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PubMed2005

Distribution within the barn owl's inferior colliculus of neurons projecting to the optic tectum and thalamus.

Behavioral studies in barn owls indicate that both the optic tectum (OT) and the auditory arcopallium (AAr) mediate sound localization through the presence of neurons that respond only when sound comes from a circumscribed direction in space. The early stages of the computations leading to these so-called space-specific neurons are shared in a common brainstem pathway, which then splits at the level of the inferior colliculus (IC) such that the last computational stage is thought to be duplicated. The study presented here addresses whether the space-specific neurons in OT and AAr are indeed partially independent of each other by using anatomical methods more precise than those used in previous studies. Specifically, projection neurons in IC were retrogradely labelled with injections of fluorescein- and rhodamine-conjugated dextran amines into OT and nucleus ovoidalis (OV), the thalamic nucleus leading to AAr. By labelling the OT-projecting and OV-projecting neurons in the same owl, it was confirmed that neurons in IC project to either OV or OT but not both. However, although a segregation was generally observed between the medially positioned OV-projecting neurons and the laterally positioned OT-projecting neurons, there was also a slight overlap between the two populations. Moreover, electrolytic lesions demarcating physiological tuning properties indicate that many OV-projecting neurons are within the area containing space-specific neurons. These results highlight the need for more detailed studies elucidating the microcircuitry and corresponding physiology of IC, such as have been done in the cortices of the mammalian cerebellum and cerebrum.

Animals↗
PubMed2005

Characteristics of auditory agnosia in a child with severe traumatic brain injury: a case report.

We present a case that is unusual in many respects from other documented incidences of auditory agnosia, including the mechanism of injury, age of the individual, and location of neurological insult. The clinical presentation is one of disturbance in the perception of spoken language, music, pitch, emotional prosody, and temporal auditory processing in the absence of significant deficits in the comprehension of written language, expressive language production, or peripheral auditory function. Furthermore, the patient demonstrates relatively preserved function in other aspects of audition such as sound localization, voice recognition, and perception of animal noises and environmental sounds. This case study demonstrates that auditory agnosia is possible following traumatic brain injury in a child, and illustrates the necessity of assessment with a wide variety of auditory stimuli to fully characterize auditory agnosia in a single individual.

Agnosia↗
PubMed2000

Abnormal auditory experience induces frequency-specific adjustments in unit tuning for binaural localization cues in the optic tectum of juvenile owls.

Early auditory experience shapes the auditory spatial tuning of neurons in the barn owl's optic tectum in a frequency-dependent manner. We examined the basis for this adaptive plasticity in terms of changes in tuning for frequency-specific interaural time differences (ITDs) and level differences (ILDs), the dominant sound localization cues. We characterized broadband and narrowband ITD and ILD tuning in normal owls and in owls raised with an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. In normal owls, units were tuned to frequency-specific ITD and ILD values that matched those produced by sound sources located in their visual receptive fields. In contrast, in device-reared owls, ITD tuning at most sites was shifted from normal by approximately 55 microsec toward open-ear leading for 4 kHz stimuli and 15 microsec toward the opposite-ear leading for 8 kHz stimuli, reflecting the acoustic effects of the device. ILD tuning was shifted in the adaptive direction by approximately 3 dB for 4 kHz stimuli and 8 dB for 8 kHz stimuli, but these shifts were substantially smaller than expected based on the acoustic effects of the device. Most sites also exhibited conspicuously abnormal frequency-response functions, including a strong dependence on stimulus ITD and a reduction of normally robust responses to 6 kHz stimuli. The results demonstrate that the response properties of high-order auditory neurons in the optic tectum are adjusted during development to reflect the influence of frequency-specific features of the binaural localization cues experienced by the individual.

Acoustic Stimulation↗
PubMed1979

Neurological development of kittens.

