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PubMed1984

Auditory space representation in the superior colliculus of the big brown bat, Eptesicus fuscus.

The auditory response areas of 123 superior collicular (SC) units of Eptesicus fuscus were studied under free-field acoustic stimulus conditions. A stimulus was delivered from a loudspeaker placed 14 cm in front of a bat. The best frequency of a unit was determined by changing the stimulus frequency until the minimum threshold was measured. A best frequency stimulus was then delivered as the loud-speaker was moved across the auditory space to determine the response center of the auditory response area of each unit. The response center was defined as the direction at which the unit had its lowest minimum threshold. The stimulus intensity was then raised 2-20 dB above the lowest minimum threshold of the unit and the response area for each stimulus intensity was determined. The response area of a unit expands with stimulus intensity, but the expansion is not even in all directions. The size of the response area of a unit does not correlate with its minimum threshold, best frequency, or recording depth. Response centers of 7 units were located directly in front of the animal, but most response centers were located in a limited portion of the contralateral auditory space. Although each unit has a response center which is the point of maximal sensitivity, the point-to-point representation of the auditory space is not systematically organized. We suggest that an animal with highly mobile external pinnae may not need an orderly auditory space map in its neural tissue for accurate sound localization.

Acoustic Stimulation↗
PubMed1988

Cortical auditory neuron interactions during presentation of 3-tone sequences: effective connectivity.

The role of cat's primary auditory cortex (AI) in both pattern discrimination and sound localization has been demonstrated by observing that ablations of it disrupt these functions. This research studied effective connectivity variations as a function of modifications in the temporal pattern of acoustic stimulation. Recordings of 10-15 neurons (simultaneously and separably) were made in AI of sedated cats. A bundle of 7 microelectrodes was stereotaxically placed along a tangential path. Stimuli were permutations of 3-tone bursts sequences. Each recorded neuron pair was analyzed off-line by cross-correlation. Cross-correlation of spike trains from neuron pairs showed signatures of direct and/or shared input. These appeared individually or in combination and for most pairs were present in spontaneous conditions. However, in stimulated conditions these spontaneous interactions were strongly modulated. The analysis detected differences in neuronal interaction during presentation of different tones. Similar differences occurred during presentation of any single particular stimulus if there was a history of different immediately previous tones. When individual neuron pair cross-correlations were put together to form an effective connectivity diagram among all recorded neurons, they turned out as different diagrams for different stimulus conditions.

Acoustic Stimulation↗
PubMed1990

Auditory response properties and directional sensitivity of cerebellar neurons of the echolocating bat, Eptesicus fuscus.

Auditory response properties and directional sensitivity of cerebellar neurons of Eptesicus fuscus were studied under free-field stimulation conditions. The best frequency (BF) and minimum threshold (MT) of a recorded neuron were first determined with a sound delivered in front of the bat. Discharge pattern and MT were studied with both BF stimuli and one-octave downward and upward sweep FM (frequency-modulated) stimuli. The directional sensitivity of cerebellar neurons was then studied by determining the variation of MT and response latency with BF and FM stimuli broadcast from each of 15 loudspeakers attached to a semicircular wooden track in front of the bat. All 85 cerebellar neurons recorded discharged phasically to acoustic stimuli. Only 20 were spontaneously active. Cerebellar neurons were generally more sensitive to FM stimuli than to pure tone pulses. Thus, they discharged more vigorously and had a lower MT to the former than the latter stimulus. Directional sensitivity of 47 neurons (BF = 23.4-81.1 kHz) was studied. All neurons varied their MTs with sound direction. Most neurons (n = 37, 79%) showed a lowest MT to a frontal sound. Directional sensitivity of cerebellar neurons appears to be sharper when determined with BF tone pulses than with FM stimuli. Thus the directional slope and the difference in MT between the best and worst angles of these neurons were larger when determined with the BF stimulus. Directional sensitivity of cerebellar neurons is not dependent upon stimulus frequency, unlike that of the inferior and cortical neurons of the same bat. Cerebellar neurons also varied their response latency with sound direction. Such a variation may provide the bat with another neural code for sound localization.

Animals↗
PubMed1990

Infants' perception of timbre: classification of complex tones by spectral structure.

