Microstructural Changes in the Brainstem Auditory Pathway in Children With Hearing Loss.

OBJECTIVE: To assess the utility of diffusion tensor imaging of the auditory pathway in children with sensorineural hearing loss (SNHL). STUDY DESIGN: Retrospective cohort study. SETTING: A single academic tertiary children's hospital. PATIENTS: Sixteen pediatric patients with bilateral SNHL of at least moderate severity in the poorer ear (eight male; mean age, 5.3 ± 4.9 yrs). Controls consisted of age- and sex-matched children with normal hearing who were imaged for nonotologic, non-neurologic medical concerns and found to have normal magnetic resonance imaging (MRI). INTERVENTIONS: Three Tesla MRI scanners were used for diffusion tensor imaging. MAIN OUTCOME MEASURES: Quantitative diffusion tensor metrics were extracted from the superior olivary nucleus (SON), inferior colliculus (IC), and ipsilateral fiber tracts between the SON and IC delineated by tractography. RESULTS: We identified differences in fractional anisotropy of the SON between the SNHL cohort and controls (0.377 ± 0.056 vs. 0.422 ± 0.052; p = 0.009), but not in the IC. There were no differences in the mean diffusivity (MD) values in the IC and SON. Among younger children (≤5 yrs), MD was decreased in the SNHL cohort compared with controls in the IC (0.918 ± 0.051 vs. 1.120 ± 0.142; p < 0.001). However, among older children (>5 yrs), there were no differences in MD (1.124 ± 0.198 vs. 0.997 ± 0.103; p = 0.119). There were no differences in MD or fractional anisotropy in the white matter fibers of the IC-SON tract. CONCLUSIONS: Our results suggest abnormal neural tracts along the central auditory pathway among children with SNHL. Longitudinal studies should assess the prognostic value of these MRI-based findings for assessing long-term outcomes and determining intervention efficacy.

Diffusion Tensor Imaging of Auditory Pathway: A Comparison of Pediatric Cochlear Implant Candidates and Healthy Cases.

BACKGROUND: We aimed to investigate the changes that may occur in the auditory neural network in pediatric congenital hearing loss cases. METHODS: Fifty-four cochlear implant candidates and 47 normal-hearing controls were included in this retrospective study. Fractional anisotropy, radial diffusivity, and apparent diffusion coefficient maps were generated. We placed region of interest on the cochlear nucleus, superior olivary nucleus, lateral lemniscus, medial geniculate body, auditory radiation, Heschl's gyrus, inferior fronto-occipital fasciculus, superior longitudinal fascicle, and corpus callosum splenium. The area of the cochlear nerve was measured. Diffusion tensor imaging metrics, children's ages, and cochlear nerve area were compared. RESULTS: Apparent diffusion coefficient and radial diffusivity values of patients were higher than the control group in all places except the radial diffusivity values of medial geniculate body. The fractional anisotropy values of the patients in lateral lemniscus, auditory radiation, Heschl's gyrus, inferior fronto-occipital fasciculus, superior longitudinal fascicle, and corpus callosum splenium were lower than the control group. There is a positive correlation between fractional anisotropy and age in both patient and control groups for all locations. The cochlear nerve area is lower in patients (0.88 ± 0.29) than in the control group (1.18 ± 0.14) (P = .000). The cochlear nerve area has a positive correlation with age in the patient group (P = .000) but has not in the control group. The cochlear nerve area positively correlates with fractional anisotropy values of all locations except fractional anisotropy values of medial geniculate body. CONCLUSION: The alterations of diffusion tensor imaging metrics on the auditory pathway reflect the microstructural changes of white matter tracts.

The differences of functional brain network in processing auditory phonological tasks between Cantonese-Mandarin bilinguals and Mandarin monolinguals.