The neurological development of the kitten was studied from birth to 120 days of age. Three motor features present at birth disappeared within the 1st 45 days of life. The labyrinthine head and body righting reactions were present at birth; the latter matured only by 25 days of age when the air righting reaction (mature by 35 days) started to appear. Limb placing reactions developed progressively with proprioceptive components being present at birth whereas tactile components evolved slowly from early contact lifting to forepaws contact placing (60 days) and narrow plank walking (75 days). Standing and walking were well developed by 45 days. Eye opening occurred at 9.5 days and several eyeblink reactions (including blink to light) were present at birth. Adequate binocular coordination was seen by 47 days. Vision progressed parallel to the clearing of the ocular media which were fairly transparent by 32 days. Visually guided paw placing and the visuopalpebral blink reflex matured later (by 37 and 59 days, respectively). The external auditory canal and pinna were fully developed by 12 and 31 days, respectively; spontaneous and tactile pinna movements were present at birth; orienting to animal and nonanimal sounds was well developed by 6.5 and 18 days, respectively; spatial sound localization was developed by 16 days and differential responding to animal sounds matured by 24.5 days. Somatic responses and olfaction were present at birth but matured further thereafter. Playful interactions between kittens started by 10--15 days and well developed play behavior was seen by 36--40 days. In brief, all neurological functions mature progressively during the 1st 3 postnatal months.

Animals↗
PubMed2001

Adaptive adjustment of connectivity in the inferior colliculus revealed by focal pharmacological inactivation.

In the midbrain sound localization pathway of the barn owl, a map of auditory space is synthesized in the external nucleus of the inferior colliculus (ICX) and transmitted to the optic tectum. Early auditory experience shapes these maps of auditory space in part by modifying the tuning of the constituent neurons for interaural time difference (ITD), a primary cue for sound-source azimuth. Here we show that these adaptive modifications in ITD tuning correspond to changes in the pattern of connectivity within the inferior colliculus. We raised owls with an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. As reported previously, device rearing shifted the representation of ITD in the ICX and tectum but not in the primary source of input to the ICX, the central nucleus of the inferior colliculus (ICC). We applied the local anesthetic lidocaine (QX-314) iontophoretically in the ICC to inactivate small populations of neurons that represented particular values of frequency and ITD. We measured the effect of this inactivation in the optic tecta of a normal owl and owls raised with the device. In the normal owl, inactivation at a critical site in the ICC eliminated responses in the tectum to the frequency-specific ITD value represented at the site of inactivation in the ICC. The location of this site was consistent with the known pattern of ICC-ICX-tectum connectivity. In the device-reared owls, adaptive changes in the representation of ITD in the tectum corresponded to dramatic and predictable changes in the locations of the critical sites of inactivation in the ICC. Given that the abnormal representation of ITD in the tectum depended on frequency and was likely conveyed directly from the ICX, these results suggest that experience causes large-scale, frequency-specific adjustments in the pattern of connectivity between the ICC and the ICX.

Adaptation, Physiological↗
PubMed2000

Localization of multiple sound sources with two microphones.

This paper presents a two-microphone technique for localization of multiple sound sources. Its fundamental structure is adopted from a binaural signal-processing scheme employed in biological systems for the localization of sources using interaural time differences (ITD). The two input signals are transformed to the frequency domain and analyzed for coincidences along left/right-channel delay-line pairs. The coincidence information is enhanced by a nonlinear operation followed by a temporal integration. The azimuths of the sound sources are estimated by integrating the coincidence locations across the broadband of frequencies in speech signals (the "direct" method). Further improvement is achieved by using a novel "stencil" filter pattern recognition procedure. This includes coincidences due to phase delays of greater than 2pi, which are generally regarded as ambiguous information. It is demonstrated that the stencil method can greatly enhance localization of lateral sources over the direct method. Also discussed and analyzed are two limitations involved in both methods, namely missed and artifactual sound sources. Anechoic chamber tests as well as computer simulation experiments showed that the signal-processing system generally worked well in detecting the spatial azimuths of four or six simultaneously competing sound sources.

Adult↗
PubMed2001

Distinct pathways involved in sound recognition and localization: a human fMRI study.