Infants 7 to 8.5 months of age were tested for their discrimination of timbre or sound quality differences in the context of variable exemplars. They were familiarized with a set of complex tones with specified spectral structure; members of the set varied in fundamental frequency, intensity, or duration. Infants were then tested for their detection of tones that contrasted in spectral structure but were similar in other respects. They successfully differentiated the two spectral structures in the context of these variations, indicating that they can classify tonal stimuli on the basis of timbre. When the stimuli were organized into arbitrary categories, infants were unable to differentiate these categories, indicating that their performance with nonarbitrary categories was not attributable to memorization of the familiarized set.

Female↗
PubMed1995

Extent and limits of callosal plasticity: presence of disconnection symptoms in callosal agenesis.

Although earlier studies have emphasized the absence of 'split-brain' symptoms in callosal agenesis patients, the notion of an 'asymptomatic' acallosal brain has lately been challenged. We report a number of findings that are indicative of an interruption of interhemispheric communication and integration in individuals lacking the corpus callosum. Several groups of patients with callosal pathology (acallosals, patients with commissurotomy or callosotomy, either complete or partial) were compared to matched controls. Interhemispheric transfer was tested in two different experiments involving pointing to a light source while maintaining central fixation. In the first experiment, a learning paradigm was used to measure transfer of a motor skill from the trained to the untrained hand. In the second experiment, subjects pointed to visual targets at different locations on a perimeter. Midline fusion, a recurrent theme when describing callosal function, was assessed using tasks which included depth perception with binocular and/or monocular cues, two-point discrimination thresholds and sound localization in the peri-central and lateral fields. Subjects with callosal pathology were impaired on all tasks involving transfer of motor and visuo-spatial skills and on some of the tasks requiring sensory integration of visual and tactile information across the body midline. We conclude that these functions require an intact corpus callosum since none of these deficits were seen in controls equated for IQ.

Adolescent↗
PubMed1990

Some of the factors involved in the Sarvazyan method for recording ultrasound field distributions with special reference to the application of ultrasound in physiotherapy.

The mapping of ultrasound fields using the dye paper method is described. The mode of action by which the dye concentration is increased has been investigated. Microbubbles on the paper surface and microstreaming of dye solution seem to be essential features of the mapping method. The streaming is thought to inhibit the formation of a dye depletion layer near the paper to an extent dependent on local sound intensity A linear relation between streaming velocity and intensity is derived.

Coloring Agents↗
PubMedDate not supplied

Binaural interaction in the brain-stem auditory evoked potential: evidence for a delay line coincidence detection mechanism.

The binaural interaction component (BIC) of the brain-stem auditory evoked potential (BAEP) was studied in 13 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. Eight or 16 electrodes on the head and neck were referred to a non-cephalic site, the binaural stimuli were delivered either simultaneously or with an inter-aural time difference (delta t) of 0.2-1.6 msec, and masking noise was presented to the non-stimulated ear. With simultaneous binaural clicks a BIC was identifiable in every subject, the most consistent peaks being a scalp-positive potential (P1) peaking approximately 0.2 msec after wave V and a scalp negativity (N1) 0.7 msec later. Similar potentials were identifiable in 6/7 subjects with delta t of 0.4 msec, 5/7 at 0.8 msec but only 1/7 at 1.2 msec. This suggests that the BIC may be associated with sound localization mechanisms which are sensitive to a similar range of delta t. On increasing delta t from 0.0 to 0.8 msec, the BIC was progressively delayed by approximately half the inter-aural time difference, with no suggestion of increasing temporal dispersion. This supports the notion of a 'delay line coincidence detection' mechanism in which the BIC represents the output of binaurally responsive neurones, probably in the superior olivary complex, which are 'tuned' to a particular delta t by the relative lengths of presynaptic axons relaying input from either ear. The distribution of the BIC in sagittal and coronal electrode chains was compared with that of binaural BAEP components I-VI and found to bear the closest resemblance to wave IV. It is suggested that both components may originate largely in the lateral lemnisci.

Acoustic Stimulation↗
PubMed1987

Sound reception in two anabantid fishes.