OBJECTIVE: To explore the differences of functional brain networks in processing auditory phonological tasks between Cantonese-Mandarin bilinguals and Mandarin monolinguals. METHODS: 31 Cantonese-Mandarin bilinguals and 31 Mandarin monolinguals performed auditory rhyming tasks under a functional magnetic resonance imaging (fMRI) scanning. Bilinguals performed two language tasks (Cantonese, CC; Mandarin, CM), while monolinguals performed only one task (Mandarin, MM). Graph theory and network-based statistic (NBS) analyses were used to reveal the differences of functional brain networks among CC, CM and MM. RESULTS: The functional brain networks of CC, CM and MM were all small-world, and there were no differences in network properties, but widespread differences in functional connectivity among distributed brain regions. CONCLUSIONS: The Cantonese-Mandarin bilingualism and Mandarin monolingualism shared similar efficient topological structure of brain network for auditory phonological processing, but varied in widespread functional connectivity. And the two languages within bilingualism employed different functional brain networks for auditory phonological processing.

Altered processing of communication signals in the subcortical auditory sensory pathway in autism.

Autism spectrum disorder (ASD) is characterised by social communication difficulties. These difficulties have been mainly explained by cognitive, motivational, and emotional alterations in ASD. The communication difficulties could, however, also be associated with altered sensory processing of communication signals. Here, we assessed the functional integrity of auditory sensory pathway nuclei in ASD in three independent functional magnetic resonance imaging experiments. We focused on two aspects of auditory communication that are impaired in ASD: voice identity perception, and recognising speech-in-noise. We found reduced processing in adults with ASD as compared to typically developed control groups (pairwise matched on sex, age, and full-scale IQ) in the central midbrain structure of the auditory pathway (inferior colliculus [IC]). The right IC responded less in the ASD as compared to the control group for voice identity, in contrast to speech recognition. The right IC also responded less in the ASD as compared to the control group when passively listening to vocal in contrast to non-vocal sounds. Within the control group, the left and right IC responded more when recognising speech-in-noise as compared to when recognising speech without additional noise. In the ASD group, this was only the case in the left, but not the right IC. The results show that communication signal processing in ASD is associated with reduced subcortical sensory functioning in the midbrain. The results highlight the importance of considering sensory processing alterations in explaining communication difficulties, which are at the core of ASD.

MRI Correlates of Ototoxicity in the Auditory Pathway in Children Treated for Medulloblastoma.

OBJECTIVE: To assess diffusion and perfusion changes of the auditory pathway in pediatric medulloblastoma patients exposed to ototoxic therapies. STUDY DESIGN: Retrospective cohort study. SETTING: A single academic tertiary children's hospital. PATIENTS: Twenty pediatric medulloblastoma patients (13 men; mean age 12.0 ±â€Š4.8 yr) treated with platinum-based chemotherapy with or without radiation and 18 age-and-sex matched controls were included. Ototoxicity scores were determined using Chang Ototoxicity Grading Scale. INTERVENTIONS: Three Tesla magnetic resonance was used for diffusion tensor and arterial spin labeling perfusion imaging. MAIN OUTCOME MEASURES: Quantitative diffusion tensor metrics were extracted from the Heschl's gyrus, auditory radiation, and inferior colliculus. Arterial spin labeling perfusion of the Heschl's gyrus was also examined. RESULTS: Nine patients had clinically significant hearing loss, or Chang grades more than or equal to 2a; 11 patients had mild/no hearing loss, or Chang grades less than 2a. The clinically significant hearing loss group showed reduced mean diffusivity in the Heschl's gyrus (p = 0.018) and auditory radiation (p = 0.037), and decreased perfusion in the Heschl's gyrus (p = 0.001). Mild/no hearing loss group showed reduced mean diffusivity (p = 0.036) in Heschl's gyrus only, with a decrease in perfusion (p = 0.008). There were no differences between groups in the inferior colliculus. There was no difference in fractional anisotropy between patients exposed to ototoxic therapies and controls. CONCLUSIONS: Patients exposed to ototoxic therapies demonstrated microstructural and physiological alteration of the auditory pathway. The present study shows proof-of-concept use of diffusion tensor imaging to gauge ototoxicity along the auditory pathway. Future larger cohort studies are needed to assess significance of changes in diffusion tensor imaging longitudinally, and the relationship between these changes and hearing loss severity and longitudinal changes of the developing auditory white matter.

Encoding of acquired sound-sequence salience by auditory cortical offset responses.