Evidence from psychophysical studies in normal and brain-damaged subjects suggests that auditory information relevant to recognition and localization are processed by distinct neuronal populations. We report here on anatomical segregation of these populations. Brain activation associated with performance in sound identification and localization was investigated in 18 normal subjects using fMRI. Three conditions were used: (i) comparison of spatial stimuli simulated with interaural time differences; (ii) identification of environmental sounds; and (iii) rest. Conditions (i) and (ii) required acknowledgment of predefined targets by pressing a button. After coregistering, images were normalized and smoothed. Activation patterns were analyzed using SPM99 for individual subjects and for the whole group. Sound recognition and localization activated, as compared to rest, inferior colliculus, medial geniculate body, Heschl gyrus, and parts of the temporal, parietal, and frontal convexity bilaterally. The activation pattern on the fronto-temporo-parietal convexity differed in the two conditions. Middle temporal gyrus and precuneus bilaterally and the posterior part of left inferior frontal gyrus were more activated by recognition than by localization. Lower part of inferior parietal lobule and posterior parts of middle and inferior frontal gyri were more activated, bilaterally, by localization than by recognition. Regions selectively activated by sound recognition, but not those selectively activated by localization, were significantly larger in women. Passive listening paradigm revealed segregated pathways on superior temporal gyrus and inferior parietal lobule. Thus, anatomically distinct networks are involved in sound recognition and sound localization.

Adult↗
PubMed2002

Detection of large interaural delays and its implication for models of binaural interaction.

The interaural time difference (ITD) is a major cue to sound localization along the horizontal plane. The maximum natural ITD occurs when a sound source is positioned opposite to one ear. We examined the ability of owls and humans to detect large ITDs in sounds presented through headphones. Stimuli consisted of either broad or narrow bands of Gaussian noise, 100 ms in duration. Using headphones allowed presentation of ITDs that are greater than the maximum natural ITD. Owls were able to discriminate a sound leading to the left ear from one leading to the right ear, for ITDs that are 5 times the maximum natural delay. Neural recordings from optic-tectum neurons, however, show that best ITDs are usually well within the natural range and are never as large as ITDs that are behaviorally discriminable. A model of binaural crosscorrelation with short delay lines is shown to explain behavioral detection of large ITDs. The model uses curved trajectories of a cross-correlation pattern as the basis for detection. These trajectories represent side peaks of neural ITD-tuning curves and successfully predict localization reversals by both owls and human subjects.

Animals↗
PubMed1999

Communicative ability in an audiological perspective. Theory and application to post-secondary school students.

The underlying assumption in the present study is that the individual's speech and hearing communicative ability is composed of three components, each corresponding to different functional systems of the brain: afferent functions (A) represent the auditory activity and sound perception largely corresponding to activity in the ascending auditory pathways. The central functions (C) include cortical auditory and language abilities controlled in parts of the left temporal lobe and subcortical centres. The efferent functions (E) consist of speech motor processes and articulation. A test battery of 20 tests measuring several aspects of afferent, central and efferent functions was applied to 11 hearing-impaired post-secondary school students and several control groups. All data were normalized with the normal materials as references. Individual communicative profiles were obtained from these primary data, which consisted of audiometric tests (tone and speech audiometry, impedance tests, brainstem response audiometry and phase audiometry), sound environmental tests with hearing aids (directional speech-in-noise, word localization, sound environment identification test), and language tests (reading tests, prosody, auditory memory and recall, phonology and articulation). Since the central functions cannot truly and directly be determined in hearing-imparied subjects, they were assessed under optimal listening conditions. Furthermore, central functions were estimated according to three different models: distributive, parallel model (model 1), multiplicative, serial model (model 2) and compensatory model (model 3). On the basis of these models, a three-component description of the communicative ability consisting of A,C and E functions was obtained. It was found that C and E functions were largely independent of the adult afferent functions, but C functions were negatively correctly to hearing in childhood. A preliminary comparison between the tests and a comparison between the models was performed by predicting benefit of hearing aid. Model 3 gave the best prediction. Beyond the three-component A,C, and E characterization of the students, a total communicative ability score could be calculated giving values from 37% to 79% of the normal mean. On the basis of the conceptual and statistical analyses, the test battery could be reduced to include tone 0-12 years, tone adults, word localization test (afferent); word chain, lecture test (central), articulatory test (efferent) and audiovisual test. The simple algorithm of adding the normalized loss of afferent (peripheral) function to the normalized results of the acoustic central tests seems to be promising for isolation for the central auditory capacity even in cases with peripheral impairment. It is concluded that a wider perspective is desirable in the diagnostic evaluation of the hearing-impaired individual in order to understand his communicative abilities and form a cornerstone in the planning of rehabilitation in conjunction with social and psychological factors.

