Publications using TDT

Objectives: To model the contribution of implant material and insertion trauma on loss of acoustic hearing after cochlear implantation in an appropriate animal model. Methods: Sixty-five C57Bl/6J mice underwent unilateral implantation with implant grade materials: 2 implant grade silicones and a third uncoated platinum wire. A sham surgery group was included as a control. Serial auditory brainstem response (ABR) thresholds and distortion product otoacoustic emissions (DPOAEs) were used to discern effects on hearing over 22 weeks. Histologic measurements of damage to the organ of Corti and spiral ganglion were correlated with degree of hearing loss and material type. Results: Organ of Corti damage correlated with rate of hearing loss soon after implantation (0-2 weeks) but not subsequently (2-22 weeks). Organ of Corti damage did not depend on implant type and was present even in sham surgery subjects when hearing was severely damaged. Spiral ganglia appeared unaffected. There was no evidence of an inflammatory or toxic effect of the materials beyond the site of implant insertion. Conclusions: Hearing loss and cochlear damage appear to be related to insertion trauma, with minimal effect on delayed hearing loss caused by different materials. In the C57Bl/6J mouse model, the sensory epithelium appears to be the location of damage after cochlear implantation.

Mutations in the potassium channel subunit KCNQ1 cause the human severe congenital deafness Jervell and Lange‐Nielsen (JLN) syndrome. We applied a gene therapy approach in a mouse model of JLN syndrome ( Kcnq1 −/− mice) to prevent the development of deafness in the adult stage. A modified adeno‐associated virus construct carrying a Kcnq1 expression cassette was injected postnatally (P0–P2) into the endolymph, which resulted in Kcnq1 expression in most cochlear marginal cells where native Kcnq1 is exclusively expressed. We also found that extensive ectopic virally mediated Kcnq1 transgene expression did not affect normal cochlear functions. Examination of cochlear morphology showed that the collapse of the Reissner's membrane and degeneration of hair cells (HCs) and cells in the spiral ganglia were corrected in Kcnq1 −/− mice. Electrophysiological tests showed normal endocochlear potential in treated ears. In addition, auditory brainstem responses showed significant hearing preservation in the injected ears, ranging from 20 dB improvement to complete correction of the deafness phenotype. Our results demonstrate the first successful gene therapy treatment for gene defects specifically affecting the function of the stria vascularis, which is a major site affected by genetic mutations in inherited hearing loss.

We present a noninvasive method of photoacoustic tomography (PAT) for imaging cerebral hemodynamics in awake-moving rats. The wearable PAT (wPAT) system has a size of 15 mm in height and 33 mm in diameter, and a weight of ~ 8 g (excluding cabling). The wPAT achieved an imaging rate of 3.33 frames/s with a lateral resolution of 243 μm. Animal experiments were designed to show wPAT feasibility for imaging cerebral hemodynamics on awake-moving animals. Results showed that the cerebral oxy-hemoglobin and deoxy-hemoglobin changed significantly in response to hyperoxia; and, after the injection of pentylenetetrazol (PTZ), cerebral blood volume changed faster over time and larger in amplitude for rats in awake-moving state compared with rats under anesthesia. By providing a light-weight, high-resolution technology for in vivo monitoring of cerebral hemodynamics in awake-behaving animals, it will be possible to develop a comprehensive understanding on how activity alters hemodynamics in normal and diseased states.

Objectives: Assess postcranial irradiation: (1) short-term threshold shift, (2) short-term peripheral auditory histopathology, and (3) the mouse as an experimental model. Methods: Adult mice were exposed to single-dose radiation of 10 to 60 Gy. Pre- and post-irradiation (baseline, 2-8 days) audiometric brainstem response data were recorded with analysis of cochlear ultrastructure. Results: Significant threshold shift occurred at all test frequencies in mice exposed to ≥20 Gy at 4 to 6 days post-irradiation. Ultrastructurally in Rosenthal’s canal and the spiral lamina, neuronal density and extracellular matrix decreased dramatically. There was overall preservation of hair cells, stria vascularis, and vasculature. No difference within Gy group was noted in the frequency or severity of pathology along the length of the cochlea. Conclusions: The initial impact of radiation in the first week post-exposure focuses on spiral ganglion cell bodies and peripheral projections, resulting in significant threshold shift for irradiation dosages ≥20 Gy. This study demonstrates that the mouse is a viable model for study of short-term peripheral auditory effects using single-dose cranial irradiation. Additionally, with access to a precise animal irradiator, the mouse may be used as an experimental model for a fractionated irradiation dosage of 10 Gy, simulating stereotactic therapeutic cranial irradiation.