Behaviorally relevant sounds are often composed of distinct acoustic units organized into specific temporal sequences. The meaning of such sound sequences can therefore be fully recognized only when they have terminated. However, the neural mechanisms underlying the perception of sound sequences remain unclear. Here, we use two-photon calcium imaging in the auditory cortex of behaving mice to test the hypothesis that neural responses to termination of sound sequences ("Off-responses") encode their acoustic history and behavioral salience. We find that auditory cortical Off-responses encode preceding sound sequences and that learning to associate a sound sequence with a reward induces enhancement of Off-responses relative to responses during the sound sequence ("On-responses"). Furthermore, learning enhances network-level discriminability of sound sequences by Off-responses. Last, learning-induced plasticity of Off-responses but not On-responses lasts to the next day. These findings identify auditory cortical Off-responses as a key neural signature of acquired sound-sequence salience.

Glucose hypometabolism in the Auditory Pathway in Age Related Hearing Loss in the ADNI cohort.

PURPOSE: Hearing loss (HL) is one of the most common age-related diseases. Here, we investigate the central auditory correlates of HL in people with normal cognition and mild cognitive impairment (MCI) and test their association with genetic markers with the aim of revealing pathogenic mechanisms. METHODS: Brain glucose metabolism based on FDG-PET, self-reported HL status, and genetic data were obtained from the Alzheimer's Disease Neuroimaging Initiative (ADNI) cohort. FDG-PET data was analysed from 742 control subjects (non-HL with normal cognition or MCI) and 162 cases (HL with normal cognition or MCI) with age ranges of 72.2 ± 7.1 and 77.4 ± 6.4, respectively. Voxel-wise statistics of FDG uptake differences between cases and controls were computed using the generalised linear model in SPM12. An additional 1515 FDG-PET scans of 618 participants were analysed using linear mixed effect models to assess longitudinal HL effects. Furthermore, a quantitative trait genome-wide association study (GWAS) was conducted on the glucose uptake within regions of interest (ROIs), which were defined by the voxel-wise comparison, using genotyping data with 5,082,878 variants available for HL cases and HL controls (N = 817). RESULTS: The HL group exhibited hypometabolism in the bilateral Heschl's gyrus (kleft = 323; kright = 151; Tleft = 4.55; Tright = 4.14; peak Puncorr < 0.001), the inferior colliculus (k = 219;T = 3.53; peak Puncorr < 0.001) and cochlear nucleus (k = 18;T = 3.55; peak Puncorr < 0.001) after age correction and using a cluster forming height threshold P < 0.005 (FWE-uncorrected). Moreover, in an age-matched subset, the cluster comprising the left Heschl's gyrus survived the FWE-correction (kleft = 1903; Tleft = 4.39; cluster PFWE-corr = 0.001). The quantitative trait GWAS identified no genome-wide significant locus in the three HL ROIs. However, various loci were associated at the suggestive threshold (p < 1e-05). CONCLUSION: Compared to the non-HL group, glucose metabolism in the HL group was lower in the auditory cortex, the inferior colliculus, and the cochlear nucleus although the effect sizes were small. The GWAS identified candidate genes that might influence FDG uptake in these regions. However, the specific biological pathway(s) underlying the role of these genes in FDG-hypometabolism in the auditory pathway requires further investigation.

Evaluation of the whole auditory pathway using high-resolution and functional MRI at 7T parallel-transmit.

PURPOSE: The aim of the present study is to show a MR procedure for the evaluation of simultaneous left and right auditory functions with functional MRI, and high-resolution acquisition of anatomical auditory pathway using parallel-transmit (pTx) methods at 7T. METHODS: The time-efficient MR acquisition included two steps: RF weights were optimized for the regions-of-interest and high-resolution MR images of the inner-ear were acquired for the first 30 min (400 µm-iso resolution) followed by functional MRI acquisitions along the whole auditory pathway during the next 20 minutes. Data was processed with a linear cross-correlation analysis to define frequency preferences for each voxel in the auditory relays. RESULTS: Tonotopic maps revealed ordered bilateral frequency gradients in the auditory relays whereas at the level of the cochlear nuclei and superior olivary complexes the frequency gradients were less evident. A 21% increase in transmit-field efficiency was achieved over the left/right inner-ear regions and thus its main structures were clearly discernible using the pTx methods, compared to a single transmit RF coil. CONCLUSION: Using 7T pTx allows a fast (less than 60 min in total) and qualitative evaluation of the simultaneous left and right auditory response along the entire auditory pathway, together with high-resolution anatomical images of the inner-ear. This could be further used for patient examination at 7T.