Adolescent↗
PubMed1995

Plasticity in human directional hearing.

Interaural time difference (ITD), the main cue for localization of low-frequency sound in azimuth, is widely thought to be evaluated according to Jeffress' model. This theory proposes that each of an array of neurons detects coinciding input from both ears, conducted along axonal delay lines, with the azimuth angle corresponding to the activation of selected neurons. Thus, sound source localization is assumed to depend on axon conduction velocities, a relatively fixed parameter. Clinical experience suggests that directional hearing is adaptable. We investigated if sound localization in azimuth could adapt plastically to altered ITDs. We equipped binaural insert hearing aids with adjustable electronic delay lines. Subjects with normal hearing were required to wear these devices during all waking hours for several days. Localization of an invisible sound source was measured in an anechoic room before and at various intervals after introduction of a constant delay in one ear between 171 and 684 mus. Test sounds were high-pass, low-pass and broad-band noises. Introduction of a delay in one ear lead to an immediate displacement of the perceived sound location towards the opposite side. Within hours of exposure, the displacement was reduced, and further normalization of the perceived localization occurred over several days. After removal of the delays sound localization normalized rapidly. We conclude that ITD alterations can lead to plastic adaptation of directional hearing, which cannot rely exclusively on fixed axon conduction velocities. Our results suggest additional mechanisms for directional hearing on the basis ITD.

Acoustic Stimulation↗
PubMed1975

[Audiological observations of a patient with auditory agnosia].

A case is reported in which a patient was become unable to identify all kinds of auditory material, including speech, despite only subnormal audiometric thresholds (i.e. "auditory agnosia"). The following audiological disorders were shown: some fragility in the perception of brief sound stimuli, difficulties to apprehending prolonged series of stimuli (rhythms), defective sound localization, an increase in the homolateral and in the contralateral effects of masking sounds. On the other hand, differential thresholds for frequency and for intensity, as well as fusion thresholds, were little affected, or unaffected. A complete dissociation appeared between the auditory evoked potentials and the audiometric thresholds; a year after the first examination, and the following years, the auditory potentials were largely reduced, or even have elapsed for one ear, while the audiometric thresholds seem to remain nearly the same.

Agnosia↗
PubMed2006

Sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis.

The directional selectivity of auditory neurons is one of the essential response properties that underlie sound localization, an important task performed by the mammalian auditory system. Here we evaluated the sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis, at the age of 15 days under free-field stimulation conditions. Compared with those in adult bats, neurons in juvenile bats were broadly tuned to sound azimuth angles as indicated by both the type and width of the azimuth selectivity curves. Their best azimuth was distributed over a wide range and, moreover, the adult-like relationship between the best azimuth and the best frequency of these neurons was not developed. These data indicate that the directional selectivity of inferior collicular neurons, like other response properties examined previously, undergoes considerable change during postnatal maturation.

Acoustic Stimulation↗
PubMed1996

Responses to simulated echoes by neurons in the barn owl's auditory space map.