Previous research suggests that within Mandarin-speaking congenital amusics, only a subgroup has behavioral lexical tone perception impairments (tone agnosia), whereas the rest of amusics do not. The purpose of the current study was to investigate the categorical nature of lexical tone perception in Mandarin-speaking amusics with and without behavioral lexical tone deficits. Three groups of listeners (controls, pure amusics, and amusics with tone agnosia) participated in tone identification and discrimination tasks. Indexes of the categorical perception (CP) of a physical continuum of fundamental frequencies ranging from a rising to level tone were measured. Specifically, the stimulus durations were manipulated at 100 and 200 ms. For both stimulus durations, all groups exhibited similar categorical boundaries. The pure amusics showed sharp identification slopes and significantly peaked discrimination functions similar to those of normal controls. However, such essential characteristics for the CP of lexical tones were not observed in amusics with tone agnosia. An enlarged step-size from 20 to 35 Hz was not able to produce any discrimination peaks in tone agnosics either. The current study revealed that only amusics with tone agnosia showed a lack of categorical tone perception, while the pure amusics demonstrated typical CP of lexical tones, indicating that the deficit of pitch processing in music does not necessarily result in the deficit in the CP of lexical tones. The different performance between congenital amusics with and without tone agnosia provides a new perspective on the proposition of the relationship between music and speech perception.

The vibratory responses to tones of the stapes and incus were measured in the middle ears of deeply anesthetized chinchillas using a wide-band acoustic-stimulus system and a laser velocimeter coupled to a microscope. With the laser beam at an angle of about 40 ° relative to the axis of stapes piston-like motion, the sensitivity-vs.-frequency curves of vibrations at the head of the stapes and the incus lenticular process were very similar to each other but larger, in the range 15-30 kHz, than the vibrations of the incus just peripheral to the pedicle. With the laser beam aligned with the axis of piston-like stapes motion, vibrations of the incus just peripheral to its pedicle were very similar to the vibrations of the lenticular process or the stapes head measured at the 40 ° angle. Thus, the pedicle prevents transmission to the stapes of components of incus vibration not aligned with the axis of stapes piston-like motion. The mean magnitude curve of stapes velocities is fairly flat over a wide frequency range, with a mean value of about 0.19 mm(.)(s Pa(-1)), has a high-frequency cutoff of 25 kHz (measured at -3 dB re the mean value), and decreases with a slope of about -60 dB/octave at higher frequencies. According to our measurements, the chinchilla middle ear transmits acoustic signals into the cochlea at frequencies exceeding both the bandwidth of responses of auditory-nerve fibers and the upper cutoff of hearing. The phase lags of stapes velocity relative to ear-canal pressure increase approximately linearly, with slopes equivalent to pure delays of about 57-76 μs.

Georg von Békésy observed that the onset times of responses to brief-duration stimuli vary as a function of distance from the stapes, with basal regions starting to move earlier than apical ones. He noticed that the speed of signal propagation along the cochlea is slow when compared with the speed of sound in water. Fast traveling waves have been recorded in the cochlea, but their existence is interpreted as the result of an experiment artifact. Accounts of the timing of vibration onsets at the base of the cochlea generally agree with Békésy's results. Some authors, however, have argued that the measured delays are too short for consistency with Békésy's theory. To investigate the speed of the traveling wave at the base of the cochlea, we analyzed basilar membrane (BM) responses to clicks recorded at several locations in the base of the chinchilla cochlea. The initial component of the BM response matches remarkably well the initial component of the stapes response, after a 4-μs delay of the latter. A similar conclusion is reached by analyzing onset times of time-domain gain functions, which correspond to BM click responses normalized by middle-ear input. Our results suggest that BM responses to clicks arise from a combination of fast and slow traveling waves.