Neural dynamics underlying the acquisition of distinct auditory category structures.

Despite the multidimensional and temporally fleeting nature of auditory signals we quickly learn to assign novel sounds to behaviorally relevant categories. The neural systems underlying the learning and representation of novel auditory categories are far from understood. Current models argue for a rigid specialization of hierarchically organized core regions that are fine-tuned to extracting and mapping relevant auditory dimensions to meaningful categories. Scaffolded within a dual-learning systems approach, we test a competing hypothesis: the spatial and temporal dynamics of emerging auditory-category representations are not driven by the underlying dimensions but are constrained by category structure and learning strategies. To test these competing models, we used functional Magnetic Resonance Imaging (fMRI) to assess representational dynamics during the feedback-based acquisition of novel non-speech auditory categories with identical dimensions but differing category structures: rule-based (RB) categories, hypothesized to involve an explicit sound-to-rule mapping network, and information integration (II) based categories, involving pre-decisional integration of dimensions via a procedural-based sound-to-reward mapping network. Adults were assigned to either the RB (n = 30, 19 females) or II (n = 30, 22 females) learning tasks. Despite similar behavioral learning accuracies, learning strategies derived from computational modeling and involvements of corticostriatal systems during feedback processing differed across tasks. Spatiotemporal multivariate representational similarity analysis revealed an emerging representation within an auditory sensory-motor pathway exclusively for the II learning task, prominently involving the superior temporal gyrus (STG), inferior frontal gyrus (IFG), and posterior precentral gyrus. In contrast, the RB learning task yielded distributed neural representations within regions involved in cognitive-control and attentional processes that emerged at different time points of learning. Our results unequivocally demonstrate that auditory learners' neural systems are highly flexible and show distinct spatial and temporal patterns that are not dimension-specific but reflect underlying category structures and learning strategies.

White matter variability in auditory callosal pathways contributes to variation in the cultural transmission of auditory symbolic systems.

The cultural transmission of spoken language and music relies on human capacities for encoding and recalling auditory patterns. In this experiment, we show that interindividual differences in this ability are associated with variation in the organization of cross-callosal white matter pathways. First, high-angular resolution diffusion MRI (dMRI) data were analyzed in a large participant sample (N = 51). Subsequently, these participants underwent a behavioral test that models in the laboratory the cultural transmission of auditory symbolic systems: the signaling game. Cross-callosal and intrahemispheric (arcuate fasciculus) pathways were reconstructed and analyzed using conventional diffusion tensor imaging (DTI) as well as a more advanced dMRI technique: fixel-based analysis (FBA). The DTI metric of fractional anisotropy (FA) in auditory callosal pathways predicted-weeks after scanning-the fidelity of transmission of an artificial tone system. The ability to coherently transmit auditory signals in one signaling game, irrespective of the signals learned during the previous game, was predicted by morphological properties of the fiber bundles in the most anterior portions of the corpus callosum. The current study is the first application of dMRI in the field of cultural transmission, and the first to connect individual characteristics of callosal pathways to core behaviors in the transmission of auditory symbolic systems.

Neurocognitive processing efficiency for discriminating human non-alarm rather than alarm scream calls.

Across many species, scream calls signal the affective significance of events to other agents. Scream calls were often thought to be of generic alarming and fearful nature, to signal potential threats, with instantaneous, involuntary, and accurate recognition by perceivers. However, scream calls are more diverse in their affective signaling nature than being limited to fearfully alarming a threat, and thus the broader sociobiological relevance of various scream types is unclear. Here we used 4 different psychoacoustic, perceptual decision-making, and neuroimaging experiments in humans to demonstrate the existence of at least 6 psychoacoustically distinctive types of scream calls of both alarming and non-alarming nature, rather than there being only screams caused by fear or aggression. Second, based on perceptual and processing sensitivity measures for decision-making during scream recognition, we found that alarm screams (with some exceptions) were overall discriminated the worst, were responded to the slowest, and were associated with a lower perceptual sensitivity for their recognition compared with non-alarm screams. Third, the neural processing of alarm compared with non-alarm screams during an implicit processing task elicited only minimal neural signal and connectivity in perceivers, contrary to the frequent assumption of a threat processing bias of the primate neural system. These findings show that scream calls are more diverse in their signaling and communicative nature in humans than previously assumed, and, in contrast to a commonly observed threat processing bias in perceptual discriminations and neural processes, we found that especially non-alarm screams, and positive screams in particular, seem to have higher efficiency in speeded discriminations and the implicit neural processing of various scream types in humans.

