Auditory Research publications

Interaural level differences (ILDs) are the dominant cue for localizing the sources of high frequency sounds that differ in azimuth. Neurons in the primary auditory cortex (A1) respond differentially to ILDs of simple stimuli such as tones and noise bands, but the extent to which this applies to complex natural sounds, such as vocalizations, is not known. In sufentanil/N2O anesthetized marmosets, we compared the responses of 76 A1 neurons to three vocalizations (Ock, Tsik, and Twitter) and pure tones at cells' characteristic frequency. Each stimulus was presented with ILDs ranging from 20 dB favoring the contralateral ear to 20 dB favoring the ipsilateral ear to cover most of the frontal azimuthal space. The response to each stimulus was tested at three average binaural levels (ABLs). Most neurons were sensitive to ILDs of vocalizations and pure tones. For all stimuli, the majority of cells had monotonic ILD sensitivity functions favoring the contralateral ear, but we also observed ILD sensitivity functions that peaked near the midline and functions favoring the ipsilateral ear. Representation of ILD in A1 was better for pure tones and the Ock vocalization in comparison to the Tsik and Twitter calls; this was reflected by higher discrimination indices and greater modulation ranges. ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes. Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones. They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

The kinase Akt is a key downstream mediator of the phosphoinositide-3-kinase signaling pathway and participates in a variety of cellular processes. Akt comprises three isoforms each encoded by a separate gene. There is evidence to indicate that Akt is involved in the survival and protection of auditory hair cells in vitro. However, little is known about the physiological role of Akt in the inner ear-especially in the intact animal. To elucidate this issue, we first analyzed the mRNA expression of the three Akt isoforms in the inner ear of C57/BL6 mice by real-time PCR. Next, we tested the susceptibility to gentamicin-induced auditory hair cell loss in isoform-specific Akt knockout mice compared to wild-types (C57/BL6) in vitro. To analyze the effect of gene deletion in vivo, hearing and cochlear microanatomy were evaluated in Akt isoform knockout animals. In this study, we found that all three Akt isoforms are expressed in the cochlea. Our results further indicate that Akt2 and Akt3 enhance hair cell resistance to ototoxicity, while Akt1 does not. Finally, we determined that untreated Akt1 and Akt2/Akt3 double knockout mice display significant hearing loss, indicating a role for these isoforms in normal hearing. Taken together, our results indicate that each of the Akt isoforms plays a distinct role in the mammalian inner ear.

A significant challenge in tinnitus research lies in explaining how acoustic insult leads to tinnitus in some individuals, but not others. One possibility is genetic variability in the expression and function of neuromodulators - components of neural signaling that alter the balance of excitation and inhibition in neural circuits. An example is nitric oxide (NO) - a free radical and potent neuromodulator in the mammalian brain - that regulates plasticity via both pre-synaptic and postsynaptic mechanisms. Changes in NO have previously been implicated in tinnitus generation, specifically in the ventral cochlear nucleus (VCN). Here, we examined nitric oxide synthase (NOS) - the enzyme responsible for NO production - in the guinea pig VCN following acoustic trauma. NOS was present in most cell types - including spherical and globular bushy cells, small, medium, and large multipolar cells, and octopus cells - spanning the entire extent of the VCN. The staining pattern was symmetrical in control animals. Unilateral acoustic over-exposure (AOE) resulted in marked asymmetries between ipsilateral and contralateral sides of the VCN in terms of the distribution of NOS across the cochlear nuclei in animals with behavioral evidence of tinnitus: fewer NOS-positive cells and a reduced level of NOS staining was present across the whole extent of the contralateral VCN, relative to the ipsilateral VCN. The asymmetric pattern of NOS-containing cells was observed as early as 1 day after AOE and was also present in some animals at 3, 7, and 21 days after AOE. However, it was not until 8 weeks after AOE, when tinnitus had developed, that asymmetries were significant overall, compared with control animals. Asymmetrical NOS expression was not correlated with shifts in the threshold hearing levels. Variability in NOS expression between animals may represent one underlying difference that can be linked to whether or not tinnitus develops after noise exposure.