1. Pure tone displacement sensitivity and bandwidth were measured from the saccule of the ear in two anabantid species (Trichogaster trichopterus and Helostoma temincki) using microphonic potentials with a 1 microV RMS threshold for the second harmonic of the stimulus frequency. 2. Saccular microphonics were recorded in both species from 80 to 1600 Hz, with lowest thresholds between 100 and 200 Hz. The overall microphonic response curves (sensitivity and bandwidth) of the two species were statistically similar to one another with an analysis of variance, although there were statistically different thresholds at 100 and 800 Hz. 3. The hair cell orientation patterns of the saccular epithelia differ in the two species. Consequently, the comparative sizes of the saccular sensory epithelium and numbers of sensory hair cells were examined. The saccular sensory epithelium of Helostoma is about 40% larger and contains nearly 50% more hair cells than the saccular epithelium of a comparably sized Trichogaster. 4. An extracranial air bubble, located in the suprabranchial chamber, is found in both species. The bubble has direct access to the saccular chamber in Trichogaster through a foramen which is absent in Helostoma. Despite the difference in morphology and the larger numbers of sensory hair cells in Helostoma, hearing sensitivity and bandwidth is similar in the two species. Although the structural differences in the auditory periphery do not affect pure tone sensitivity and bandwidth, other aspects of fish hearing such as frequency discrimination, discrimination of signals in the presence of noise, and/or sound localization ability may be affected by these structural differences.

Acoustic Stimulation↗
PubMed1988

Comparative psychoacoustics.

Psychophysical data on unspecialized mammals commonly used in auditory research were compiled from the literature, and an attempt was made to compare the hearing capacities of these species with man. Binaural hearing and sound localization were not considered. The most complete psychoacoustic data exist for chinchilla, cat, various primates, and the mouse. The existing data include audiograms, frequency and intensity discrimination thresholds, critical masking ratios, critical bandwidths, temporal summation functions at threshold, psychophysical tuning curves, gap detection thresholds, temporal modulation transfer functions, temporal discriminations, and auditory filter shapes. In general, the qualitative forms of most all psychoacoustic functions for these mammals are similar to those for man, and there is little reason to believe that the mechanisms underlying these capacities are different across mammals. Although the discriminative capacities of humans are generally more acute than those of non-humans, the database on the capacities of non-humans is not yet sufficient for systematic comparisons across species to be made with confidence.

Animals↗
PubMedDate not supplied

Central auditory aging: GABA changes in the inferior colliculus.

Age-related hearing loss (presbycusis) is a complex state that reflects pathologic changes along the entire auditory neuraxis. Loss of speech understanding, decreased ability to localize sounds, and a decreased ability to detect and extract signals in noise are characteristic problems encountered by the elderly. Central (neural) presbycusis frequently results in a dramatic loss in speech understanding without a parallel change in pure-tone thresholds. In spite of evidence that suggests these deficits cannot be fully explained by peripheral changes alone, few studies have examined the neurochemical basis of central auditory dysfunction in aging. Age-related alterations in neural circuits involved in the processing of acoustic information could reflect changes in the synthesis, degradation, uptake, release, and receptor sensitivity of neurotransmitters, perhaps secondary to cell loss and/or progressive deafferentation. A series of studies designed to test this hypothesis has examined aging in the central auditory system of the F344 rate. Age-related changes associated with GABA neurotransmitter function in an important auditory midbrain structure, the inferior colliculus, have been investigated. These studies found: (1) decreased numbers of GABA immunoreactive neurons; (2) decreased basal levels (concentrations) of GABA; (3) decreased GABA release; (4) decreased glutamic acid decarboxylase activity; (5) decreased GABAB receptor binding; (6) decreased numbers of presynaptic terminals; and (7) subtle GABAA receptor binding changes. Collectively, these age-related changes suggest altered GABA neurotransmitter function in the IC. Identification of specific neurotransmitter changes in structures important in speech processing could eventually lead to the development of pharmacotherapy for selective types of age-related hearing loss.

Aging↗
PubMed2004

Calmodulin and Munc13 form a Ca2+ sensor/effector complex that controls short-term synaptic plasticity.

The efficacy of synaptic transmission between neurons can be altered transiently during neuronal network activity. This phenomenon of short-term plasticity is a key determinant of network properties; is involved in many physiological processes such as motor control, sound localization, or sensory adaptation; and is critically dependent on cytosolic [Ca2+]. However, the underlying molecular mechanisms and the identity of the Ca2+ sensor/effector complexes involved are unclear. We now identify a conserved calmodulin binding site in UNC-13/Munc13s, which are essential regulators of synaptic vesicle priming and synaptic efficacy. Ca2+ sensor/effector complexes consisting of calmodulin and Munc13s regulate synaptic vesicle priming and synaptic efficacy in response to a residual [Ca2+] signal and thus shape short-term plasticity characteristics during periods of sustained synaptic activity.

Animals↗
PubMed2005

Fish otolith mass asymmetry: morphometry and influence on acoustic functionality.

