Functional Magnetic Resonance Imaging of Activation in Subcortical Auditory Pathway

Integration of Functional Human Auditory Neural Circuits Based on a 3D Carbon Nanotube System

The physiological interactions between the peripheral and central auditory systems are crucial for auditory information transmission and perception, while reliable models for auditory neural circuits are currently lacking. To address this issue, mouse and human neural pathways are generated by utilizing a carbon nanotube nanofiber system. The super-aligned pattern of the scaffold renders the axons of the bipolar and multipolar neurons extending in a parallel direction. In addition, the electrical conductivity of the scaffold maintains the electrophysiological activity of the primary mouse auditory neurons. The mouse and human primary neurons from peripheral and central auditory units in the system are then co-cultured and showed that the two kinds of neurons form synaptic connections. Moreover, neural progenitor cells of the cochlea and auditory cortex are derived from human embryos to generate region-specific organoids and these organoids are assembled in the nanofiber-combined 3D system. Using optogenetic stimulation, calcium imaging, and electrophysiological recording, it is revealed that functional synaptic connections are formed between peripheral neurons and central neurons, as evidenced by calcium spiking and postsynaptic currents. The auditory circuit model will enable the study of the auditory neural pathway and advance the search for treatment strategies for disorders of neuronal connectivity in sensorineural hearing loss.

A specialized channel for encoding auditory transients in the magnocellular division of the human medial geniculate nucleus

We test the hypothesis that there exists a generalized magnocellular system in the brain optimized for temporal processing. In the visual system, it is well known that the magnocellular layers in the lateral geniculate nucleus (LGN) are strongly activated by transients and quickly habituate. However, little is known about the perhaps analogous magnocellular division of the medial geniculate nucleus (MGN), the auditory relay in the thalamus. We measured the functional responses of the MGN in 11 subjects who passively listened to sustained and transient nonlinguistic sounds, using functional MRI. We observed that voxels in the ventromedial portion of the MGN, corresponding to the magnocellular division, exhibited a robust preference to transient sounds, consistently across subjects, whereas the remainder of the MGN did not discriminate between sustained and transient sounds. We conclude that the magnocellular neurons in the MGN parallel the magnocellular neurons in its visual counterpart, LGN, and constitute an information stream specialized for encoding auditory dynamics.

Influence of Acoustic Overstimulation on the Central Auditory System: An Functional Magnetic Resonance Imaging (fMRI) Study

Background

The goal of the fMRI experiment was to explore the involvement of central auditory structures in pathomechanisms of a behaviorally manifested auditory temporary threshold shift in humans.

Material/Methods

The material included 18 healthy volunteers with normal hearing. Subjects in the exposure group were presented with 15 min of binaural acoustic overstimulation of narrowband noise (3 kHz central frequency) at 95 dB(A). The control group was not exposed to noise but instead relaxed in silence. Auditory fMRI was performed in 1 session before and 3 sessions after acoustic overstimulation and involved 3.5–4.5 kHz sweeps.

Results

The outcomes of the study indicate a possible effect of acoustic overstimulation on central processing, with decreased brain responses to auditory stimulation up to 20 min after exposure to noise. The effect can be seen already in the primary auditory cortex. Decreased BOLD signal change can be due to increased excitation thresholds and/or increased spontaneous activity of auditory neurons throughout the auditory system.

Conclusions

The trial shows that fMRI can be a valuable tool in acoustic overstimulation studies but has to be used with caution and considered complimentary to audiological measures. Further methodological improvements are needed to distinguish the effects of TTS and neuronal habituation to repetitive stimulation.

Auditory and visual interhemispheric communication in musicians and non-musicians

The corpus callosum (CC) is a brain structure composed of axon fibres linking the right and left hemispheres. Musical training is associated with larger midsagittal cross-sectional area of the CC, suggesting that interhemispheric communication may be faster in musicians. Here we compared interhemispheric transmission times (ITTs) for musicians and non-musicians. ITT was measured by comparing simple reaction times to stimuli presented to the same hemisphere that controlled a button-press response (uncrossed reaction time), or to the contralateral hemisphere (crossed reaction time). Both visual and auditory stimuli were tested. We predicted that the crossed-uncrossed difference (CUD) for musicians would be smaller than for non-musicians as a result of faster interhemispheric transfer times. We did not expect a difference in CUDs between the visual and auditory modalities for either musicians or non-musicians, as previous work indicates that interhemispheric transfer may happen through the genu of the CC, which contains motor fibres rather than sensory fibres. There were no significant differences in CUDs between musicians and non-musicians. However, auditory CUDs were significantly smaller than visual CUDs. Although this auditory-visual difference was larger in musicians than non-musicians, the interaction between modality and musical training was not significant. Therefore, although musical training does not significantly affect ITT, the crossing of auditory information between hemispheres appears to be faster than visual information, perhaps because subcortical pathways play a greater role for auditory interhemispheric transfer.

