ABR/DPOAE publications

The insulin receptor substrate (IRS) proteins are key mediators of insulin and insulinlike growth factor 1 (IGF-1) signaling. Protein tyrosine phosphatase (PTP)-1B dephosphorylates and inactivates both insulin and IGF-1 receptors. IRS2-deficient mice present altered hepatic insulin signaling and β-cell failure and develop type 2-like diabetes. In addition, IRS2 deficiency leads to developmental defects in the nervous system. IGF1 gene mutations cause syndromic sensorineural hearing loss in humans and mice. However, the involvement of IRS2 and PTP1B, two IGF-1 downstream signaling mediators, in hearing onset and loss has not been studied. Our objective was to study the hearing function and cochlear morphology of Irs2-null mice and the impact of PTP1B deficiency. We have studied the auditory brainstem responses and the cochlear morphology of systemic Irs2⁻/⁻Ptpn1⁺/⁺, Irs2⁺/⁺Ptpn1⁻/⁻ and Irs2⁻/⁻Ptpn1⁻/⁻ mice at different postnatal ages. The results indicated that Irs2⁻/⁻Ptpn1⁺/⁺ mice present a profound congenital sensorineural deafness before the onset of diabetes and altered cochlear morphology with hypoinnervation of the cochlear ganglion and aberrant stria vascularis, compared with wild-type mice. Simultaneous PTP1B deficiency in Irs2⁻/⁻Ptpn1⁻/⁻ mice delays the onset of deafness. We show for the first time that IRS2 is essential for hearing and that PTP1B inhibition may be useful for treating deafness associated with hyperglycemia and type 2 diabetes.

Hypothesis: Impairments of the medial olivocochlear system (MOCS) increase the risk of environmentally induced auditory neuropathy spectrum disorder (ANSD). Background: ANSD is a problem in the neural transmission of auditory information that accounts for 10% to 15% of the cases of pediatric hearing loss. The underlying mechanisms of the disorder remain poorly understood, but noise exposure is an important risk factor. The goal of this study was to identify environmental conditions and genetic predispositions that lead to ANSD. Our approach was based on the assumption that noise induces ANSD by impeding the functional maturation of the brain's sound coding pathways. Because the MOCS adjusts the sensitivity of the inner ear to noise, impairments of this feedback are predicted to increase the disruptive effects of environmental exposures. Methods: An animal model of ANSD was created by rearing mice in noise. MOCS protection was assessed by comparing the incidence of noise-induced ANSD among knockout mice lacking feedback and wild-type (WT) controls. The mice were screened for ANSD with distortion product otoacoustic emissions, auditory brainstem responses, and behavioral measures of gap detection. Single-unit recording procedures were used to link these deficits to impaired synaptic transmission in the ventral cochlear nucleus. Results: ANSD manifested in noise-reared mice as intact distortion product otoacoustic emissions, abnormal auditory brainstem responses, and impaired gap detection. The phenotype was not observed among quiet-reared WT mice but was occasionally noted among noise-reared WT mice. The incidence of ANSD significantly increased among knockout mice, especially when they were reared in noise. Conclusion: Noise promotes ANSD by altering the functional maturation of the brain's temporal pathways. Noise-induced impairments are reduced by the sound-attenuating effects of the MOCS. Noise levels do not need to be unnaturally loud to constitute significant risk in MOCS-compromised individuals.

Heterozygous mutations in the gene encoding chromodomain-DNA-binding-protein 7 (CHD7) cause CHARGE syndrome, a multiple anomaly condition which includes vestibular dysfunction and hearing loss. Mice with heterozygous Chd7 mutations exhibit semicircular canal dysgenesis and abnormal inner ear neurogenesis, and are an excellent model of CHARGE syndrome. Here we characterized Chd7 expression in mature middle and inner ears, analyzed morphological features of mutant ears and tested whether Chd7 mutant mice have altered responses to noise exposure and correlated those responses to inner and middle ear structure. We found that Chd7 is highly expressed in mature inner and outer hair cells, spiral ganglion neurons, vestibular sensory epithelia and middle ear ossicles. There were no obvious defects in individual hair cell morphology by prestin immunostaining or scanning electron microscopy, and cochlear innervation appeared normal in Chd7(Gt)(/+) mice. Hearing thresholds by auditory brainstem response (ABR) testing were elevated at 4 and 16 kHz in Chd7(Gt)(/+) mice, and there were reduced distortion product otoacoustic emissions (DPOAE). Exposure of Chd7(Gt)(/+) mice to broadband noise resulted in variable degrees of hair cell loss which inversely correlated with severity of stapedial defects. The degrees of hair cell loss and threshold shifts after noise exposure were more severe in wild type mice than in mutants. Together, these data indicate that Chd7(Gt)(/+) mice have combined conductive and sensorineural hearing loss, correlating with changes in both middle and inner ears.

