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To localize a sound source in space, the auditory system detects minute differences in the arrival time of a sound between the two ears. It has long been assumed that delay lines and coincidence detectors turn these time differences into a labelled line code for source position, but recent studies challenge this view.
BACKGROUND: The prevalence of nosocomial infection in Australian hospitals is estimated to be between 5.5% and 6.3%. Since 1989, infection control professionals (ICPs) in hospitals accredited by the Australian Council on Health Care Standards (ACHS) have been encouraged to collect nosocomial infection data according to ACHS methodology. METHOD: In 1996, we surveyed members of the Australian Infection Control Association to examine the time spent on surveillance, the practice of surveillance of all hospital infections (hospital-wide surveillance), case-finding methods, case definitions, and reporting routinely used by ICPs in acute care hospitals. We also examined the ICPs' education and experience in infection control (IC). RESULTS: The survey was completed and returned by 65% (644 of 993) of Australian Infection Control Association members. Of the ICPs who completed the survey, 47.8% (308 of 644; 95% CI, 43.9%-51.7%) met the criteria for inclusion, because they coordinated an IC program in an acute care or surgical hospital and performed surveillance for either surgical wound infection, intravascular device-related bacteremia, or non-device-related bacteremia. Of the ICPs who reported their facility's accreditation status, 93.5% participated in ACHS system. Most (97.6%) ICPs had completed hospital-based general registered nurse training. Only 1.9% (6 of 308) of ICPs reported completion of continuing education relating to hospital epidemiology. The number of years of IC experience ranged from zero to 35 years, with a median of 4 years. ICPs spent a substantial proportion of their total weekly IC time on surveillance irrespective of ACHS accreditation; 19.5 hours in ACHS hospitals and 15.6 hours in non-ACHS hospitals (P =.33). More than three quarters (76.0%) of ICPs performed hospital-wide surveillance. The case-finding methods, definitions of infections, and reporting formats varied greatly. The definition most commonly applied by ICPs (6.8%; 95% CI, 4.1%-10.4%) to define surgical wound infection was infection within 30 days after the operative procedure, plus purulent drainage, plus isolation of organisms from a culture from the incision site, plus diagnosis by a medical officer. A 5-item definition of a patient being asymptomatic, plus afebrile on admission, plus infection occurring at least 48 hours after admission, plus the patient having a fever of >38 degrees C, plus a recognized culture from one or more bottles was used by 15.7% (95% CI, 11.3%-21.0%) of ICPs to define a case of bacteremia. CONCLUSION: Surveillance is the core business of Australian ICPs and consumes a substantial proportion of their time. The importance of surveillance, the epidemiologic limitations of the current ACHS system, and the nonstandard methods we report indicate that improved methodology is required for case finding and reporting of nosocomial infections. Australian ICPs should complete training in the principles of surveillance and epidemiology. With this training, ICPs can work collaboratively with other health care professionals to develop epidemiologically sound, local, nosocomial surveillance systems and lobby for a voluntary, national, standardized, risk-adjusted system of targeted nosocomial surveillance.
The dorsal nucleus of the lateral lemniscus (DNLL) is a distinct auditory neuronal group located ventral to the inferior colliculus (IC). It receives excitatory and inhibitory afferent inputs from various structures of the auditory lower brainstem and sends GABAergic inhibitory efferents mainly to the contralateral DNLL and the bilateral IC. The synaptic excitation in DNLL neurons consists of two components, an early fast depolarization and a later long lasting one. Glutamate is the probable excitatory neurotransmitter for DNLL neurons. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors mediate the early part of the excitation while N-Methyl-D-aspartate (NMDA) receptors mediate the long lasting component. The long lasting NMDA receptor-mediated component in the DNLL may contribute to a prolonged inhibition in the IC. The DNLL is thought to be a structure for processing binaural information. Most DNLL neurons in rat and bat are sensitive to interaural intensity differences (IIDs). They are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear, showing an excitatory/inhibitory (EI) binaural response pattern. The EI pattern can be attributed to synaptic inputs that originate from various structures in the lower auditory brainstem and impinge on the DNLL neurons. In cat some DNLL neurons are sensitive to IIDs and some are sensitive to interaural time differences. In addition, DNLL neurons exhibit different temporal response patterns to contralateral tonal stimulation and respond to amplitude modulated tones, implying that DNLL may contribute to processing temporally complex acoustic information. DNLL neurons shape binaural responses in the contralateral inferior colliculus and auditory cortex through their inhibitory brainstem projections and contribute to the accuracy with which animals localize sounds in space.