Functional magnetic resonance imaging of the ascending stages of the auditory system in dogs

Background

Functional magnetic resonance imaging (fMRI) is a technique able to localize neural activity in the brain by detecting associated changes in blood flow. It is an essential tool for studying human functional neuroanatomy including the auditory system. There are only a few studies, however, using fMRI to study canine brain functions. In the current study ten anesthetized dogs were scanned during auditory stimulation. Two functional sequences, each in combination with a suitable stimulation paradigm, were used in each subject. Sequence 1 provided periods of silence during which acoustic stimuli could be presented unmasked by scanner noise (sparse temporal sampling) whereas in sequence 2 the scanner noise was present throughout the entire session (continuous imaging). The results obtained with the two different functional sequences were compared.

Results

This study shows that with the proper experimental setup it is possible to detect neural activity in the auditory system of dogs. In contrast to human fMRI studies the strongest activity was found in the subcortical parts of the auditory pathways. Especially sequence 1 showed a high reliability in detecting activated voxels in brain regions associated with the auditory system.

Conclusion

These results indicate that fMRI is applicable for studying the canine auditory system and could become an additional method for the clinical evaluation of the auditory function of dogs. Additionally, fMRI is an interesting technique for future studies concerned with canine functional neuroanatomy.

Hearing without listening: functional connectivity reveals the engagement of multiple nonauditory networks during basic sound processing

The present functional magnetic resonance imaging (fMRI) study presents data challenging the traditional view that sound is processed almost exclusively in the classical auditory pathway unless imbued with behavioral significance. In a first experiment, subjects were presented with broadband noise in on/off fashion as they performed an unrelated visual task. A conventional analysis assuming predictable sound-evoked responses demonstrated a typical activation pattern that was confined to classical auditory centers. In contrast, spatial independent component analysis (sICA) disclosed multiple networks of acoustically responsive brain centers. One network comprised classical auditory centers, but four others included nominally "nonauditory" areas: cingulo-insular cortex, mediotemporal limbic lobe, basal ganglia, and posterior orbitofrontal cortex, respectively. Functional connectivity analyses confirmed the sICA results by demonstrating coordinated activity between the involved brain structures. In a second experiment, fMRI data obtained from unstimulated (i.e., resting) subjects revealed largely similar networks. Together, these two experiments suggest the existence of a coordinated system of multiple acoustically responsive intrinsic brain networks, comprising classical auditory centers but also other brain areas. Our results suggest that nonauditory centers play a role in sound processing at a very basic level, even when the sound is not intertwined with behaviors requiring the well-known functionality of these regions.

Perspective of functional magnetic resonance imaging in middle ear research

Functional magnetic resonance imaging (MRI) studies have frequently been applied to study sensory system such as vision, language, and cognition, but have proceeded at a considerably slower speed in investigating middle ear and central auditory processing. This is due to several factors, including the intrinsic anatomy of the middle ear system and inherent acoustic noise during acquisition of MRI data. However, accumulating evidences have demonstrated that clarification of some fundamental neural underpinnings of audition associated with middle ear mechanics can be achieved using functional MRI methods. This mini review attempted to take a narrow snapshot of the currently available functional MRI procedures and gave examples of what may be learned about hearing from their application. It is hoped that with these technical advancements, many new high impact applications in audition would follow. In particular, because the fMRI can be used in humans and in animals, fMRI may represent a unique tool that should promote translational research by enabling parallel analyses of physiological and pathological processes in the human and animal auditory system. This article is part of a special issue entitled "MEMRO 2012".