Noise exposure is a major cause of hearing loss. Classical methods of studying protein involvement have provided a basis for understanding signaling pathways that mediate hearing loss and damage repair but do not lend themselves to studying large networks of proteins that are likely to increase or decrease during noise trauma. To address this issue, antibody microarrays were used to quantify the very early changes in protein expression in three distinct regions of the chinchilla cochlea 2h after exposure to a 0.5-8 kHz band of noise for 2h at 112 dB SPL. The noise exposure caused significant functional impairment 2h post-exposure which only partially recovered. Distortion product otoacoustic emissions were abolished 2h after the exposure, but at 4 weeks post-exposure, otoacoustic emissions were present, but still greatly depressed. Cochleograms obtained 4 weeks post-exposure demonstrated significant loss of outer hair cells in the basal 60% of the cochlea corresponding to frequencies in the noise spectrum. A comparative analysis of the very early (2h post-exposure) noise-induced proteomic changes indicated that the sensory epithelium, lateral wall and modiolus differ in their biological response to noise. Bioinformatic analysis of the cochlear protein profile using "The Database for Annotation, Visualization and Integrated Discovery 2008" (DAVID - http://david.abcc. ncifcrf.gov) revealed the initiation of the cell death process in sensory epithelium and modiolus. An increase in Fas and phosphorylation of FAK and p38/MAPK in the sensory epithelium suggest that noise-induced stress signals at the cell membrane are transmitted to the nucleus by Fas and focal adhesion signaling through the p38/MAPK signaling pathway. Up-regulation of downstream nuclear proteins E2F3 and WSTF in immunoblots and microarrays along with their immunolocalization in the outer hair cells supported the pivotal role of p38/MAPK signaling in the mechanism underlying noise-induced hearing loss.

This study examines the test-retest reliability of distortion-product otoacoustic emissions (DPOAEs) in ketamine-anesthetized common marmosets (Callithrix jacchus). DPOAE gain functions were measured at 16 f(2)-frequencies between 3 and 24 kHz. Test-retest reliability was assessed at the following time intervals: (1) Interleaved, in which two gain functions were obtained at each frequency before advancing to the next frequency, (2) Immediate, wherein one gain function was collected at all f(2)-frequencies and the retest was immediately performed without removing the probe tip, (3) Short-term, in which the retest followed a 10-min period with the probe removed, and (4) Long-term, wherein the retest was performed at least one week after the initial test. Reliability was assessed using four correlation coefficients used in the literature. Test-retest reliability was best in the interleaved interval and worst in the short-term interval. In general, reliability was best when primary-tone levels were high. Correlation coefficients decreased at frequencies above 12-kHz in the short-term and long-term intervals, but the decrease was more substantial in females than in males in the long-term interval. At frequencies below 12 kHz, same-day measurements (2, 3) were less repeatable, regardless of whether the probe was removed, which may be due to time under anesthesia. These results have implications for DPOAE studies where repeated measures are required and when treatment or group differences are small.

Neurotrophins prevent spiral ganglion neuron (SGN) degeneration in animal models of ototoxin-induced deafness and may be used in the future to improve the hearing of cochlear implant patients. It is increasingly common for patients with residual hearing to undergo cochlear implantation. However, the effect of neurotrophin treatment on acoustic hearing is not known. In this study, brain-derived neurotrophic factor (BDNF) was applied to the round window membrane of adult guinea pigs for 4 weeks using a cannula attached to a mini-osmotic pump. SGN survival was first assessed in ototoxically deafened guinea pigs to establish that the delivery method was effective. Increased survival of SGNs was observed in the basal and middle cochlear turns of deafened guinea pigs treated with BDNF, confirming that delivery to the cochlea was successful. The effects of BDNF treatment in animals with normal hearing were then assessed using distortion product otoacoustic emissions (DPOAEs), pure tone, and click-evoked auditory brainstem responses (ABRs). DPOAE assessment indicated a mild deficit of 5 dB SPL in treated and control groups at 1 and 4 weeks after cannula placement. In contrast, ABR evaluation showed that BDNF lowered thresholds at specific frequencies (8 and 16 kHz) after 1 and 4 weeks posttreatment when compared to the control cohort receiving Ringer's solution. Longer treatment for 4 weeks not only widened the range of frequencies ameliorated from 2 to 32 kHz but also lowered the threshold by at least 28 dB SPL at frequencies ≥16 kHz. BDNF treatment for 4 weeks also increased the amplitude of the ABR response when compared to either the control cohort or prior to treatment. We show that BDNF applied to the round window reduces auditory thresholds and could potentially be used clinically to protect residual hearing following cochlear implantation.