We determined the audiogram of the Jamaican fruit-eating bat (Phyllostomidae: Artibeus jamaicensis), a relatively large (40-50 g) species that, like other phyllostomids, uses low-intensity echolocation calls. A conditioned suppression/avoidance procedure with a fruit juice reward was used for testing. At 60 dB SPL the hearing range of A. jamaicensis extends from 2.8 to 131 kHz, with an average best sensitivity of 8.5 dB SPL at 16 kHz. Although their echolocation calls are low-intensity, the absolute sensitivity of A. jamaicensis and other 'whispering' bats does not differ from that of other mammals, including other bats. The high-frequency hearing of A. jamaicensis and other Microchiroptera is slightly higher than expected on the basis of selective pressure for passive sound localization. Analysis suggests that the evolution of echolocation may have been accompanied by the extension of their high-frequency hearing by an average of one-half octave. With respect to low-frequency hearing, all bats tested so far belong to the group of mammals with poor low-frequency hearing, i.e., those unable to hear below 500 Hz.
The barn owl (Tyto alba) uses interaural level difference (ILD) as a cue for the localization of sound. The first site of binaural convergence in the pathway that processes ILD is the ventral lateral lemniscus pars posterior (VLVp). Neurons in VLVp receive excitatory input from the contralateral nucleus angularis, and inhibitory input from the contralateral VLVp. Within the lateral shell of the inferior colliculus are ILD sensitive neurons that show maximum spike rate at a specific ILD value, with response falling off sharply on each side. Adolphs has developed a model of such lateral shell neurons based on anatomic and physiological data. In his model, lateral shell neurons receive inhibitory input from VLVp on both sides, and this inhibition, applied against a constant excitatory input, produces the observed two-sided response curves. We simulated, in Matlab 4, Adolphs' model, and obtained supporting results. Our simulation suggests that VLVp provides a repository of simple ILD filters from which higher centers construct more complex filters, including the single-peaked curves observed by Adolphs. The VLVp filters are organized along the inhibitory gradient, with broad filters ventral, sharp filters dorsal.
Temporal changes in the excitability of inferior colliculus (IC) neurons will shape their responses to complex stimuli. Single-unit responses of rat IC neurons to the second (probe) of a pair of tones exhibited suppression, facilitation and delay tuned effects. Responses to probe tones were markedly suppressed (by 76% for contralateral stimulation with equal intensity tone pairs) during contralateral and binaural stimulation in 60% of IC neurons. Suppression developed rapidly as a function of the duration of the initial tone, and approached maximum for tones of less than 200 ms. Suppression decreased as the interval between tones increased, and this recovery of responsiveness was often exponential (time constants: mean: 271.4 ms; median: 72.8 ms; n = 47), and independent of the duration and intensity of preceding stimulation. Facilitation of responses to probe tones was observed chiefly in neurons with 'pauser/buildup' response patterns, and decreased as the intertone interval increased. The greatest suppression of responses to probe tones occurred only after intertone intervals of 32 ms (delayed minimum; n = 8) in 11% of IC neurons. Other IC neurons exhibited an increased excitability to probe tones presented 128 ms after stimulation (delayed maximum; n = 7). The latencies of the later neurons' responses were longer (mean: 29.5 ms) than other IC neurons. The role of suppression in sound localization and echo suppression, and the relationship between 'delay tuning' effects and encoding of complex stimuli are discussed.