Musician developed right putaminal hemorrhage but conserved absolute pitch ability -Case report.

Absolute pitch (AP) is known as the ability to recognize and label the pitch chroma of a given tone without external reference. The neural mechanism and its asymmetry of AP musicians remain unclear. We herein report a 41-year-old AP musician who developed a right putaminal hemorrhage. On a postoperative day 5, a fluid-attenuated inversion recovery image revealed the rest of the hematoma and edematous lesion at the right white matter between the Heschl's gyrus and other cortices. Diffusion tensor tractography with the region of interest at the Heschl's gyrus was performed. In the left hemisphere, the anterior part of the arcuate fiber and middle longitudinal fasciculus were observed. However, these connections were absent in the right hemisphere, but her AP ability was maintained. Our case suggested that the fibers from the right Heschl's gyrus to the right frontal lobe via the right ventral stream is not associated with AP.

Dissociable Auditory Cortico-Cerebellar Pathways in the Human Brain Estimated by Intrinsic Functional Connectivity.

The cerebellum, a structure historically associated with motor control, has more recently been implicated in several higher-order auditory-cognitive functions. However, the exact functional pathways that mediate cerebellar influences on auditory cortex (AC) remain unclear. Here, we sought to identify auditory cortico-cerebellar pathways based on intrinsic functional connectivity magnetic resonance imaging. In contrast to previous connectivity studies that principally consider the AC as a single functionally homogenous unit, we mapped the cerebellar connectivity across different parts of the AC. Our results reveal that auditory subareas demonstrating different levels of interindividual functional variability are functionally coupled with distinct cerebellar regions. Moreover, auditory and sensorimotor areas show divergent cortico-cerebellar connectivity patterns, although sensorimotor areas proximal to the AC are often functionally grouped with the AC in previous connectivity-based network analyses. Lastly, we found that the AC can be functionally segmented into highly similar subareas based on either cortico-cerebellar or cortico-cortical functional connectivity, suggesting the existence of multiple parallel auditory cortico-cerebellar circuits that involve different subareas of the AC. Overall, the present study revealed multiple auditory cortico-cerebellar pathways and provided a fine-grained map of AC subareas, indicative of the critical role of the cerebellum in auditory processing and multisensory integration.

Mass spectrometry imaging of blast overpressure induced modulation of GABA/glutamate levels in the central auditory neuraxis of Chinchilla.

Acoustic trauma damages inner ear neural structures including cochlear hair cells which result in hearing loss and neurotransmitter imbalances within the synapses of the central auditory pathway. Disruption of GABA/glutamate levels underlies, tinnitus, a phantom perception of sound that persists post-exposure to blast noise which may manifest in tandem with acute/chronic loss of hearing. Many putative theories explain tinnitus physiology based on indirect and direct assays in animal models and humans, although there is no comprehensive evidence to explain the phenomenon. Here, GABA/glutamate levels were imaged and quantified in a blast overpressure model of chinchillas using Fourier transform ion cyclotron resonance mass spectrometry imaging. The direct measurement from whole-brain sections identified the relative levels of GABA/glutamate in the central auditory neuraxis centers including the cochlear nucleus, inferior colliculus, and auditory cortex. These preliminary results provide insight on the homeostasis of GABA/glutamate within whole-brain sections of chinchilla for investigation of the pathomechanism of blast-induced tinnitus.

Individual Variability in Functional Organization of the Human and Monkey Auditory Cortex.

