-phase-locked bursting. Some bursting responses have not been previously observed in vertebrate otolithic nerve fibers. Lagenar fibers could respond to accelerations as small as 1.1 mm s(-2). Like saccular fibers, lagenar fibers were directionally responsive and decreased directional selectivity with stimulus level. Best response axes of the lagenar fibers clustered around the lagenar longitudinal axis in the horizontal plane, but distributed in a diversity of axes in the mid-sagittal plane, which generally reflect morphological polarizations of hair cells in the lagena. We conclude that the lagena of D. latifrons plays a role in sound localization in elevation, particularly at high stimulus intensities where responses of most saccular fibers are saturated.

Acoustic Stimulation↗
PubMed2001

Direct comparison of the impact of head tracking, reverberation, and individualized head-related transfer functions on the spatial perception of a virtual speech source.

A study of sound localization performance was conducted using headphone-delivered virtual speech stimuli, rendered via HRTF-based acoustic auralization software and hardware, and blocked-meatus HRTF measurements. The independent variables were chosen to evaluate commonly held assumptions in the literature regarding improved localization: inclusion of head tracking, individualized HRTFs, and early and diffuse reflections. Significant effects were found for azimuth and elevation error, reversal rates, and externalization.

Acoustics↗