Chronic tinnitus, or "ringing of the ears", affects upwards of 15% of the adult population. Identifying a cost-effective and objective measure of tinnitus is needed due to legal concerns and disability issues, as well as for facilitating the effort to assess neural biomarkers. We developed a modified gap-in-noise (GIN) paradigm to assess tinnitus in mice using the auditory brainstem response (ABR). We then compared the commonly used acoustic startle reflex gap-prepulse inhibition (gap-PPI) and the ABR GIN paradigm in young adult CBA/CaJ mice before and after administrating sodium salicylate (SS), which is known to reliably induce a 16 kHz tinnitus percept in rodents. Post-SS, gap-PPI was significantly reduced at 12 and 16 kHz, consistent with previous studies demonstrating a tinnitus-induced gap-PPI reduction in this frequency range. ABR audiograms indicated thresholds were significantly elevated post-SS, also consistent with previous studies. There was a significant increase in the peak 2 (P2) to peak 1 (P1) and peak 4 (P4) to P1 amplitude ratios in the mid-frequency range, along with decreased latency of P4 at higher intensities. For the ABR GIN, peak amplitudes of the response to the second noise burst were calculated as a percentage of the first noise burst response amplitudes to quantify neural gap processing. A significant decrease in this ratio (i.e. recovery) was seen only at 16 kHz for P1, indicating the presence of tinnitus near this frequency. Thus, this study demonstrates that GIN ABRs can be used as an efficient, non-invasive, and objective method of identifying the approximate pitch and presence of tinnitus in a mouse model. This technique has the potential for application in human subjects and also indicates significant, albeit different, deficits in temporal processing in peripheral and brainstem circuits following drug induced tinnitus.

Fragile X Syndrome (FXS), a neurodevelopmental disorder, is the most prevalent single-gene cause of autism spectrum disorder. Autism has been associated with impaired auditory processing, abnormalities in the auditory brainstem response (ABR), and reduced cell number and size in the auditory brainstem nuclei. FXS is characterized by elevated cortical responses to sound stimuli, with some evidence for aberrant ABRs. Here, we assessed ABRs and auditory brainstem anatomy in Fmr1-/- mice, an animal model of FXS. We found that Fmr1-/- mice showed elevated response thresholds to both click and tone stimuli. Amplitudes of ABR responses were reduced in Fmr1-/- mice for early peaks of the ABR. The growth of the peak I response with sound intensity was less steep in mutants that in wild type mice. In contrast, amplitudes and response growth in peaks IV and V did not differ between these groups. We did not observe differences in peak latencies or in interpeak latencies. Cell size was reduced in Fmr1-/- mice in the ventral cochlear nucleus (VCN) and in the medial nucleus of the trapezoid body (MNTB). We quantified levels of inhibitory and excitatory synaptic inputs in these nuclei using markers for presynaptic proteins. We measured VGAT and VGLUT immunolabeling in VCN, MNTB, and the lateral superior olive (LSO). VGAT expression in MNTB was significantly greater in the Fmr1-/- mouse than in wild type mice. Together, these observations demonstrate that FXS affects peripheral and central aspects of hearing and alters the balance of excitation and inhibition in the auditory brainstem.