In this experiment, the critical band (CB)-widths were measured using two narrow-band noises (NBNs) with steep cut-off slopes as a masker, varying their spectral level and center-frequency. The following results were obtained: (i) The CB-widths at all center-frequencies were narrower than the classical CB-widths estimated by Zwicker et al [J Acoust Soc Am 29:548-557 (1957)], and decreased continuously toward the lower frequency at the center-frequencies below 500 Hz, though Zwicker's data were constant in the same range. These results barely coincided with the frequency dependence of the equivalent rectangular bandwidth (ERB) which was estimated from the shape of the auditory filter measured directly. (ii) Level dependence of masking noise in CB was constant beyond 20 dB of the masked threshold. This depends on the use of NBN as a masker. (iii) The CB was influenced by distortion products and off-frequency listening. Accordingly, the bandwidths might differ with the experimental method.
The first binaural integration within the auditory system responsible for sound localization depends upon commissural neurons that connect the two symmetrical cochlear nuclei. These cells in the deep polymorphic layer of the rat dorsal cochlear nucleus were identified with the electron microscope after injection of the retrograde tracer, Wheat Germ Agglutinin conjugated to Horseradish Peroxydase, into the contralateral cochlear nucleus. Commissural neurons are multipolar or bipolar with an oval to fusiform shape. Few commissural neurons, most inhibitory but also excitatory, connect most of the divisions of the rat cochlear nuclei. The most common type is a glycinergic, sometimes GABAergic, moderately large cell. Its ergastoplasm is organized into peripheral stacks of cisternae, and few axo-somatic synaptic boutons are present. Another type of commissural neuron is a medium-sized, spindle-shaped cell, glycine and GABA-negative, with sparse ergastoplasm and synaptic coverage. A giant, rare type of commissural neuron is glycine-positive and GABA-negative, with short peripheral stacks of ergastoplasmic cisternae. It is covered with synaptic boutons, many of which contain round synaptic vesicles. Another rare type of commissural neuron is a moderately large cell, oval to fusiform in shape, immunonegative for both glycine and GABA, and contacted by many axo-somatic boutons. It contains large dense mitochondria and numerous dense core vesicles of peptidergic type. Some labelled boutons, mostly inhibitory and probably derived from commissural neurons, contact pyramidal, cartwheel, giant and tuberculo-ventral neurons. The prevalent inhibition of electrical activity in a cochlear nucleus observed after stimulation of the contralateral cochlear nucleus may be due to commissural inhibitory terminals which contact excitatory neurons such as pyramidal and giant cells. Other inhibitory commissural terminals which contact inhibitory neurons such as cartwheel and tuberculo-ventral neurons, may explain the stimulation of electrical activity in the DCN after contralateral stimulation.
Human brain imaging studies have suggested that posterior temporo-parietal regions are involved in auditory spatial processing. We used electroencephalography to investigate the dynamics of temporo-parietal networks during working memory for moving sounds. A delayed matching-to-sample task required a decision on the identity of positions and trajectories of two moving sounds S1 and S2 presented with delays of 927 or 1427 ms. Moving sounds consisted of noise bursts positioned at successive angles to create the impression of one of six possible trajectories at variable spatial positions. Stimuli in the equally difficult control condition were identical to the memory task up to S2, which was replaced by a spatial displacement in the otherwise stationary background sound whose direction had to be detected. Event-related potentials were recorded from 31 scalp electrodes in 15 subjects. Scalp current density estimates allowed to identify the following components. The fronto-central negative variation preceding S2 did not differ between tasks. In contrast, the sustained negative current during the presentation of S1 originating from superior temporal cortex was more pronounced for the memory task, probably reflecting enhanced attention allocation and foreground-background discrimination. Most importantly, the memory task activated current sources over bilateral posterior parietal regions between the middle of S1 and the end of the delay phase. This component was completely absent in the control condition. In summary, the present study disclosed varying degrees of memorization-related, top-down driven influences on the processing of moving sounds at different stages of an auditory network involving temporal and parietal regions.