Accumulating evidence shows that auditory cortex (AC) of humans, and other primates, is involved in more complex cognitive processes than feature segregation only, which are shaped by experience-dependent plasticity and thus likely show substantial individual variability. However, thus far, individual variability of ACs has been considered a methodological impediment rather than a phenomenon of theoretical importance. Here, we examined the variability of ACs using intrinsic functional connectivity patterns in humans and macaques. Our results demonstrate that in humans, interindividual variability is greater near the nonprimary than primary ACs, indicating that variability dramatically increases across the processing hierarchy. ACs are also more variable than comparable visual areas and show higher variability in the left than in the right hemisphere, which may be related to the left lateralization of auditory-related functions such as language. Intriguingly, remarkably similar modality differences and lateralization of variability were also observed in macaques. These connectivity-based findings are consistent with a confirmatory task-based functional magnetic resonance imaging analysis. The quantification of variability in auditory function, and the similar findings in both humans and macaques, will have strong implications for understanding the evolution of advanced auditory functions in humans.

Data-Driven Classification of Spectral Profiles Reveals Brain Region-Specific Plasticity in Blindness.

Congenital blindness has been shown to result in behavioral adaptation and neuronal reorganization, but the underlying neuronal mechanisms are largely unknown. Brain rhythms are characteristic for anatomically defined brain regions and provide a putative mechanistic link to cognitive processes. In a novel approach, using magnetoencephalography resting state data of congenitally blind and sighted humans, deprivation-related changes in spectral profiles were mapped to the cortex using clustering and classification procedures. Altered spectral profiles in visual areas suggest changes in visual alpha-gamma band inhibitory-excitatory circuits. Remarkably, spectral profiles were also altered in auditory and right frontal areas showing increased power in theta-to-beta frequency bands in blind compared with sighted individuals, possibly related to adaptive auditory and higher cognitive processing. Moreover, occipital alpha correlated with microstructural white matter properties extending bilaterally across posterior parts of the brain. We provide evidence that visual deprivation selectively modulates spectral profiles, possibly reflecting structural and functional adaptation.

Imaging Parameters of the Ipsilateral Medial Geniculate Body May Predict Prognosis of Patients with Idiopathic Unilateral Sudden Sensorineural Hearing Loss on the Basis of Diffusion Spectrum Imaging.

BACKGROUND AND PURPOSE: Idiopathic sudden sensorineural hearing loss is an acute unexplained onset of hearing loss. We examined the central auditory pathway abnormalities in patients with unilateral idiopathic sudden sensorineural hearing loss using diffusion spectrum imaging and the relationships between hearing recovery and diffusion spectrum imaging parameters. MATERIALS AND METHODS: Forty-eight patients with unilateral idiopathic sudden sensorineural hearing loss with a duration of ≤2 weeks (range, 8.9 ± 4.3 days) and 20 healthy subjects underwent diffusion spectrum imaging tractography. Hearing levels were evaluated using a pure-tone average at initial presentation and 3-month follow-up. Clinical characteristics and MR imaging findings were assessed. RESULTS: Compared with healthy control subjects, the generalized fractional anisotropy values of patients decreased significantly in the bilateral posterior limbs of the internal capsule, with no differences between the ipsilateral and contralateral sides. The quantitative anisotropy values decreased in the Brodmann area 41, contralateral medial geniculate body, bilateral lateral lemniscus, anterior limb of internal capsule, middle temporal gyrus, and anterior corona radiata. Furthermore, at 3-month follow-up, 14 patients had <15 dB of hearing gain. Receiver operating characteristic curve analysis demonstrated that generalized fractional anisotropy in the ipsilateral medial geniculate body was related to prognosis (sensitivity = 64.7%; specificity = 85.7%; area under the curve = 0.796, 95% CI, 0.661-0.931; P < .01). CONCLUSIONS: Diffusion spectrum imaging can detect abnormalities of white matter microstructure along the central auditory pathway in patients with unilateral idiopathic sudden sensorineural hearing loss. The generalized fractional anisotropy value of the ipsilateral medial geniculate body may help to predict recovery outcomes.

Magnetic resonance diffusion tensor tractography of a midbrain auditory circuit in Alligator.