Tinnitus is often identified in animal models by using the gap prepulse inhibition of acoustic startle. Impaired gap detection following acoustic over-exposure (AOE) is thought to be caused by tinnitus "filling in" the gap, thus, reducing its salience. This presumably involves altered perception, and could conceivably be caused by changes at the level of the neocortex, i.e., cortical reorganization. Alternatively, reduced gap detection ability might reflect poorer temporal processing in the brainstem, caused by AOE; in which case, impaired gap detection would not be a reliable indicator of tinnitus. We tested the latter hypothesis by examining gap detection in inferior colliculus (IC) neurons following AOE. Seven of nine unilaterally noise-exposed guinea pigs exhibited behavioral evidence of tinnitus. In these tinnitus animals, neural gap detection thresholds (GDTs) in the IC significantly increased in response to broadband noise stimuli, but not to pure tones or narrow-band noise. In addition, when IC neurons were sub-divided according to temporal response profile (onset vs. sustained firing patterns), we found a significant increase in the proportion of onset-type responses after AOE. Importantly, however, GDTs were still considerably shorter than gap durations commonly used in objective behavioral tests for tinnitus. These data indicate that the neural changes observed in the IC are insufficient to explain deficits in behavioral gap detection that are commonly attributed to tinnitus. The subtle changes in IC neuron response profiles following AOE warrant further investigation.

Animal models of tinnitus allow us to study the relationship between changes in neural activity and the tinnitus percept. Here, guinea pigs were subjected to unilateral noise trauma and tested behaviourally for tinnitus 8 weeks later. By comparing animals with tinnitus with those without, all of which were noise‐exposed, we were able to identify changes unique to the tinnitus group. Three physiological markers known to change following noise exposure were examined: spontaneous firing rates ( SFR s) and burst firing in the inferior colliculus ( IC ), evoked auditory brainstem responses ( ABR s), and the number of neurons in the cochlear nucleus containing nitric oxide synthase ( NOS ). We obtained behavioural evidence of tinnitus in 12 of 16 (75%) animals. Both SFR s and incidences of burst firing were elevated in the IC of all noise‐exposed animals, but there were no differences between tinnitus and no‐tinnitus animals. There were significant decreases in ipsilateral ABR latencies in tinnitus animals, contrary to what might be expected with a small hearing loss. Furthermore, there was an ipsilateral–contralateral asymmetry in NOS staining in the ventral cochlear nucleus ( VCN ) that was only apparent in tinnitus animals. Tinnitus animals had a significantly greater number of NOS ‐containing neurons on the noise‐exposed side, whereas no‐tinnitus animals did not. These data suggest that measuring NOS in the VCN and recording ABR s supplement behavioural methods for confirming tinnitus in animals, and that nitric oxide is involved in plastic neural changes associated with tinnitus.

The neural substrate for the perception of vocalizations is relatively well described, but how their timing and specificity are tightly coupled with accompanying physiological changes and context-appropriate behaviors remains unresolved. We hypothesized that temporally integrated vocal and emotive responses, especially the expression of fear, vigilance and aggression, originate within the amygdala. To test this hypothesis, we performed electrical microstimulation at 461 highly restricted loci within the basal and central amygdala in awake mustached bats. At a subset of these sites, high frequency stimulation with weak constant current pulses presented at near-threshold levels triggered vocalization of either echolocation pulses or social calls. At the vast majority of locations, microstimulation produced a constellation of changes in autonomic and somatomotor outputs. These changes included widespread co-activation of significant tachycardia and hyperventilation and/or rhythmic ear pinna movements (PMs). In a few locations, responses were constrained to vocalization and/or PMs despite increases in the intensity of stimulation. The probability of eliciting echolocation pulses vs. social calls decreased in a medial-posterior to anterolateral direction within the centrobasal amygdala. Microinjections of kainic acid (KA) at stimulation sites confirmed the contribution of cellular activity rather than fibers-of-passage in the control of multimodal outputs. The results suggest that localized clusters of neurons may simultaneously modulate the activity of multiple central pattern generators (CPGs) present within the brainstem.