The natural acoustical environment contains many reflective surfaces that give rise to echoes, complicating the task of sound localization and identification. The barn owl (Tyto alba), as a nocturnal predator, relies heavily on its auditory system for tracking and capturing prey in this highly echoic environment. The external nucleus of the owl's inferior colliculus (ICx) contains a retina-like map of space composed of "space-specific" auditory neurons that have spatially limited receptive fields. We recorded extracellularly from individual space-specific neurons in an attempt to understand the pattern of activity across the ICx in response to a brief direct sound and a simulated echo. Space-specific neurons responded strongly to the direct sound, but their response to a simulated echo was suppressed, typically, if the echo arrived within 5 ms or less of the direct sound. Thus we expect there to be little or no representation within the ICx of echoes arriving within such short delays. Behavioral tests using the owl's natural tendency to turn their head toward a sound source suggested that owls, like their space-specific neurons, similarly localize only the first of two brief sounds. Naive, untrained owls were presented with a pair of sounds in rapid succession from two horizontally-separated speakers. With interstimulus delays of less than 10 ms, the owl consistently turned its head toward the leading speaker. Longer delays elicited head turns to either speaker with approximately equal frequency and in some cases to both speakers sequentially.

Acoustic Stimulation↗
PubMed2001

Sensitivity of auditory cortical neurons to locations of signals and competing noise sources.

The present study examined cortical parallels to psychophysical signal detection and sound localization in the presence of background noise. The activity of single units or of small clusters of units was recorded in cortical area A2 of chloralose-anesthetized cats. Signals were 80-ms click trains that varied in location in the horizontal plane around the animal. Maskers were continuous broadband noises. In the focal masker condition, a single masker source was tested at various azimuths. In the diffuse masker condition, uncorrelated noise was presented from two speakers at +/-90 degrees lateral to the animal. For about 2/3 of units ("type A"), the presence of the masker generally reduced neural sensitivity to signals, and the effects of the masker depended on the relative locations of signal and masker sources. For the remaining 1/3 of units ("type B"), the masker reduced spike rates at low signal levels but often augmented spike rates at higher signal levels. Increases in spike rates of type B units were most common for signal sources in front of the ear contralateral to the recording site but tended to be independent of masker source location. For type A units, masker effects could be modeled as a shift toward higher levels of spike-rate- and spike-latency-versus-level functions. For a focal masker, the shift size decreased with increasing separation of signal and masker. That result resembled psychophysical spatial unmasking, i.e., improved signal detection by spatial separation of the signal from the noise source. For the diffuse masker condition, the shift size generally was constant across signal locations. For type A units, we examined the effects of maskers on cortical signaling of sound-source location, using an artificial-neural-network (ANN) algorithm. First, an ANN was trained to estimate the signal location in the quiet condition by recognizing the spike patterns of single units. Then we tested ANN responses for spike patterns recorded under various masker conditions. Addition of a masker generally altered spike patterns and disrupted ANN identification of signal location. That disruption was smaller, however, for signal and masker configurations in which the masker did not severely reduce units' spike rates. That result compared well with the psychophysical observation that listeners maintain good localization performance as long as signals are clearly audible.

Acoustic Stimulation↗
PubMed1982

Concerning the need for more sophisticated animal models in sensory behavioral toxicology.

It is necessary but not sufficient to develop laboratory animal models in sensory behavioral toxicology for screening toxic substances and for the analysis of sensory impairment at threshold levels of stimulation. It is important to develop more thorough and quantitative tests of impairment which in their greater complexity more accurately reflect the conditions and environmental demands of day-to-day life. Such greater complexity in stimulus conditions and behavior may also aid in monitoring not merely the state of the receptor organ but more central nervous processes which are the focus of assault by many known toxic substances. Techniques are described for studying such acoustic behaviors as intensity discrimination and frequency selectivity in guinea pig and monkey by use of operant conditioning procedures coupled with sensory testing (psychophysical) methods. Impaired auditory selectively and discrimination is shown to be correlated with histopathological changes in the inner ear. Slight modification of these procedures in animals may be used to investigate acoustically more intricate behaviors such as sound localization and the perception of frequency modulated acoustic signals as elements of speech and communication sounds.