A series of noise signals was synthesized with spectral notches whose central frequencies moved regularly along the frequency range (from low frequencies to high and from high frequencies to low), imitating movement of the sound source in the vertical plane. The spike responses of neurons in the inferior colliculus of the mouse (Mus musculus) to noise signals changed as the spectral notch moved relative to the excitatory and inhibitory areas of the receptive fields of the neurons and depended on the notch width. Disinhibition reactions in the inhibitory zones were more marked when a frequency notch in the inhibitory zone was followed by a frequency notch in the excitatory part of the response. It is suggested that the selectivity of neurons to the direction of movement of the spectral notch in a noise signal is based on the interaction of the excitatory and inhibitory inputs. The overall set of neuron responses can provide information on the movement of the sound source in acoustic space.
Four studies illustrate a new auditory illusion associated with the Doppler effect and demonstrate a new influence of dynamic intensity change on perceived pitch. Experiment 1 confirmed the existence of a popular belief that the pitch of a moving sound source rises as the source approaches. Because there is no corresponding rise in frequency, the authors refer to the perceived pitch rise as the Doppler illusion. Experiment 2 confirmed that the effect occurs perceptually, so the belief in a "naive principle" of physics has a perceptual basis. Experiment 3 confirmed the effect does not occur under matched static conditions. Experiment 4 showed that the influence of dynamic intensity change on perceived pitch occurs outside the realm of Doppler stimuli. The findings support a dynamic dimensional interaction of pitch and loudness, with marked differences in the perception of pitch and loudness under static and dynamic conditions.
The rhythm created by spacing a series of brief tones in a regular pattern can be disguised by interleaving identical distractors at irregular intervals. The disguised rhythm can be unmasked if the distractors are allocated to a separate stream from the rhythm by integration with temporally overlapping captors. Listeners identified which of 2 rhythms was presented, and the accuracy and rated clarity of their judgment was used to estimate the fusion of the distractors and captors. The extent of fusion depended primarily on onset asynchrony and degree of temporal overlap. Harmonic relations had some influence, but only an extreme difference in spatial location was effective (dichotic presentation). Both preattentive and attentionally driven processes governed performance.
Coincidence-detector neurons in the auditory brainstem of mammals and birds use interaural time differences to localize sounds. Each neuron receives many narrow-band inputs from both ears and compares the time of arrival of the inputs with an accuracy of 10-100 micros. Neurons that receive low-frequency auditory inputs (up to about 2 kHz) have bipolar dendrites, and each dendrite receives inputs from only one ear. Using a simple model that mimics the essence of the known electrophysiology and geometry of these cells, we show here that dendrites improve the coincidence-detection properties of the cells. The biophysical mechanism for this improvement is based on the nonlinear summation of excitatory inputs in each of the dendrites and the use of each dendrite as a current sink for inputs to the other dendrite. This is a rare case in which the contribution of dendrites to the known computation of a neuron may be understood. Our results show that, in these neurons, the cell morphology and the spatial distribution of the inputs enrich the computational power of these neurons beyond that expected from 'point neurons' (model neurons lacking dendrites).
At a listener's ears, sound source power and sound source distance are confounded in measures of acoustic intensity, a physical property long thought to be the primary determinate of loudness. Although the relationship between sound source loudness and power is well known when source distance is fixed, relatively little is known about source loudness under conditions of varying distance. Here we show a robust loudness constancy, similar in many ways to visual size constancy, that results under distance-varying conditions that produce inaccurate estimates of source distance. Our results suggest that the auditory system does not require accurate distance estimates to judge source loudness, even when distance is variable. We offer an alternative explanation of loudness constancy based solely on a reverberant sound energy cue.