Introduction: This study aimed to assess the top-down control of sound processing in the auditory brainstem of rats. Short latency evoked responses were analyzed after unilateral or bilateral ablation of auditory cortex. This experimental paradigm was also used towards analyzing the long-term evolution of post-lesion plasticity in the auditory system and its ability to self-repair. Method: Auditory cortex lesions were performed in rats by stereotactically guided fine-needle aspiration of the cerebrocortical surface. Auditory Brainstem Responses (ABR) were recorded at post-surgery day (PSD) 1, 7, 15 and 30. Recordings were performed under closed-field conditions, using click trains at different sound intensity levels, followed by statistical analysis of threshold values and ABR amplitude and latency variables. Subsequently, brains were sectioned and immunostained for GAD and parvalbumin to assess the location and extent of lesions accurately. Results: Alterations in ABR variables depended on the type of lesion and post-surgery time of ABR recordings. Accordingly, bilateral ablations caused a statistically significant increase in thresholds at PSD1 and 7 and a decrease in waves amplitudes at PSD1 that recover at PSD7. No effects on latency were noted at PSD1 and 7, whilst recordings at PSD15 and 30 showed statistically significant decreases in latency. Conversely, unilateral ablations had no effect on auditory thresholds or latencies, while wave amplitudes only decreased at PSD1 strictly in the ipsilateral ear. Conclusion: Post-lesion plasticity in the auditory system acts in two time periods: short-term period of decreased sound sensitivity (until PSD7), most likely resulting from axonal degeneration; and a long-term period (up to PSD7), with changes in latency responses and recovery of thresholds and amplitudes values. The cerebral cortex may have a net positive gain on the auditory pathway response to sound.

Vocal communication is an important aspect of guinea pig behaviour and a large contributor to their acoustic environment. We postulated that some cortical areas have distinctive roles in processing conspecific calls. In order to test this hypothesis we presented exemplars from all ten of their main adult vocalizations to urethane anesthetised animals while recording from each of the eight areas of the auditory cortex. We demonstrate that the primary area (AI) and three adjacent auditory belt areas contain many units that give isomorphic responses to vocalizations. These are the ventrorostral belt (VRB), the transitional belt area (T) that is ventral to AI and the small area (area S) that is rostral to AI. Area VRB has a denser representation of cells that are better at discriminating among calls by using either a rate code or a temporal code than any other area. Furthermore, 10% of VRB cells responded to communication calls but did not respond to stimuli such as clicks, broadband noise or pure tones. Area S has a sparse distribution of call responsive cells that showed excellent temporal locking, 31% of which selectively responded to a single call. AI responded well to all vocalizations and was much more responsive to vocalizations than the adjacent dorsocaudal core area. Areas VRB, AI and S contained units with the highest levels of mutual information about call stimuli. Area T also responded well to some calls but seems to be specialized for low sound levels. The two dorsal belt areas are comparatively unresponsive to vocalizations and contain little information about the calls. AI projects to areas S, VRB and T, so there may be both rostral and ventral pathways for processing vocalizations in the guinea pig.

The mammalian outer hair cells (OHCs) provide a positive mechanical feedback to enhance the cochlea's hearing sensitivity and frequency selectivity. Although the OHC-specific, somatic motor protein prestin is required for cochlear amplification, it remains unclear whether prestin can provide sufficient cycle-by-cycle feedback. In cochlear mechanical modeling, varying amounts of OHC motor activity should provide varying degrees of feedback efficiency to adjust the gain of cochlear amplifier at resonant frequencies. Here we created and characterized two new prestin-hypomorphic mouse models with reduced levels of wild-type prestin. OHCs from these mice exhibited length, total elementary charge movement (Q(max)), charge density, and electromotility intermediate between those of wild-type and prestin-null mice. Remarkably, measurements of auditory brainstem responses and distortion product otoacoustic emissions from these mice displayed wild-type like hearing sensitivities at 4-22 kHz. These results indicate that as low as 26.7% Q(max), 34.0% charge density and 44.0% electromotility in OHCs were sufficient for wild-type-like hearing sensitivity in mice at 4-22 kHz, and that these in vitro parameters of OHCs did not correlate linearly with the feedback efficiency for in vivo gain of the cochlear amplifier. Our results thus provide valuable data for modeling cochlear mechanics and will stimulate further mechanistic analysis of the cochlear amplifier.