The role of the fish otolith mass asymmetry in acoustic functionality is studied. The saccular, lagenar and utricular otoliths are weighted in two species of the Black Sea rays, 15 species of the Black Sea teleost fish and guppy fish. The dimensionless otolith mass asymmetry chi is calculated as ratio of the difference between masses of the right and left paired otoliths to average otolith mass. In the most fish studied the otolith mass asymmetry is within the range of -0.2 < chi < +0.2 (< 20%). We do not find specific fish species with extremely large or extremely small otolith asymmetry. The large otoliths do not belong solely to any particular side, left or right. The heavier otoliths of different otolithic organs can be located in different labyrinths. No relationship has been found between the magnitude of the otolith mass asymmetry and the length (mass, age) of the animal. The suggested fluctuation model of the otolith growth can interpret these results. The model supposes that the otolith growth rate varies slightly hither and thither during lifetime of the individual fish. Therefore, the sign of the relative otolith mass asymmetry can change several times in the process of the individual fish growth but within the range outlined above. Mathematical modeling shows that acoustic functionality (sensitivity, temporal processing, sound localization) of the fish can be disturbed by the otolith mass asymmetry. But this is valid only for the fish with largest otolith masses, characteristic of the bottom and littoral fish, and with highest otolith asymmetry. For most fish the values of otolith mass asymmetry is well below critical values. Thus, the most fish get around the troubles related to the otolith mass asymmetry. We suggest that a specific physicochemical mechanism of the paired otolith growth that maintains the otolith mass asymmetry at the lowest possible level should exist. However, the principle and details of this mechanism are still far from being understood.

Animals↗
PubMed2004

Relating neuronal dynamics for auditory object processing to neuroimaging activity: a computational modeling and an fMRI study.

We investigated the neural basis of auditory object processing in the cerebral cortex by combining neural modeling and functional neuroimaging. We developed a large-scale, neurobiologically realistic network model of auditory pattern recognition that relates the neuronal dynamics of cortical auditory processing of frequency modulated (FM) sweeps to functional neuroimaging data of the type obtained using PET and fMRI. Areas included in the model extend from primary auditory to prefrontal cortex. The electrical activities of the neuronal units of the model were constrained to agree with data from the neurophysiological literature regarding the perception of FM sweeps. We also conducted an fMRI experiment using stimuli and tasks similar to those used in our simulations. The integrated synaptic activity of the neuronal units in each region of the model, convolved with a hemodynamic response function, was used as a correlate of the simulated fMRI activity, and generally agreed with the experimentally observed fMRI data in the brain areas corresponding to the regions of the model. Our results demonstrate that the model is capable of exhibiting the salient features of both electrophysiological neuronal activities and fMRI values that are in agreement with empirically observed data. These findings provide support for our hypotheses concerning how auditory objects are processed by primate neocortex.

Adult↗
PubMed2006

Gamma-band activity dissociates between matching and nonmatching stimulus pairs in an auditory delayed matching-to-sample task.

Electro- and magnetoencephalography studies have suggested that increased gamma-band activity (GBA) is a correlate of activated neural stimulus representations. In this study, a delayed matching-to-sample paradigm for auditory spatial information was employed to investigate the role of magnetoencephalographic gamma-band activity in the differentiation between matching and nonmatching stimulus pairs. Twelve subjects made same-different judgments about the lateralization angle of pairs of filtered noise stimuli (S1 and S2) presented with 0.8-s delays. One half of the subjects had to respond to matching stimulus pairs, the other half to nonmatching stimulus pairs. Cortical oscillatory activity in the memory task was compared to a control task requiring the detection of background noise intensity changes. Memory-related GBA increases were revealed over midline parietal areas in the middle of the delay phase and during the presentation of S2 and over frontocentral areas at the end of the delay phase. This replicated previous findings. In addition, nonmatching trials were associated with increased GBA over right parietal areas in response to S2. The midline parietal GBA increase during S2 in the memory condition may have reflected the representation of S1 needed for a comparison between S1 and S2. When S1 and S2 were identical, no further representation was required. In contrast, for nonmatching pairs, a second representation was activated over right parietal areas.

Adult↗
PubMed2005

Convergence of auditory-nerve fiber projections onto globular bushy cells.