This study was undertaken to determine the role of adenosine signalling in the development of age-related hearing loss (ARHL). We and others have shown previously that adenosine signalling via A(1) receptors is involved in cochlear protection from noise-induced cochlear injury. Here we demonstrate that enhanced adenosine signalling in the cochlea provides partial protection from ARHL in C57BL/6J mice. We targeted adenosine kinase (ADK), the key enzyme in adenosine metabolism, using a treatment regime with the selective ADK inhibitor ABT-702 (1.5mg/kg intraperitoneally twice a week) commencing at the age of three months or six months. This treatment, intended to increase free adenosine levels in the cochlea, was maintained until the age of nine months and hearing thresholds were evaluated monthly using auditory brainstem responses (ABR). At nine months, when C57BL/6J mice normally exhibit significant ARHL, both groups treated with ABT-702 showed lower ABR threshold shifts at 10 and 16kHz compared to control animals receiving the vehicle solution. The better thresholds of the ABT-702-treated mice at these frequencies were supported by increased survival of hair cells in the apical region of the cochlea. This study provides the first evidence that ARHL can be mitigated by enhancing adenosine signalling in the cochlea.

Cochlear hair cells convert sound vibration into electrical potential, and loss of these cells diminishes auditory function. In response to mechanical stimuli, piezoelectric materials generate electricity, suggesting that they could be used in place of hair cells to create an artificial cochlear epithelium. Here, we report that a piezoelectric membrane generated electrical potentials in response to sound stimuli that were able to induce auditory brainstem responses in deafened guinea pigs, indicating its capacity to mimic basilar membrane function. In addition, sound stimuli were transmitted through the external auditory canal to a piezoelectric membrane implanted in the cochlea, inducing it to vibrate. The application of sound to the middle ear ossicle induced voltage output from the implanted piezoelectric membrane. These findings establish the fundamental principles for the development of hearing devices using piezoelectric materials, although there are many problems to be overcome before practical application.

Background: Sound production and hearing sensitivity of ectothermic animals are affected by the ambient temperature. This is the first study investigating the influence of temperature on both sound production and on hearing abilities in a fish species, namely the neotropical Striped Raphael catfish Platydoras armatulus. Methodology/principal findings: Doradid catfishes produce stridulation sounds by rubbing the pectoral spines in the shoulder girdle and drumming sounds by an elastic spring mechanism which vibrates the swimbladder. Eight fish were acclimated for at least three weeks to 22°, then to 30° and again to 22°C. Sounds were recorded in distress situations when fish were hand-held. The stridulation sounds became shorter at the higher temperature, whereas pulse number, maximum pulse period and sound pressure level did not change with temperature. The dominant frequency increased when the temperature was raised to 30°C and the minimum pulse period became longer when the temperature decreased again. The fundamental frequency of drumming sounds increased at the higher temperature. Using the auditory evoked potential (AEP) recording technique, the hearing thresholds were tested at six different frequencies from 0.1 to 4 kHz. The temporal resolution was determined by analyzing the minimum resolvable click period (0.3-5 ms). The hearing sensitivity was higher at the higher temperature and differences were more pronounced at higher frequencies. In general, latencies of AEPs in response to single clicks became shorter at the higher temperature, whereas temporal resolution in response to double-clicks did not change. Conclusions/significance: These data indicate that sound characteristics as well as hearing abilities are affected by temperatures in fishes. Constraints imposed on hearing sensitivity at different temperatures cannot be compensated even by longer acclimation periods. These changes in sound production and detection suggest that acoustic orientation and communication are affected by temperature changes in the neotropical catfish P. armatulus.