Knowledge of brain circuitry is critical for understanding the organization, function, and evolution of central nervous systems. Most commonly, brain connections have been elucidated using histological and experimental methods that require animal sacrifice. On the other hand, magnetic resonance diffusion tensor imaging and associated tractography have emerged as a preferred method to noninvasively visualize brain white matter tracts. However, existing studies have primarily examined large, heavily myelinated fiber tracts. Whether tractography can visualize fiber bundles that contain thin and poorly myelinated axons is uncertain. To address this question, the midbrain auditory pathway to the thalamus was investigated in Alligator. This species was chosen because of its evolutionary importance as it is the reptilian group most closely related to birds and because its brain contains many thin and poorly myelinated tracts. Furthermore, this auditory pathway is well documented in other reptiles, including a related crocodilian. Histological observations and experimental determination of anterograde connections confirmed this path in Alligator. Tractography identified these tracts in Alligator and provided a 3-dimensional picture that accurately identified the neural elements of this circuit. In addition, tractography identified one possible unrecognized pathway. These results demonstrate that tractography can visualize circuits containing thin, poorly myelinated fibers. These findings open the door for future studies to examine these types of pathways in other vertebrates.

Visualising the topography of the acoustic radiation in clinical diffusion tensor imaging scans.

PURPOSE: It has long been thought that the acoustic radiation (AR) white matter fibre tract from the medial geniculate body of the thalamus to the Heschl's gyrus cannot be reconstructed via single-fibre analysis of clinical diffusion tensor imaging (DTI) scans. A recently developed single-fibre probabilistic method suggests otherwise. The method uses dynamic programming (DP) to compute the most probable paths between two regions of interest. This study aims to observe the ability of single-fibre probabilistic analysis via DP to visualise the AR in clinical DTI scans from legacy pilot cohorts of subjects with normal hearing (NH) and profound hearing loss (HL). METHODS: Single-fibre probabilistic analysis via DP was applied to reconstruct 3D models of the AR in the two cohorts. DTI and T1 data at 1.5 T for subjects with NH (n = 11) and HL (n = 5), as well as 3 T for NH (n = 1) and HL (n = 1), were used. RESULTS: The topographical features of AR previously observed in post-mortem and multi-fibre analyses can be visualised in DTI scans of 16 subjects and 2 atlases with a success rate of 100%. Relative to MNI coordinates, there was no significant difference in the varifold distances between the topography of the tracts in the 1.5 T cohort. CONCLUSION: The AR can be visualised in clinical 1.5 T and 3 T DTI scans using single-fibre probabilistic analysis via DP, hence, the potential for DP to visualise the AR in medical and pre-surgical applications in pathologies such as vestibular schwannoma, multiple sclerosis, thalamic tumours and stroke as well as hearing loss.

Functional magnetic resonance imaging of enhanced central auditory gain and electrophysiological correlates in a behavioral model of hyperacusis.

Hyperacusis is a debilitating hearing condition in which normal everyday sounds are perceived as exceedingly loud, annoying, aversive or even painful. The prevalence of hyperacusis approaches 10%, making it an important, but understudied medical condition. To noninvasively identify the neural correlates of hyperacusis in an animal model, we used sound-evoked functional magnetic resonance imaging (fMRI) to locate regions of abnormal activity in the central nervous system of rats with behavioral evidence of hyperacusis induced with an ototoxic drug (sodium salicylate, 250 mg/kg, i.p.). Reaction time-intensity measures of loudness-growth revealed behavioral evidence of salicylate-induced hyperacusis at high intensities. fMRI revealed significantly enhanced sound-evoked responses in the auditory cortex (AC) to 80 dB SPL tone bursts presented at 8 and 16 kHz. Sound-evoked responses in the inferior colliculus (IC) were also enhanced, but to a lesser extent. To confirm the main results, electrophysiological recordings of spike discharges from multi-unit clusters were obtained from the central auditory pathway. Salicylate significantly enhanced tone-evoked spike-discharges from multi-unit clusters in the AC from 4 to 30 kHz at intensities ≥60 dB SPL; less enhancement occurred in the medial geniculate body (MGB), and even less in the IC. Our results demonstrate for the first time that non-invasive sound-evoked fMRI can be used to identify regions of neural hyperactivity throughout the brain in an animal model of hyperacusis.