Publications using TDT

This study investigated the extent to which language familiarity affects the perception of the indexical properties of speech by testing listeners’ identification and discrimination of bilingual talkers across two different languages. In one experiment, listeners were trained to identify bilingual talkers speaking in only one language and were then tested on their ability to identify the same talkers speaking in another language. In the second experiment, listeners discriminated between bilingual talkers across languages in an AX discrimination paradigm. The results of these experiments indicate that there is sufficient language-independent indexical information in speech for listeners to generalize knowledge of talkers’ voices across languages and to successfully discriminate between bilingual talkers regardless of the language they are speaking. However, the results of these studies also revealed that listeners do not solely rely on language-independent information when performing these tasks. Listeners use language-dependent indexical cues to identify talkers who are speaking a familiar language. Moreover, the tendency to perceive two talkers as the “same” or “different” depends on whether the talkers are speaking in the same language. The combined results of these experiments thus suggest that indexical processing relies on both language-dependent and language-independent information in the speech signal.

Listeners are relatively good at estimating the true content of each physical source in a sound mixture in most everyday situations. However, if there is a spectrotemporal element that logically could belong to more than one object, the correct way to group that element can be ambiguous. Many psychoacoustic experiments have implicitly assumed that when a sound mixture contains ambiguous sound elements, the ambiguous elements “trade” between competing sources, such that the elements contribute more to one object in conditions when they contribute less to others. However, few studies have directly tested whether such trading occurs. While some studies found trading, trading failed in some recent studies in which spatial cues were manipulated to alter the perceptual organization. The current study extended this work by exploring whether trading occurs for similar sound mixtures when frequency content, rather than spatial cues, was manipulated to alter grouping. Unlike when spatial cues were manipulated, results are roughly consistent with trading. Together, results suggest that the degree to which trading is obeyed depends on how stimuli are manipulated to affect object formation.

WD40 repeat 1 protein (WDR1) was first reported in the acoustically injured chicken inner ear, and bioinformatics revealed that WDR1 has numerous WD40 repeats, important for protein-protein interactions. It has significant homology to actin interacting protein 1 (Aip1) in several lower species such as yeast, roundworm, fruitfly and frog. Several studies have shown that Aip1 binds cofilin/actin depolymerizing factor, and that these interactions are pivotal for actin disassembly via actin filament severing and actin monomer capping. However, the role of WDR1 in auditory function has yet to be determined. WDR1 is typically restricted to hair cells of the normal avian basilar papilla, but is redistributed towards supporting cells after acoustic overstimulation, suggesting that WDR1 may be involved in inner ear response to noise stress. One aim of the present study was to resolve the question as to whether stress factors, other than intense sound, could induce changes in WDR1 presence in the affected avian inner ear. Several techniques were used to assess WDR1 presence in the inner ears of songbird strains, including Belgian Waterslager (BW) canary, an avian strain with degenerative hearing loss thought to have a genetic basis. Reverse transcription, followed by polymerase chain reactions with WDR1-specific primers, confirmed WDR1 presence in the basilar papillae of adult BW, non-BW canaries, and zebra finches. Confocal microscopy examinations, following immunocytochemistry with anti-WDR1 antibody, localized WDR1 to the hair cell cytoplasm along the avian sensory epithelium. In addition, little, if any, staining by anti-WDR1 antibody was observed among supporting cells in the chicken or songbird ear. The present observations confirm and extend the early findings of WDR1 localization in hair cells, but not in supporting cells, in the normal avian basilar papilla. However, unlike supporting cells in the acoustically damaged chicken basilar papilla, the inner ear of the BW canary showed little, if any, WDR1 up-regulation in supporting cells. This may be due to the fact that the BW canary already has established hearing loss and/or to the possibility that the mechanism(s) involved in BW hearing loss may not be related to WDR1.

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at three modulation depths (100, 50, and 20%) at 30 dB hearing level with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.

We assessed the spatial-tuning properties of units in the cat's anterior auditory field (AAF) and compared them with those observed previously in the primary (A1) and posterior auditory fields (PAF). Multi-channel, silicon-substrate probes were used to record single- and multi-unit activity from the right hemispheres of alpha-chloralose-anesthetized cats. Spatial tuning was assessed using broadband noise bursts that varied in azimuth or elevation. Response latencies were slightly, though significantly, shorter in AAF than A1, and considerably shorter in both of those fields than in PAF. Compared to PAF, spike counts and latencies were more poorly modulated by changes in stimulus location in AAF and A1, particularly at higher sound pressure levels. Moreover, units in AAF and A1 demonstrated poorer level tolerance than units in PAF with spike rates modulated as much by changes in stimulus intensity as changes in stimulus location. Finally, spike-pattern-recognition analyses indicated that units in AAF transmitted less spatial information, on average, than did units in PAF-an observation consistent with recent evidence that PAF is necessary for sound-localization behavior, whereas AAF is not.