Voltage-gated Kv7 (KCNQ) channels are voltage-dependent potassium channels that are activated at resting membrane potentials and therefore provide a powerful brake on neuronal excitability. Genetic or experience-dependent reduction of KCNQ2/3 channel activity is linked with disorders that are characterized by neuronal hyperexcitability, such as epilepsy and tinnitus. Retigabine, a small molecule that activates KCNQ2-5 channels by shifting their voltage-dependent opening to more negative voltages, is an US Food and Drug Administration (FDA) approved anti-epileptic drug. However, recently identified side effects have limited its clinical use. As a result, the development of improved KCNQ2/3 channel activators is crucial for the treatment of hyperexcitability-related disorders. By incorporating a fluorine substituent in the 3-position of the tri-aminophenyl ring of retigabine, we synthesized a small-molecule activator (SF0034) with novel properties. Heterologous expression of KCNQ2/3 channels in HEK293T cells showed that SF0034 was five times more potent than retigabine at shifting the voltage dependence of KCNQ2/3 channels to more negative voltages. Moreover, unlike retigabine, SF0034 did not shift the voltage dependence of either KCNQ4 or KCNQ5 homomeric channels. Conditional deletion of Kcnq2 from cerebral cortical pyramidal neurons showed that SF0034 requires the expression of KCNQ2/3 channels for reducing the excitability of CA1 hippocampal neurons. Behavioral studies demonstrated that SF0034 was a more potent and less toxic anticonvulsant than retigabine in rodents. Furthermore, SF0034 prevented the development of tinnitus in mice. We propose that SF0034 provides, not only a powerful tool for investigating ion channel properties, but, most importantly, it provides a clinical candidate for treating epilepsy and preventing tinnitus.

We investigated whether hearing advertisement calls over several nights, as happens in natural frog choruses, modified the responses of the peripheral auditory system in the green treefrog, Hyla cinerea . Using auditory evoked potentials (AEP), we found that exposure to 10 nights of a simulated male chorus lowered auditory thresholds in males and females, while exposure to random tones had no effect in males, but did result in lower thresholds in females. The threshold change was larger at the lower frequencies stimulating the amphibian papilla than at higher frequencies stimulating the basilar papilla. Suprathreshold responses to tonal stimuli were assessed for two peaks in the AEP recordings. For the peak P1 (assessed for 0.8–1.25 kHz), peak amplitude increased following chorus exposure. For peak P2 (assessed for 2–4 kHz), peak amplitude decreased at frequencies between 2.5 and 4.0 kHz, but remained unaltered at 2.0 kHz. Our results show for the first time, to our knowledge, that hearing dynamic social stimuli, like frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Noise-induced hearing loss (NIHL) is one of the most frequent disabilities in industrialized countries. It has been demonstrated that hair cell loss in the auditory end organ may account for the majority of ear pathological conditions. Previous studies have indicated that histone deacetylases (HDACs) play an important role in neurodegenerative diseases, including hearing impairment, in older persons. Thus, we hypothesized that the inhibition of HDACs would prevent hair cell loss and, consequently, NIHL. In the present study, a CBA/J mouse model of NIHL was established. Following an injection with the HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA), the expression levels of HDAC1, HDAC4 and acetyl-histone H3 (Lys9) (H3-AcK9) were measured. The number of hair cells was quantified and their morphology was observed. The results revealed that 1 h following exposure to 110 dB SPL broadband noise, there was a significant increase in HDAC1 and HDAC4 expression, and a marked decrease in the H3-AcK9 protein levels, as shown by western blot analysis. Pre-treatment with SAHA significantly inhibited these effects. Two weeks following exposure to noise, the mice exhibited significant hearing impairment and an obvious loss in the number of outer hair cells. An abnormal cell morphology with cilia damage was also observed. Pre-treatment with SAHA markedly attenuated these noise-induced effects. Taken together, the findings of our study suggest that HDAC expression is associated with outer hair cell function and plays a significant role in NIHL. Our data indicate that SAHA may be a potential therapeutic agent for the prevention of NIHL.