Auditory cortical plasticity can be induced through various approaches. The medial geniculate body (MGB) of the auditory thalamus gates the ascending auditory inputs to the cortex. The thalamocortical system has been proposed to play a critical role in the responses of the auditory cortex (AC). In the present study, we investigated the cellular mechanism of the cortical activity, adopting an in vivo intracellular recording technique, recording from the primary auditory cortex (AI) while presenting an acoustic stimulus to the rat and electrically stimulating its MGB. We found that low-frequency stimuli enhanced the amplitudes of sound-evoked excitatory postsynaptic potentials (EPSPs) in AI neurons, whereas high-frequency stimuli depressed these auditory responses. The degree of this modulation depended on the intensities of the train stimuli as well as the intervals between the electrical stimulations and their paired sound stimulations. These findings may have implications regarding the basic mechanisms of MGB activation of auditory cortical plasticity and cortical signal processing.

Acoustic communication has an important role in the reproductive behaviour of anurans. Although males of the concave-eared frog (Odorrana tormota) have shown an ultrasonic communication capacity adapted to the intense, predominately low-frequency ambient noise from local streams, whether the females communicate with ultrasound remains unclear. Here we present evidence that females exhibit no ultrasonic sensitivity. Acoustic playback experiments show that the calls from male evoke phonotaxis and vocal responses from gravid females, whereas the ultrasonic components (frequencies above 20 kHz) of the calls do not elicit any phonotaxis or vocalization in the females. Electrophysiological recordings from the auditory midbrain reveal an upper frequency limit at 16 kHz in females. Laser Doppler vibrometer measurements show that the velocity amplitude of the tympanic membranes peaks at 5 kHz in females and at ∼7 kHz in males. The auditory sex differences in O. tormota imply that ultrasonic hearing has evolved only in male anurans.

We investigated how physiologically observed forward suppression interacts with stimulus frequency in neuronal responses in the guinea pig auditory cortex. The temporal order and frequency proximity of sounds influence both their perception and neuronal responses. Psychophysically, preceding sounds (conditioners) can make successive sounds (probes) harder to hear. These effects are larger when the two sounds are spectrally similar. Physiological forward suppression is usually maximal for conditioner tones near to a unit's characteristic frequency (CF), the frequency to which a neuron is most sensitive. However, in most physiological studies, the frequency of the probe tone and CF are identical, so the role of unit CF and probe frequency cannot be distinguished. Here, we systemically varied the frequency of the probe tone, and found that the tuning of suppression was often more closely related to the frequency of the probe tone than to the unit's CF, i.e. suppressed tuning was specific to probe frequency. This relationship was maintained for all measured gaps between the conditioner and the probe tones. However, when the probe frequency and CF were similar, CF tended to determine suppressed tuning. In addition, the bandwidth of suppression was slightly wider for off-CF probes. Changes in tuning were also reflected in the firing rate in response to probe tones, which was maximally reduced when probe and conditioner tones were matched in frequency. These data are consistent with the idea that cortical neurons receive convergent inputs with a wide range of tuning properties that can adapt independently.

Reversible inactivation of the cortex by surface cooling is a powerful method for studying the function of a particular area. Implanted cooling cryoloops have been used to study the role of individual cortical areas in auditory processing of awake-behaving cats. Cryoloops have also been used in rodents for reversible inactivation of the cortex, but recently there has been a concern that the cryoloop may also cool non-cortical structures either directly or via the perfusion of blood, cooled as it passed close to the cooling loop. In this study we have confirmed that the loop can inactivate most of the auditory cortex without causing a significant reduction in temperature of the auditory thalamus or other subcortical structures. We placed a cryoloop on the surface of the guinea pig cortex, cooled it to 2°C and measured thermal gradients across the neocortical surface. We found that the temperature dropped to 20-24°C among cells within a radius of about 2.5 mm away from the loop. This temperature drop was sufficient to reduce activity of most cortical cells and led to the inactivation of almost the entire auditory region. When the temperature of thalamus, midbrain, and middle ear were measured directly during cortical cooling, there was a small drop in temperature (about 4°C) but this was not sufficient to directly reduce neural activity. In an effort to visualize the extent of neural inactivation we measured the uptake of thallium ions following an intravenous injection. This confirmed that there was a large reduction of activity across much of the ipsilateral cortex and only a small reduction in subcortical structures.