Globular bushy cells are a key element of brainstem circuits that mediate the early stages of sound localization. Many of their physiological properties have been attributed to convergence of inputs from the auditory nerve, many of which are large with complex geometry, but the number of these terminals contacting individual cells has not been measured directly. Herein we report, using cats as the experimental model, that this number ranged greatly (9-69) across a population of 12 cells, but over one-half of the cells (seven of 12) received between 15 and 23 inputs. In addition, we provide the first measurements of cell body surface area, which also varies considerably within this population and is uncorrelated with convergence. For one cell, we were able to document axonal structure over a distance greater than 100 microm, between the soma and the location where the axon expanded to its characteristic large diameter. These data were combined with accumulated physiological information on vesicle release, receptor kinetics and voltage-gated ionic conductances, and incorporated into computational models for four cells that are representative of the structural variation within our sample population. This predictive model reveals that basic physiological features, such as precise first spike latencies and peristimulus time histogram shapes, including primary-like with notch and onset-L, can be generated in these cells without including inhibitory inputs. However, phase-locking is not significantly enhanced over auditory-nerve fibers. These combined anatomical and computational approaches reveal additional parameters, such as active zone density, nerve terminal size, numbers and sources of inhibitory inputs and their activity patterns, that must be determined and incorporated into next-generation models to understand the physiology of globular bushy cells.

Action Potentials↗
PubMed2005

Auditory gating in schizophrenia: a pilot study of the precedence effect.

Individuals with schizophrenia are known to exhibit diminished auditory gating, as manifested by prepulse inhibition of the startle response and suppression of the P50 evoked potential. Those observations indicate that schizophrenics have deficits of inhibitory auditory processing. The precedence effect is a phenomenon in which inhibitory processing in the auditory system aids the localization of sound sources and segregation of speech in complex auditory environments. We tested the hypothesis that schizophrenic subjects would exhibit deficits in psychophysical tests of the precedence effect. The performance of 12 male medicated schizophrenic subjects was compared with that of 12 male comparison subjects. Schizophrenics and comparison subjects clearly demonstrated all three aspects of the precedence effect that were evaluated: echo suppression, buildup of echo suppression, and breakdown of echo suppression. In none of these aspects did the performance of schizophrenic subjects differ significantly from that of the comparison subjects. These findings suggest that abnormalities of auditory spatial processing and speech segregation are unlikely to contribute to the clinical pathology of schizophrenia.

Adult↗
PubMed2004

What's a cerebellar circuit doing in the auditory system?

The shapes of the head and ears of mammals are asymmetrical top-to-bottom and front-to-back. Reflections of sounds from these structures differ with the angle of incidence, producing cues for monaural sound localization in the spectra of the stimuli at the eardrum. Neurons in the dorsal cochlear nucleus (DCN) respond specifically to spectral cues and integrate them with somatosensory, vestibular and higher-level auditory information through parallel fiber inputs in a cerebellum-like circuit. Synapses between parallel fibers and their targets show long-term potentiation (LTP) and long-term depression (LTD), whereas those between auditory nerve fibers and their targets do not. This paper discusses the integration of acoustic and the proprioceptive information in terms of possible computational roles for the DCN.

Animals↗
PubMed2004

EphA4 signaling promotes axon segregation in the developing auditory system.

Precision of synaptic connections within neural circuits is essential for the accurate processing of sensory information. Specificity is exemplified at cellular and subcellular levels in the chick auditory brainstem, where nucleus magnocellularis (NM) neurons project bilaterally to nucleus laminaris (NL). Dorsal dendrites of NL neurons receive input from ipsilateral, but not contralateral, branches of NM axons whereas ventral dendrites are innervated by contralateral NM axons. This organization is analogous to that of the mammalian medial superior olive (MSO) and represents an important component of the circuitry underlying sound localization. However, the molecular mechanisms that establish segregated inputs to individual regions of NL neurons have not been identified. During synapse formation in NL, the EphA4 receptor is expressed in dorsal, but not ventral NL, neuropil, suggesting a potential role in targeting synapses to appropriate termination zones. Here, we directly tested this role by ectopically expressing EphA4 and disrupting EphA4 signaling using in ovo electroporation. We found that both misexpression of EphA4 and disruption of EphA4 signaling resulted in an increase in the number of NM axons that grow aberrantly across NL cell bodies into inappropriate regions of NL neuropil. EphA4 signaling is thus essential for targeting axons to distinct subsets of dendrites. Moreover, loss of EphA4 function resulted in morphological abnormalities of NL suggestive of errors in cell migration. These results suggest that EphA4 has multiple roles in the formation of auditory brainstem nuclei and their projections.

Animals↗