Time-locked sequences of neural activity can be found throughout the vertebrate forebrain in various species and behavioral contexts. From "time cells" in the hippocampus of rodents to cortical activity controlling movement, temporal sequence generation is integral to many forms of learned behavior. However, the mechanisms underlying sequence generation are not well known. Here, we describe a spatial and temporal organization of the songbird premotor cortical microcircuit that supports sparse sequences of neural activity. Multi-channel electrophysiology and calcium imaging reveal that neural activity in premotor cortex is correlated with a length scale of 100 µm. Within this length scale, basal-ganglia-projecting excitatory neurons, on average, fire at a specific phase of a local 30 Hz network rhythm. These results show that premotor cortical activity is inhomogeneous in time and space, and that a mesoscopic dynamical pattern underlies the generation of the neural sequences controlling song.

Signal processing in the auditory brainstem is based on an interaction of neuronal excitation and inhibition. To date, we have incomplete knowledge of how the dynamic interplay of both contributes to the processing power and temporal characteristics of signal coding. The spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their primary excitatory input through auditory nerve fibers via large, axosomatic synaptic terminals called the endbulbs of Held and by additional, acoustically driven inhibitory inputs. SBCs provide the input to downstream nuclei of the brainstem sound source localization circuitry, such as the medial and lateral superior olive, which rely on temporal precise inputs. In this study, we used juxtacellular recordings in anesthetized Mongolian gerbils to assess the effect of acoustically evoked inhibition on the SBCs input–output function and on temporal precision of SBC spiking. Acoustically evoked inhibition proved to be strong enough to suppress action potentials (APs) of SBCs in a stimulus-dependent manner. Inhibition shows slow onset and offset dynamics and increasing strength at higher sound intensities. In addition, inhibition decreases the rising slope of the EPSP and prolongs the EPSP-to-AP transition time. Both effects can be mimicked by iontophoretic application of glycine. Inhibition also improves phase locking of SBC APs to low-frequency tones by acting as a gain control to suppress poorly timed EPSPs from generating postsynaptic APs to maintain precise SBC spiking across sound intensities. The present data suggest that inhibition substantially contributes to the processing power of second-order neurons in the ascending auditory system.

Intracortical brain–machine interfaces (BMIs) may eventually restore function in those with motor disability after stroke. However, current research into the development of intracortical BMIs has focused on subjects with largely intact cortical structures, such as those with spinal cord injury. Although the stroke perilesional cortex (PLC) has been hypothesized as a potential site for a BMI, it remains unclear whether the injured motor cortical network can support neuroprosthetic control directly. Using chronic electrophysiological recordings in a rat stroke model, we demonstrate here the PLC's capacity for neuroprosthetic control and physiological plasticity. We initially found that the perilesional network demonstrated abnormally increased slow oscillations that also modulated neural firing. Despite these striking abnormalities, neurons in the perilesional network could be modulated volitionally to learn neuroprosthetic control. The rate of learning was surprisingly similar regardless of the electrode distance from the stroke site and was not significantly different from intact animals. Moreover, neurons achieved similar task-related modulation and, as an ensemble, formed cell assemblies with learning. Such control was even achieved in animals with poor motor recovery, suggesting that neuroprosthetic control is possible even in the absence of motor recovery. Interestingly, achieving successful control also reduced locking to abnormal oscillations significantly. Our results thus suggest that, despite the disrupted connectivity in the PLC, it may serve as an effective target for neuroprosthetic control in those with poor motor recovery after stroke.

Pannexin1 (Panx1) is a gap junction gene in vertebrates whose proteins mainly function as non-junctional channels on the cell surface. Panx1 channels can release ATP under physiological conditions and play critical roles in many physiological and pathological processes. Here, we report that Panx1 deficiency can reduce ATP release and endocochlear potential (EP) generation in the cochlea inducing hearing loss. Panx1 extensively expresses in the cochlea, including the cochlear lateral wall. We found that deletion of Panx1 in the cochlear lateral wall almost abolished ATP release under physiological conditions. Positive EP is a driving force for current through hair cells to produce auditory receptor potential. EP generation requires ATP. In the Panx1 deficient mice, EP and auditory receptor potential as measured by cochlear microphonics (CM) were significantly reduced. However, no apparent hair cell loss was detected. Moreover, defect of connexin hemichannels by deletion of connexin26 (Cx26) and Cx30, which are predominant connexin isoforms in the cochlea, did not reduce ATP release under physiological conditions. These data demonstrate that Panx1 channels dominate ATP release in the cochlea ensuring EP and auditory receptor potential generation and hearing. Panx1 deficiency can reduce ATP release and EP generation causing hearing loss.