Acoustic Stimulation↗
PubMed2002

The digital hearing aid, wearable computing, and electrophysiological response.

Presbycusis is the most common form of hearing loss caused by aging and long-term exposure to sound energy. This type of ailment decreases the ear's ability to perceive high frequencies and localize sound, thus making comprehension more difficult. To compensate for this loss, the choice of a digital hearing aid has become more common. However, most digital hearing aids do little more that their analogue predecessor's by providing a single, fixed hearing response. Such a fixed response is not suitable for all of a user's auditory environments and typically causes a more rapid loss of hearing. Significant advancement in processing power and reduction in size of computing hardware has produced increasingly more powerful, more portable, and more personal computing devices. These advances have spurred research and development of wearable computing devices towards integrating both man and machine. By definition, the digital hearing aid is a wearable computing device. The development of a digital hearing aid with increased onboard processing that is aware of its owner's electrophysiological and auditory environments is an obvious progression. This awareness will give the hearing aid the ability to autonomously modify its own parameters to improve audibility and comprehension. Electrophysiological signals can be classified as naturally occurring or voluntarily controlled. Employing these signals will allow the hearing aid to adapt to its owner's external and internal stimuli. Research and initial experiments into the monitoring and use of electrophysiological response for the control of the digital hearing aid shall be presented.

Electroencephalography↗
PubMed1999

Localization of moving sounds by hemispherectomized subjects.

The objective of the present study was to evaluate the localization abilities of the residual auditory structures in hemispherectomized subjects (Hs) during monaural and binaural listening. The monaural stimulus presentations were aimed at evaluating the relative contribution of crossed and uncrossed afferents to the perception of a moving auditory target. Three Hs and ten control subjects (Cs) were asked to localize a simulated moving sound source of fixed intensity presented on the horizontal plane. The moving stimulus was delivered randomly through 16 loudspeakers which were mounted at 10 degree intervals on a calibrated perimeter frame located inside an anechoic chamber. The apparent movement traveled in either direction over three different distances in the peri-central and lateral fields. Listeners had to report the movement trajectory by pointing with the index finger where they thought the beginning and the ending of the stimulus occurred on the perimeter. Hs were less accurate than Cs in the binaural condition, suggesting that the residual hemisphere and/or subcortical structures are not by themselves as efficient to accurately analyze motion as when the two hemispheres are present. In the monaural testing conditions, most of the subjects showed a prominent lateral displacement of the perceived localization towards the functional ear. However, all subjects, particularly the Cs, were able to perceive different lengths of trajectories although they did not discriminate the direction of the motion. For the Hs, the pattern of performance differed for each subject. One right Hs was more accurate for localizing the sources delivered on the side of the functional ear when relying on crossed-input. A left Hs was able to detect the appropriate side of the moving sound source even when it was presented on the side to the obstructed ear. The other left Hs who had developed normally until the age of 5, was more affected by the hemidecortication in both the binaural and the monaural listening conditions. These findings suggest that it is possible to detect moving sounds in the absence of binaural cues and without the integration carried out by both cerebral hemispheres. However it appears that interaural disparities and bihemispheric integration are required for finer analysis.

Acoustic Stimulation↗
PubMed1998

Role of synaptic inhibition in processing of dynamic binaural level stimuli.

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the inferior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD), a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ipsilateral stimulation. Sound pressure level was modulated trapezoidally at the ipsilateral "inhibitory" ear, whereas the contralateral "excitatory" level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression under static conditions, an unexpected discharge was often elicited, apparently because of an aftereffect of synaptic inhibition. In contrast, when the inhibitory stimulus was increased within a range of sound levels that produced only modest suppression under static conditions, neuronal discharge was often profoundly suppressed. In many cases the "conditioned enhancement" or "conditioned suppression" persisted for several seconds after the modulation of ILD, and such conditioned responses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory stimulus. The acoustically elicited discharge rate was potentiated markedly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad implications for processing ensuing auditory stimuli.

Acoustic Stimulation↗