Animal experiments suggest that chronic tinnitus ("ringing in the ears") may result from processes that overcompensate for lost afferent input. Abnormally elevated spontaneous neural activity has been found in the dorsal cochlear nucleus (DCN) of animals with psychophysical evidence of tinnitus. However, it has also been reported that DCN ablation fails to reduce established tinnitus. Since other auditory areas have been implicated in tinnitus, the role of the DCN is unresolved. The apparently conflicting electrophysiological and lesion data can be reconciled if the DCN serves as a necessary trigger zone rather than a chronic generator of tinnitus. The present experiment used lesion procedures identical to those that failed to decrease pre-existing tinnitus. The exception was that lesions were done prior to tinnitus induction. Young adult rats were trained and tested using a psychophysical procedure shown to detect tinnitus. Tinnitus was induced by a single unilateral high-level noise exposure. Consistent with the trigger hypothesis, bilateral dorsal DCN lesions made before high-level noise exposure prevented the development of tinnitus. A protective effect stemming from disruption of the afferent pathway could not explain the outcome because unilateral lesions ipsilateral to the noise exposure did not prevent tinnitus and unilateral lesions contralateral to the noise exposure actually exacerbated the tinnitus. The DCN trigger mechanism may involve plastic circuits that, through loss of inhibition, or upregulation of excitation, increase spontaneous neural output to rostral areas such as the inferior colliculus. The increased drive could produce persistent pathological changes in the rostral areas, such as high-frequency bursting and decreased interspike variance, that comprise the chronic tinnitus signal.

It is well known that ethacrynic acid (EA) can potentiate the ototoxicity of aminoglycoside antibiotics (AmAn) such as kanamycin (KM), if they were applied at the same time. Currently, to create the model of EA-KM-induced cochlear lesion in rats, adult rats received a single injection of EA (75 mg/kg, intravenous injection), or followed immediately by KM (500 mg/kg, intramuscular injection). The hearing function was assessed by auditory brainstem response (ABR) measurement in response to click and/or tone bursts at 4, 8, 12, 16, 20, 24, and 32 kHz. The static microcirculation status in the stria vascularis after a single EA injection was evaluated with eosin staining. The pathological changes in cochlear and vestibular hair cells were also quantified after co-administration of EA and KM. After a single EA injection, blood flow in vessels supplying the stria vascularis rapidly diminished. However, the blood supply to the cochlear lateral wall partially recovered 5 h after EA treatment. Threshold changes in ABR were basically parallel to the microcirculation changes in stria vascularis after single EA treatment. Importantly, disposable co-administration of EA and KM resulted in a permanent hearing loss and severe damage to the cochlear hair cells, but spared the vestibular hair cells. Since the cochlear lateral wall is the important part of the blood-cochlea barrier, EA-induced anoxic damage to the epithelium of stria vascularis may enhance the entry of KM to the cochlea. Thus, experimental animal model of selective cochlear damage with normal vestibular systems can be reliably created through co-administration of EA and KM.

Objective The degeneration of hair cells and spiral ganglion neurons (SGNs) is an important pathologic process in the development of sensorineural hearing loss. In a murine model, predictable and reproducible damage to SGNs occurs through the application of ouabain to the round window. Recent evidence has shown that the chemokine stromal cell–derived factor‐1 (SDF‐1) is a potent chemoattractant of hematopoietic stem cells (HSCs) and provides trophic support to injured tissues during development and maturation. The hypothesis for the current study is that expression of SDF‐1 plays an important role in protecting SGNs and preventing further degeneration in the setting of cochlear injury. Study Design Prospective, controlled. Setting Academic research laboratory. Subject and Methods Auditory brainstem response (ABR) and the expression of SDF‐1 mRNA and protein were examined 1, 3, 7, 14, and 30 days after application of ouabain in 35 adult mice. Results Following ouabain application, real‐time reverse‐transcription polymerase chain reaction for SDF demonstrates increased mRNA expression following ouabain injury in nontransplanted mice. A significant increase in SDF protein expression was also observed using immunolabeling techniques and Western blot analysis. Conclusions SDF‐1 expression is increased in the auditory nerve following cochlear injury. Further knowledge about the cochlear microenvironment, including SDF‐1, is critical to maximizing HSC engraftment in the injured cochlea and providing a therapeutic option for sensorineural hearing loss.