Sensory experiences contribute to the development and specialization of signal processing capacities in the mammalian auditory system during a "critical period" of postnatal development. Earlier studies have shown that passive exposure to tonal stimuli during this postnatal epoch induces a large-scale expansion of the representations of those stimuli within the primary auditory cortex (A1) [Zhang LI, Bao S, Merzenich MM (2001) Persistent and specific influences of early acoustic environments on primary auditory cortex. Nat Neurosci 4:1123-1130]. Here, we show that exposing rat pups through the normal critical period epoch and beyond to continuous, un-modulated, moderate-level tones induces no such representational distortion, and in fact disrupts the normal development of frequency response selectivity and tonotopicity all across area A1. The area of cortex responding selectively to continuously exposed sound frequencies was actually reduced, when compared with rats reared in normal environments. Strong exposure-driven plasticity characteristic of the critical period could be induced well beyond the normal end of the critical period, by simply modulating the tonal stimulus. Thus, continuous tone exposure, like continuous noise exposure [Chang EF, Merzenich MM (2003) Environmental noise retards auditory cortical development. Science 300:498-502], ineffectively induces critical period plasticity, and indefinitely blocks the closure of a normally-brief critical period window. These findings again demonstrate the crucial role of temporally structured inputs for inducing the progressive cortical maturational changes that result in the closure of the critical period window.

Auditory localization experiments typically either require subjects to judge the location of a sound source from a discrete set of response alternatives or involve measurements of the accuracy of orienting responses made toward the source location. To compare the results obtained by both methods, we trained ferrets by positive conditioning to stand on a platform at the center of a circular arena prior to stimulus presentation and then approach the source of a broadband noise burst delivered from 1 of 12 loudspeakers arranged at 30 degrees intervals in the horizontal plane. Animals were rewarded for making a correct choice. We also obtained a non-categorized measure of localization accuracy by recording head-orienting movements made during the first second following stimulus onset. The accuracy of the approach-to-target responses declined as the stimulus duration was reduced, particularly for lateral and posterior locations, although responses to sounds presented in the frontal region of space and directly behind the animal remained quite accurate. Head movements had a latency of approximately 200 ms and varied systematically in amplitude with stimulus direction. However, the final head bearing progressively undershot the target with increasing eccentricity and rarely exceeded 60 degrees to each side of the midline. In contrast to the approach-to-target responses, the accuracy of the head orienting responses did not change much with stimulus duration, suggesting that the improvement in percent correct scores with longer stimuli was due, at least in part, to re-sampling of the acoustical stimulus after the initial head turn had been made. Nevertheless, for incorrect trials, head orienting responses were more closely correlated with the direction approached by the animals than with the actual target direction, implying that at least part of the neural circuitry for translating sensory spatial signals into motor commands is shared by these two behaviors.

Rodent studies using cortical removal techniques, ranging from transient deactivation to surgical ablation of cortex, reveal the importance of auditory cortical integrity in detecting short silent gaps in white noise (2-15 ms). Processing limits for longer gaps under decorticate conditions in rats remain unknown. Determining the temporal threshold for subcortical resolution of gaps in noise could, however, shed light on both normal hierarchical processing of acoustic temporal stimuli, as well as the etiology of processing anomalies following developmental cortical disruption. To address these important issues, we assessed whether intact rats, as well as those with induced developmental cortical disruptions (microgyria) could resolve silent gaps of 20-100 ms in duration when embedded in white noise, during functional deactivation of auditory cortex. Results showed that both intact rats, as well as those with cortical malformations resulting from early focal disruptions of neuronal migration could resolve silent gaps of 100-ms duration under cortical deactivation (KCl). However, only intact rats could reliably detect 75-ms gaps, suggesting possible subcortical anomalies in subjects with early cortical disturbances.