Vestibular evoked myogenic potentials (VEMPs) were measured in 22 unilateral Menière patients with monaural and binaural stimulation with 250 and 500 Hz tone bursts. For all measurement situations significantly lower VEMP amplitudes were on average measured at the affected side compared to the unaffected side. Unilateral Menière patients have, in contrast to normal subjects, asymmetric VEMPs, indicating a permanently affected vestibular (most likely otolith) system at the side of hearing loss. The diagnostic value of VEMP amplitude asymmetry measurement in individual patients is low, because of the large overlap of the VEMP amplitude asymmetry range for unilateral Menière patients with that for normal subjects.

Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

Neurons in the auditory midbrain are sensitive to differences in the timing of sounds at the two ears—an important sound localization cue. We used broadband noise stimuli to investigate the interaural-delay sensitivity of low-frequency neurons in two midbrain nuclei: the inferior colliculus (IC) and the dorsal nucleus of the lateral lemniscus. Noise-delay functions showed asymmetries not predicted from a linear dependence on interaural correlation: a stretching along the firing-rate dimension ( rate asymmetry), and a skewing along the interaural-delay dimension ( delay asymmetry). These asymmetries were produced by an envelope-sensitive component to the response that could not entirely be accounted for by monaural or binaural nonlinearities, instead indicating an enhancement of envelope sensitivity at or after the level of the superior olivary complex. In IC, the skew-like asymmetry was consistent with intermediate-type responses produced by the convergence of ipsilateral peak-type inputs and contralateral trough-type inputs. This suggests a stereotyped pattern of input to the IC. In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays. These findings have important consequences for the neural representation of interaural timing differences and interaural correlation—cues critical to the perception of acoustic space.

Background: Mucopolysaccharidosis (MPS) IIIB (Sanfilippo Syndrome type B) is caused by a deficiency in the lysosomal enzyme N-acetyl-glucosaminidase (Naglu). Children with MPS IIIB develop disturbances of sleep, activity levels, coordination, vision, hearing, and mental functioning culminating in early death. The murine model of MPS IIIB demonstrates lysosomal distention in multiple tissues, a shortened life span, and behavioral changes. Principal findings: To more thoroughly assess MPS IIIB in mice, alterations in circadian rhythm, activity level, motor function, vision, and hearing were tested. The suprachiasmatic nucleus (SCN) developed pathologic changes and locomotor analysis showed that MPS IIIB mice start their daily activity later and have a lower proportion of activity during the night than wild-type controls. Rotarod assessment of motor function revealed a progressive inability to coordinate movement in a rocking paradigm. Purkinje cell counts were significantly reduced in the MPS IIIB animals compared to age matched controls. By electroretinography (ERG), MPS IIIB mice had a progressive decrease in the amplitude of the dark-adapted b-wave response. Corresponding pathology revealed shortening of the outer segments, thinning of the outer nuclear layer, and inclusions in the retinal pigmented epithelium. Auditory-evoked brainstem responses (ABR) demonstrated progressive hearing deficits consistent with the observed loss of hair cells in the inner ear and histologic abnormalities in the middle ear. Conclusions/significance: The mouse model of MPS IIIB has several quantifiable phenotypic alterations and is similar to the human disease. These physiologic and histologic changes provide insights into the progression of this disease and will serve as important parameters when evaluating various therapies.