Objective: Single carbon fiber electrodes (d = 8.4 μm) insulated with parylene-c and functionalized with Pedot: pTS have been shown to record single unit activity but manual implantation of these devices with forceps can be difficult. Without an improvement in the insertion method any increase in the channel count by fabricating carbon fiber arrays would be impractical. In this study, we utilize a water soluble coating and structural backbones that allow us to create, implant, and record from fully functionalized arrays of carbon fibers with ∼150 μm pitch. Approach: Two approaches were tested for the insertion of carbon fiber arrays. The first method used a poly(ethylene glycol) (PEG) coating that temporarily stiffened the fibers while leaving a small portion at the tip exposed. The small exposed portion (500 μm-1 mm) readily penetrated the brain allowing for an insertion that did not require the handling of each fiber by forceps. The second method involved the fabrication of silicon support structures with individual shanks spaced 150 μm apart. Each shank consisted of a small groove that held an individual carbon fiber. Main results: Our results showed that the PEG coating allowed for the chronic implantation of carbon fiber arrays in five rats with unit activity detected at 31 days post-implant. The silicon support structures recorded single unit activity in three acute rat surgeries. In one of those surgeries a stacked device with three layers of silicon support structures and carbon fibers was built and shown to readily insert into the brain with unit activity on select sites. Significance: From these studies we have found that carbon fibers spaced at ∼150 μm readily insert into the brain. This greatly increases the recording density of chronic neural probes and paves the way for even higher density devices that have a minimal scarring response.

Behavioral and EEG studies suggest spatial attention is allocated as a gradient in which processing benefits decrease away from an attended location. Yet the spatiotemporal dynamics of cortical processes that contribute to attentional gradients are unclear. We measured EEG while participants (n = 35) performed an auditory spatial attention task that required a button press to sounds at one target location on either the left or right. Distractor sounds were randomly presented at four non-target locations evenly spaced up to 180° from the target location. Attentional gradients were quantified by regressing ERP amplitudes elicited by distractors against their spatial location relative to the target. Independent component analysis was applied to each subject's scalp channel data, allowing isolation of distinct cortical sources. Results from scalp ERPs showed a tri-phasic response with gradient slope peaks at ~300 ms (frontal, positive), ~430 ms (posterior, negative), and a plateau starting at ~550 ms (frontal, positive). Corresponding to the first slope peak, a positive gradient was found within a central component when attending to both target locations and for two lateral frontal components when contralateral to the target location. Similarly, a central posterior component had a negative gradient that corresponded to the second slope peak regardless of target location. A right posterior component had both an ipsilateral followed by a contralateral gradient. Lateral posterior clusters also had decreases in α and β oscillatory power with a negative slope and contralateral tuning. Only the left posterior component (120-200 ms) corresponded to absolute sound location. The findings indicate a rapid, temporally-organized sequence of gradients thought to reflect interplay between frontal and parietal regions. We conclude these gradients support a target-based saliency map exhibiting aspects of both right-hemisphere dominance and opponent process models.

Significance Sound encoding relies on Ca 2+ -regulated transmitter release from inner hair cells (IHCs). Here we demonstrate a role of Ras-related in brain 3 (Rab3)–interacting molecule 2 (RIM2) in Ca 2+ channel-clustering and vesicle-tethering at the active zones of IHCs. Active zones of RIM2α-deficient IHCs cluster fewer synaptic voltage-gated Ca V 1.3 Ca 2+ channels, resulting in reduced synaptic Ca 2+ influx. Exocytosis was diminished in RIM2α-deficient IHCs, likely contributing to the mild hearing impairment of RIM2α knockout mice. Hair cell-specific disruption of all RIM2 isoforms caused a stronger decrease of Ca 2+ current and exocytosis in IHCs and impaired the encoding of sound onset in spiral ganglion neurons. We conclude that RIM2α and RIM2β promote synaptic clustering of Ca 2+ channels at IHC active zones and are required for normal hearing.