Developmental exposure to polychlorinated biphenyls (PCBs) causes hearing loss that may be due to reduced thyroxine during cochlear development. Polybrominated diphenyl ethers (PBDEs) are structurally similar to PCBs and reduce thyroxine. This study utilized an environmental PCB mixture and a commercial PBDE mixture, DE-71, that represents the PBDEs found in humans to assess the potential for additive effects of PCBs and PBDEs on cochlear function. Female Long-Evans rats were dosed with corn oil vehicle, PCBs (3 or 6 mg/kg), molar equivalent doses of PBDEs (5.7 or 11.4 mg/kg), 3 mg/kg PCBs + 5.7 mg/kg PBDEs, or 6 mg/kg PCBs + 11.4 mg/kg PBDEs throughout gestation and lactation. At weaning, pup blood was taken to assess thyroxine concentrations. One male and one female from each litter were maintained until adulthood for distortion product otoacoustic emission (DPOAE) measurements of cochlear function. DPOAE amplitudes were decreased and thresholds were elevated in the 6 mg/kg PCB group. Exposure to PBDEs did not cause DPOAE deficits. There was an interactive effect from combined exposure such that the individual low doses of PCBs and PBDEs did not result in DPOAE deficits, but the two combined produced a deficit similar to that in the high-dose PCB group. Serum thyroxine concentrations of all groups were reduced compared with controls, but PBDEs produced a less dramatic reduction than PCBs, which could explain the lack of DPOAE effects. Importantly, there was evidence that the co-exposure to subthreshold doses of PCBs and PBDEs can have an additive effect on cochlear function.

The deafness locus DFNB1 contains GJB2, the gene encoding connexin26 and GJB6, encoding connexin30, which appear to be coordinately regulated in the inner ear. In this work, we investigated the expression and function of connexin26 and connexin30 from postnatal day 5 to adult age in double transgenic Cx26(Sox10Cre) mice, which we obtained by crossing connexin26 floxed mice with a deleter Sox10-Cre line. Cx26(Sox10Cre) mice presented with complete connexin26 ablation in the epithelial gap junction network of the cochlea, whereas connexin30 expression was developmentally delayed; immunolabeling patterns for both connexins were normal in the cochlear lateral wall. In vivo electrophysiological measurements in Cx26(Sox10Cre) mice revealed profound hearing loss accompanied by reduction of endocochlear potential, and functional experiments performed in postnatal cochlear organotypic cultures showed impaired gap junction coupling. Transduction of these cultures with a bovine adeno associated virus vector restored connexin26 protein expression and rescued gap junction coupling. These results suggest that restoration of normal connexin levels by gene delivery via recombinant adeno associated virus could be a way to rescue hearing function in DFNB1 mouse models and, in future, lead to the development of therapeutic interventions in humans.

Combined measurements of middle ear transfer function and auditory brainstem response (ABR) in live guinea pigs with middle ear effusion (MEE) are reported in this paper. The MEE model was created by injecting saline into the middle ear cavity. Vibrations of the tympanic membrane (TM), the tip of the incus, and the round window membrane (RWM) were measured with a laser vibrometer at frequencies of 0.2-40 kHz when the middle ear fluid increased from 0 to 0.2 ml (i.e., full fill of the cavity). The click and pure tone ABRs were recorded as the middle ear fluid increased. Fluid introduction reduced mobility of the TM, incus and RWM mainly at high frequencies (f > 1 kHz). The magnitude of this reduction was related to the volume of fluid. The displacement transmission ratio of the TM to incus varied with frequency and fluid level. The volume displacement ratio of the oval window to round window was approximately 1.0 over most frequencies. Elevation of ABR thresholds and prolongation of ABR latencies were observed as fluid level increased. Reduction of TM displacement correlated well with elevation of ABR threshold at 0.5-8 kHz. Alterations in the ratio of ossicular displacements before and after fluid induction are consistent with fluid-induced changes in complex ossicular motions.

The development of sound-evoked responses in Chinchilla lanigera was studied from postnatal ages P0-1 (first 24 h) to adult. Cochlear microphonic (CMs) and compound action potentials (CAPs), representing ensemble sound-evoked activities of hair cells and auditory nerve fibers, respectively, were present as early as age P0-1. The data indicate that CM thresholds and sensitivities were generally adult-like (i.e., fall into adult ranges) at birth, but suprathreshold CM amplitudes remained below adult ranges through P28. CAP thresholds reached adult-like values between P7–P14, but the suprathreshold CAP amplitude continued to increase until ∼P28. The results confirm the auditory precociousness of the chinchilla.