We investigated encoding properties of identified auditory interneurons in two not closely related grasshopper species (Acrididae). The neurons can be homologized on the basis of their similar morphologies and physiologies. As test stimuli, we used the species-specific stridulation signals of Chorthippus biguttulus , which evidently are not relevant for the other species, Locusta migratoria . We recorded spike trains produced in response to these signals from several neuron types at the first levels of the auditory pathway in both species. Using a spike train metric to quantify differences between neuronal responses, we found a high similarity in the responses of homologous neurons: interspecific differences between the responses of homologous neurons in the two species were not significantly larger than intraspecific differences (between several specimens of a neuron in one species). These results suggest that the elements of the thoracic auditory pathway have been strongly conserved during the evolutionary divergence of these species. According to the ‘efficient coding’ hypothesis, an adaptation of the thoracic auditory pathway to the specific needs of acoustic communication could be expected. We conclude that there must have been stabilizing selective forces at work that conserved coding characteristics and prevented such an adaptation.

The auditory center in the cerebrum, the auditory cortex, consists of multiple interconnected areas. The functional role of these interconnections is poorly understood. The auditory cortex of the mustached bat consists of at least nine areas, including the frequency modulation–frequency modulation (FF) and dorsal fringe (DF) areas. The FF and DF areas consist of neurons tuned to specific echo delays carrying target-distance information. The DF area is hierarchically at a higher level than the FF area. Here, we show that the feedback projection from the DF area to the FF area shifts the delay-tuning of FF neurons toward that of the stimulated DF neurons. In contrast, the feed-forward projection from the FF area to the DF area shifts the delay-tuning of DF neurons away from that of the stimulated FF neurons. The lateral projection within the DF area shifts the delay-tuning of DF neurons toward that of the stimulated DF neurons. In contrast, the lateral projection within the FF area shifts the delay-tuning of FF neurons away from that of the stimulated FF neurons. The delay-tuning shift evoked by the DF stimulation was 2.5 times larger than that evoked by the FF stimulation. Our data indicate that the FF–DF feed-forward and FF–FF lateral projections shape the highly selective neural representation of the tuning of the excited DF neurons, whereas the DF–FF feedback and DF–DF lateral projections enhance the representation of the selected tuning, perhaps, for focal processing of information carried by the excited FF neurons.

The transformation of auditory information from the cochlea to the cortex is a highly nonlinear process. Studies using tone stimuli have revealed that changes in even the most basic parameters of the auditory stimulus can alter neural response properties; for example, a change in stimulus intensity can cause a shift in a neuron's preferred frequency. However, it is not yet clear how such nonlinearities contribute to the processing of spectrotemporal features in complex sounds. Here, we use spectrotemporal receptive fields (STRFs) to characterize the effects of stimulus intensity on feature selectivity in the mammalian inferior colliculus (IC). At low intensities, we find that STRFs are relatively simple, typically consisting of a single excitatory region, indicating that the neural response is simply a reflection of the stimulus amplitude at the preferred frequency. In contrast, we find that STRFs at high intensities typically consist of a combination of an excitatory region and one or more inhibitory regions, often in a spectrotemporally inseparable arrangement, indicating selectivity for complex auditory features. We show that a linear–nonlinear model with the appropriate STRF can predict neural responses to stimuli with a fixed intensity, and we demonstrate that a simple extension of the model with an intensity-dependent STRF can predict responses to stimuli with varying intensity. These results illustrate the complexity of auditory feature selectivity in the IC, but also provide encouraging evidence that the prediction of nonlinear responses to complex stimuli is a tractable problem.

Sound duration can play a pivotal role in the reproductive behavior of anuran amphibians. Here, we report the first whole-cell recordings from duration-selective neurons in the anuran torus semicircularis, in vivo . We show that most short-pass duration-selective cells exhibited short-latency inhibition and delayed excitation. The duration of the inhibition increased with tone burst duration. Hence, for long-duration tone bursts, inhibition overlapped with excitation, reducing or eliminating spikes; no postinhibitory rebound was present. Other short-pass cells, however, showed inhibition only for long-duration tone bursts. Bandpass duration selectivity also involved interplay between inhibition and excitation; inhibition negated excitation with tone bursts that exceeded the optimum duration. Additionally, however, bandpass selectivity arose from stimulus-dependent excitation; tone bursts of sufficiently long duration were required to elicit excitation. Similarly, long-pass neurons showed inhibition and duration-dependent enhancement of excitation; long-pass selectivity resulted from enhanced excitation outlasting the transient inhibition or, in some cases, excitation overriding concurrent inhibition. Last, we evaluated the stimulus specificity of duration-selective neurons to variations in pulse repetition rate. We show that (1) most neurons that exhibited long-pass selectivity for tone-burst duration nonetheless responded to short-duration pulses when repeated at particular rates, and (2) some neurons that showed selectivity for tone burst duration also showed selectivity for pulse train duration. These novel response profiles appear to result from interplay between inhibition and time- and activity-dependent changes in excitation strength. These findings are discussed in the context of prevailing models of duration selectivity and acoustic communication in anurans.