Activity-dependent synapse refinement is crucial for the formation of precise excitatory and inhibitory neuronal circuits. Whereas the mechanisms that guide refinement of excitatory circuits are becoming increasingly clear, the mechanisms guiding inhibitory circuits have remained obscure. In the lateral superior olive (LSO), a nucleus in the mammalian sound localization system that receives inhibitory input from the medial nucleus of the trapezoid body (MNTB), specific elimination and strengthening of synapses that are both GABAergic and glycinergic (GABA/glycinergic synapses) is essential for the formation of a precise tonotopic map. We provide evidence that immature GABA/glycinergic synapses in the rat LSO also release the excitatory neurotransmitter glutamate, which activates postsynaptic NMDA receptors (NMDARs). Immunohistochemical studies demonstrate synaptic colocalization of the vesicular glutamate transporter 3 with the vesicular GABA transporter, indicating that GABA, glycine and glutamate are released from single MNTB terminals. Glutamatergic transmission at MNTB-LSO synapses is most prominent during the period of synapse elimination. Synapse-specific activation of NMDARs by glutamate release at GABAergic and glycinergic synapses could be important in activity-dependent refinement of inhibitory circuits.
The lateral superior olive (LSO), a binaural nucleus involved in sound localization, receives tonotopically organized inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). During development, the tonotopic organization of this glycinergic/GABAergic MNTB-LSO pathway is established by activity-dependent axonal reorganization. However, the underlying mechanisms by which this reorganization takes place have remained largely unknown. As cytosolic calcium is one of the most important second messengers responsible for inducing synaptic plasticity and reorganization, we examined whether and how activity in the MNTB-LSO pathway changes the intracellular calcium concentration ([Ca2+]i) in developing LSO neurons. By applying calcium imaging techniques to Fura-2-labelled slices from neonatal rats and mice, we found that glycine and GABA (gamma-aminobutyric acid) affect [Ca2+]i in LSO neurons in an age-dependent manner; during the first postnatal week, the period at which glycine and GABA are depolarizing in the LSO, glycine and GABA always increased [Ca2+]i. However, in 2-week-old animals, the time around hearing onset when glycine and GABA are hyperpolarizing, glycine and GABA slightly decreased [Ca2+]i. Calcium responses could also be elicited by stimulation of afferent fibres from the MNTB, and these synaptic responses were mediated by glycine and GABA(A) receptors. Furthermore, GABA, which is a neurotransmitter only in the immature MNTB-LSO pathway, played a major role in generating MNTB-elicited Ca2+ responses. The direct link of glycinergic/GABAergic synaptic activity to intracellular calcium signalling during the period of inhibitory synaptic plasticity could be one of the mechanisms by which tonotopic MNTB-LSO connections become established.
Two experiments were carried out to investigate the perception of complex auditory-spatial patterns. Subjects were asked to identify alphanumeric characters whose patterns could be outlined acoustically through the sequential activation of specific units in a speaker array. Signal bandwidths were varied systematically in both experiments. Signals in experiment 1 had sharp onsets and offsets; envelope shapes in experiment 2 were much more gradual. Subjects showed considerable ability in recognizing alphanumeric patterns traced with signals of varying acoustical composition. Reductions in the steepness of signal attack and decay produced limited declines in pattern recognition ability. Systematic trends in the relation between patterns and the distribution of incorrect responses suggest that subjects performed a pattern-matching task, in which identifications were made on the basis of component features. The unexpected pattern recognition abilities that subjects demonstrated in both experiments suggest that spatial hearing, like vision, has access to mechanisms for amodal spatial representations.
Several studies in the auditory-perception literature hint that listeners may be able to anticipate the time of arrival of an approaching sound source. Two experiments are reported in which listeners judged the time of arrival of an approaching car on the basis of various portions of its auditory signal. Subjects pressed a computer key to indicate when the car would have just passed them, assuming that the car maintained a constant approach velocity. A number of variables were tested including (a) the time between the offset of the signal and the virtual time of passage, (b) duration of the signal, and (c) feedback concerning judgment accuracy. Results indicate that increasing the time between signal offset and virtual time of passage decreases judgment accuracy whereas the actual duration of the signal had no significant effect. Feedback significantly improved performance overall.