The auditory portion of the inner ear, the cochlea, is an ideal organ for local gene transfection owing to its relative isolation. Various carriers have been tested for cochlear gene transfection. To date, viral vectors appear to have much higher transfection efficacy than non-viral mechanisms. Among these vectors, recombinant adeno-associated virus (rAAV) vectors have several advantages such as being non-pathogenic and the ability to produce prolonged gene expression in various cell types. However, rAAV vectors cannot pass through the intact round window membrane (RWM), otherwise a very attractive approach to access the human inner ear. In this study, performed in guinea-pigs, we describe a method to increase the permeability of RWM to rAAV vectors by partial digestion with collagenase solution. Elevated delivery of rAAV across the partially digested RWM increased transfection efficacy to a satisfactory level, even though it was still lower than that achieved by direct cochleostomy injection. Functional tests (auditory brainstem responses) showed that this enzymatic manipulation did not cause permanent hearing loss if applied appropriately. Morphological observations suggested that the damage to RWM caused by partial digestion healed within four weeks. Taken together, these findings suggest that partial digestion of the RWM is a safe and effective method for increasing the transfection of cochlear sensory cells with rAAV.

Adult mammalian auditory hair cells (HCs) and their associated supporting cells (SCs) do not proliferate, and HC death leads to irreversible neurosensory hearing loss and balance impairment. In nonmammalian vertebrates, loss of HCs induces mitotic proliferation of adjacent nonsensory SCs and/or direct SC transdifferentiation to generate replacement cells. This results in the structural and functional recovery of the nonmammalian sensory systems. Potential replacement of mammalian auditory HCs, either by transplanting cells or by transforming existing cells through molecular therapy, has long been proposed. However, HC replacement strategies with clear therapeutic potential remain elusive. The retinoblastoma (pRB) family of cell cycle regulators, Rb1, Rbl1 (p107), and Rbl2 (p130), regulate the G(1)- to S-phase transition in proliferating cells. In the inner ear, the biochemical and molecular pathways involving pRBs, particularly p107 and p130, are relatively unexplored and their therapeutic suitability is yet to be determined. In this study, we analyzed the cochleae of adult p130 knock-out (p130(-/-)) mice and showed that lack of the p130 gene results in extra rows of HCs and SCs in the more apical regions of the cochlea. No evidence of transdifferentiation of these supernumerary SCs into HCs was observed in the p130(-/-) mouse. Nevertheless, unscheduled proliferation of SCs in the adult p130(-/-) cochlea coupled to downregulation of bona fide cell cycle inhibitors provides a mechanistic basis for the role of p130 as a regulator of SC and HC mitotic quiescence in the more apical regions of the cochlea. Interestingly, p130(-/-) mice exhibited nearly normal peripheral auditory sensitivity.

With increasing efforts to develop and utilize mouse models of a variety of neuro-developmental diseases, there is an urgent need for sensitive neuroimaging methods that enable in vivo analysis of subtle alterations in brain anatomy and function in mice. Previous studies have shown that the brains of Fibroblast Growth Factor 17 null mutants (Fgf17(-/-)) have anatomical abnormalities in the inferior colliculus (IC)-the auditory midbrain-and minor foliation defects in the cerebellum. In addition, changes in the expression domains of several cortical patterning genes were detected, without overt changes in forebrain morphology. Recently, it has also been reported that Fgf17(-/-) mutants have abnormal vocalization and social behaviors, phenotypes that could reflect molecular changes in the cortex and/or altered auditory processing / perception in these mice. We used manganese (Mn)-enhanced magnetic resonance imaging (MEMRI) to analyze the anatomical phenotype of Fgf17(-/-) mutants in more detail than achieved previously, detecting changes in IC, cerebellum, olfactory bulb, hypothalamus and frontal cortex. We also used MEMRI to characterize sound-evoked activity patterns, demonstrating a significant reduction of the active IC volume in Fgf17(-/-) mice. Furthermore, tone-specific (16- and 40-kHz) activity patterns in the IC of Fgf17(-/-) mice were observed to be largely overlapping, in contrast to the normal pattern, separated along the dorsal-ventral axis. These results demonstrate that Fgf17 plays important roles in both the anatomical and functional development of the auditory midbrain, and show the utility of MEMRI for in vivo analyses of mutant mice with subtle brain defects.