Animals, including humans, use interaural time differences (ITDs) that arise from different sound path lengths to the two ears as a cue of horizontal sound source location. The nature of the neural code for ITD is still controversial. Current models differentiate between two population codes: either a map-like rate-place code of ITD along an array of neurons, consistent with a large body of data in the barn owl, or a population rate code, consistent with data from small mammals. Recently, it was proposed that these different codes reflect optimal coding strategies that depend on head size and sound frequency. The chicken makes an excellent test case of this proposal because its physical prerequisites are similar to small mammals, yet it shares a more recent common ancestry with the owl. We show here that, like in the barn owl, the brainstem nucleus laminaris in mature chickens displayed the major features of a place code of ITD. ITD was topographically represented in the maximal responses of neurons along each isofrequency band, covering approximately the contralateral acoustic hemisphere. Furthermore, the represented ITD range appeared to change with frequency, consistent with a pressure gradient receiver mechanism in the avian middle ear. At very low frequencies, below 400 Hz, maximal neural responses were symmetrically distributed around zero ITD and it remained unclear whether there was a topographic representation. These findings do not agree with the above predictions for optimal coding and thus revive the discussion as to what determines the neural coding strategies for ITDs.

Auditory information is processed in a fine-to-crude hierarchical scheme, from low-level acoustic information to high-level abstract representations, such as phonological labels. We now ask whether fine acoustic information, which is not retained at high levels, can still be used to extract speech from noise. Previous theories suggested either full availability of low-level information or availability that is limited by task difficulty. We propose a third alternative, based on the Reverse Hierarchy Theory (RHT), originally derived to describe the relations between the processing hierarchy and visual perception. RHT asserts that only the higher levels of the hierarchy are immediately available for perception. Direct access to low-level information requires specific conditions, and can be achieved only at the cost of concurrent comprehension. We tested the predictions of these three views in a series of experiments in which we measured the benefits from utilizing low-level binaural information for speech perception, and compared it to that predicted from a model of the early auditory system. Only auditory RHT could account for the full pattern of the results, suggesting that similar defaults and tradeoffs underlie the relations between hierarchical processing and perception in the visual and auditory modalities.

Two previous studies manipulated spatial cues to alter the perceptual organization of a sound mixture containing an ambiguous sound element (a pure tone; the "target") that could belong to two competing auditory objects (a sequential tone stream and a simultaneous harmonic complex). In both studies, the sum of the contributions of the target to the two objects was less than the physical target level in the mixture. However, many listeners had difficulties making consistent judgments about the perceptual contribution of the target to the harmonic complex. The current study used stimuli similar to those used in the previous study, but with a target made up of five tones rather than a single pure tone. In addition, listeners performed a direct matching task to indicate the perceptual contribution of the target to the competing objects rather than relying on an indirect mapping procedure. The matching task proved to be efficient and reliable. However, the complex-tone target was perceptually stronger in the harmonic complex and weaker in the sequential tone stream than in past studies. As a result, the sum of the target contributions to the two objects roughly equaled the physical target level for all tested spatial configurations, unlike in the previous studies.

The lateral superior olive (LSO) is one of the earliest sites in the auditory pathway that is involved in processing acoustical cues to sound location. Here, we tested the hypothesis that LSO neurons can signal small changes in interaural level differences (ILDs), a cue to horizontal sound location, of pure tones based on discharge rate consistent with psychophysical performance in the discrimination of ILDs. Neural thresholds for ILD discrimination were determined from the discharge rates and associated response variability of single units in response to 300 ms tones in the LSO of barbiturate-anesthetized cats using detection theory. Neural response variability was well described by a power function of the mean rate, both in individual neurons and collectively; LSO neurons were less variable than expected from a Poisson process. Compared with psychophysical data, the best-threshold ILDs of single LSO neurons were comparable with or better than behavior over the full range of frequencies (0.3–35 kHz) and pedestal ILDs (±25 dB) explored in this study. With a pedestal ILD of 0 dB, ILD increments of 1 dB could be discriminated by some neurons, with a median of 4.35 dB across neurons. For pedestal ILDs away from 0 dB, the best-threshold ILDs were as low as 0.5 dB, with a median of 2.3 dB. These findings support the hypothesis that the LSO plays a role in the extraction of ILD, and that the representation of ILD by LSO neurons may set a lower bound on the behavioral sensitivity to ILDs.