Current-displacement (I-X) and the force-displacement (F-X) relationships characterize hair-cell mechano-transduction in the inner ear. A common technique for measuring these relationships is to deliver mechanical stimulations to individual hair bundles with microprobes and measure whole cell transduction currents through patch pipette electrodes at the basolateral membrane. The sensitivity of hair-cell mechano-transduction is determined by two fundamental biophysical properties of the mechano-transduction channel, the stiffness of the putative gating spring and the gating swing, which are derived from the I-X and F-X relationships. Although the hair-cell stereocilia in vivo deflect <100 nm even at high sound pressure levels, often it takes >500 nm of stereocilia displacement to saturate hair-cell mechano-transduction in experiments with individual hair cells in vitro. Despite such discrepancy between in vivo and in vitro data, key biophysical properties of hair-cell mechano-transduction to define the transduction sensitivity have been estimated from in vitro experiments. Using three-dimensional finite-element methods, we modeled an inner hair-cell and an outer hair-cell stereocilia bundle and simulated the effect of probe stimulation. Unlike the natural situation where the tectorial membrane stimulates hair-cell stereocilia evenly, probes deflect stereocilia unevenly. Because of uneven stimulation, 1) the operating range (the 10-90% width of the I-X relationship) increases by a factor of 2-8 depending on probe shapes, 2) the I-X relationship changes from a symmetric to an asymmetric function, and 3) the bundle stiffness is underestimated. Our results indicate that the generally accepted assumption of parallel stimulation leads to an overestimation of the gating swing and underestimation of the gating spring stiffness by an order of magnitude.

In rats, forelimb movements can be evoked from two distinct cortical regions, the rostral (RFA) and the caudal (CFA) forelimb areas. RFA and CFA have numerous reciprocal connections, and their projections reach several common targets, which allows them to interact at multiple levels of the motor axis. Lesions affecting these areas result in profound and persistent deficits, supporting their essential role for the production of arm and hand movements. Whereas rats are widely used to study motor control and recovery following lesions, little is known as to how cortical motor areas in this model interact to generate movements. To study interactions between RFA and CFA, we used paired-pulse protocols with intracortical microstimulation techniques (ICMS). A conditioning stimulus (C) in RFA was applied simultaneously, or before a test stimulus (T) in CFA. The impact of RFA conditioning on CFA outputs was quantified by recording electromyographic signals (EMG) signals from the contralateral arm muscles. We found that stimulation of RFA substantially modulates the intensity of CFA outputs while only mildly affecting the latency. In general, the effect of RFA conditioning changed from predominantly facilitatory to inhibitory with increasing delays between the C and the T stimulus. However, inspection of individual cortical sites revealed that RFA has a wide range of influence on CFA outputs with each interstimulation delay we used. Our results show that RFA has powerful and complex modulatory effects on CFA outputs that can allow it to play a major role in the cortical control of forelimb movements.

The speech-evoked auditory brain stem response (speech ABR) is widely considered to provide an index of the quality of neural temporal encoding in the central auditory pathway. The aim of the present study was to evaluate the extent to which the speech ABR is shaped by spectral processing in the cochlea. High-pass noise masking was used to record speech ABRs from delimited octave-wide frequency bands between 0.5 and 8 kHz in normal-hearing young adults. The latency of the frequency-delimited responses decreased from the lowest to the highest frequency band by up to 3.6 ms. The observed frequency-latency function was compatible with model predictions based on wave V of the click ABR. The frequency-delimited speech ABR amplitude was largest in the 2- to 4-kHz frequency band and decreased toward both higher and lower frequency bands despite the predominance of low-frequency energy in the speech stimulus. We argue that the frequency dependence of speech ABR latency and amplitude results from the decrease in cochlear filter width with decreasing frequency. The results suggest that the amplitude and latency of the speech ABR may reflect interindividual differences in cochlear, as well as central, processing. The high-pass noise-masking technique provides a useful tool for differentiating between peripheral and central effects on the speech ABR. It can be used for further elucidating the neural basis of the perceptual speech deficits that have been associated with individual differences in speech ABR characteristics.