Recent measurements of three-dimensional stapes motion in gerbil indicated that the piston component of stapes motion was the primary contributor to intracochlear pressure. In order to make a detailed correlation between stapes piston motion and intracochlear pressure behind the stapes, simultaneous pressure and motion measurements were undertaken. We found that the scala vestibuli pressure followed the piston component of the stapes velocity with high fidelity, reinforcing our previous finding that the piston motion of the stapes was the main stimulus to the cochlea. The present data allowed us to calculate cochlear input impedance and power flow into the cochlea. Both the amplitude and phase of the impedance were quite flat with frequency from 3 kHz to at least 30 kHz, with a phase that was primarily resistive. With constant stimulus pressure in the ear canal the intracochlear pressure at the stapes has been previously shown to be approximately flat with frequency through a wide range, and coupling that result with the present findings indicates that the power that flows into the cochlea is quite flat from about 3 to 30 kHz. The observed wide-band intracochlear pressure and power flow are consistent with the wide-band audiogram of the gerbil.

The results reported here for CBA/CaJ mice describe the effects of regular dosing with a common antiretroviral drug combination on outer hair cell (OHC) function using measures of 2f1-f2 distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs). Specifically, experimental mice were treated daily over a 3-mo period with the nucleoside reverse transcriptase inhibitors (NRTIs), zidovudine (ZDV) and lamivudine (3TC), dissolved in their drinking water, while their control counterparts received untreated water. DPOAE levels and ABR detection thresholds prior to and after 12 wk of NRTI treatment did not differ between experimental and control groups. To assess whether NRTI treatment potentiates the adverse effects of noise over-exposure on OHC function, both experimental and control mice were exposed 1 wk later, while still on the drug regimen, to a 10-kHz octave-band noise (OBN) at 105-dB SPL for 1h. A major outcome of the sound over-exposure episode was that the NRTI-pretreated mice showed significantly greater permanent OBN-induced reductions in DPOAE levels at 2 wk postexposure than were observed for the untreated control animals. These findings support the notion that a synergistic relationship exists between certain NRTIs and intense sounds in that such preexposure drug treatments produced greater noise-induced decreases in DPOAE activity than did noise exposure alone. This drug/noise interaction is consistent with the known harmful effects of NRTIs on cellular mitochondrial activity.

Application of ouabain to the round window membrane of the gerbil selectively induces the death of most spiral ganglion neurons and thus provides an excellent model for investigating the survival and differentiation of embryonic stem cells (ESCs) introduced into the inner ear. In this study, mouse ESCs were pretreated with a neural-induction protocol and transplanted into Rosenthal's canal (RC), perilymph, or endolymph of Mongolian gerbils either 1-3 days (early post-injury transplant group) or 7 days or longer (late post-injury transplant group) after ouabain injury. Overall, ESC survival in RC and perilymphatic spaces was significantly greater in the early post-injury microenvironment as compared to the later post-injury condition. Viable clusters of ESCs within RC and perilymphatic spaces appeared to be associated with neovascularization in the early post-injury group. A small number of ESCs transplanted within RC stained for mature neuronal or glial cell markers. ESCs introduced into perilymph survived in several locations, but most differentiated into glia-like cells. ESCs transplanted into endolymph survived poorly if at all. These experiments demonstrate that there is an optimal time window for engraftment and survival of ESCs that occurs in the early post-injury period.

Glucose transporter 5 (Glut5) is a high-affinity fructose transporter. It was proposed to be a motor protein or part of the motor complex required for cochlear amplification in outer hair cells (OHCs). Here we show that, in contrast to previous reports, Glut5 is undetectable, and possibly absent, in OHCs harvested from wildtype mice. Further, Glut5-deficient mice display normal OHC morphology and motor function (i.e., nonlinear capacitance and electromotility) and normal cochlear sensitivity and frequency selectivity. We conclude that Glut5 is not required for OHC motility or cochlear amplification.