BOLD fMRI investigation of the rat auditory pathway and tonotopic organization

Rodents share general anatomical, physiological and behavioral features in the central auditory system with humans. In this study, monaural broadband noise and pure tone sounds are presented to normal rats and the resulting hemodynamic responses are measured with blood oxygenation level-dependent (BOLD) fMRI using a standard spin-echo echo planar imaging sequence (without sparse temporal sampling). The cochlear nucleus (CN), superior olivary complex, lateral lemniscus, inferior colliculus (IC), medial geniculate body and primary auditory cortex, all major auditory structures, are activated by broadband stimulation. The CN and IC BOLD signal changes increase monotonically with sound pressure level. Pure tone stimulation with three distinct frequencies (7, 20 and 40 kHz) reveals the tonotopic organization of the IC. The activated regions shift from dorsolateral to ventromedial IC with increasing frequency. These results agree with electrophysiology and immunohistochemistry findings, indicating the feasibility of auditory fMRI in rats. This is the first fMRI study of the rodent ascending auditory pathway.

The effect of slice orientation on auditory FMRI at the level of the brainstem

Although auditory information is processed in several subcortical nuclei, most fMRI studies focus solely on the auditory cortex and do not take brainstem responses into account. One common difficulty in obtaining clear functional brainstem recordings is due to heartbeat related motion, manifested in the rostro-caudal and in the ventro-dorsal directions in the contraction phase of the heart. The aim of this study was to investigate the effect of slice orientation on auditory functional magnetic resonance imagining (fMRI) measurements with respect to the pattern of brainstem oscillation. Fourteen healthy volunteers listened monaurally to modulated pink noise. Blood oxygenation level dependent (BOLD) contrast was performed with an echo-planar image (EPI) sequence using a 3T MRI system. Three different slice orientations were compared: approximately parallel, at 45 degrees , and orthogonal to the brainstem. The standard deviation of the residuals, the effect size, the median t-values, and the number of activated voxels were calculated to quantify variability in activation between orientations. The data for the inferior colliculi indicated that a slice orientation with a 45 degrees angle to the brainstem yielded the lowest sensitivity to motion (reflected in the standard deviation of the residuals). By contrast, the results did not suggest differences between the three imaging planes on the scanning of the auditory cortex. Findings indicate that the 45 degrees slice orientation is the optimum orientation for accurate measurement at the upper brainstem level.

Activation of Cortical and Subcortical Auditory Structures at 3 T by Means of a Functional Magnetic Resonance Imaging Paradigm Suitable for Clinical Use

OBJECTIVES

Visualization of functional magnetic resonance imaging (fMRI) activation of subcortical auditory structures remains challenging because of the cardiac-related pulsatile movement of both the brainstem and the cerebrospinal fluid and involved, until now, special scanning, pre- and postprocessing techniques, which are not convenient in clinical settings. The aim of this study is to examine the activation in both cortical and subcortical auditory structures by means of an fMRI paradigm, which is suitable for clinical use.

MATERIALS AND METHODS

Twenty subjects (13 volunteers and 7 patients) were examined on a 3 T imaging system with binaural musical stimulation.

RESULTS

Both cortical and subcortical auditory structures are successfully visualized in volunteers and patients.

CONCLUSIONS

Activation of both the cortical and subcortical auditory structures can be visualized by means of an appropriate fMRI setup at 3 T. This paradigm can easily be used in patients with tumors and/or hearing disorders.

Lateralization, connectivity and plasticity in the human central auditory system

Although it is known that responses in the auditory cortex are evoked predominantly contralateral to the side of stimulation, the lateralization of responses at lower levels in the human central auditory system has hardly been studied. Furthermore, little is known on the functional interactions between the involved processing centers. In this study, functional MRI was performed using sound stimuli of varying left and right intensities. In normal hearing subjects, contralateral activation was consistently detected in the temporal lobe, thalamus and midbrain. Connectivity analyses showed that auditory information crosses to the contralateral side in the lower brainstem followed by ipsilateral signal conduction towards the auditory cortex, similar to the flow of auditory signals in other mammals. In unilaterally deaf subjects, activation was more symmetrical for the cortices but remained contralateral in the midbrain and thalamus. Input connection strengths were different only at cortical levels, and there was no evidence for plastic reorganization at subcortical levels.

Functional magnetic resonance imaging: contemporary and future use

PURPOSE OF REVIEW

Functional magnetic resonance imaging is a relatively new neuroimaging technique that is being used in both research and clinical applications. Increasing work has been done to elucidate the auditory cortex.

RECENT FINDINGS

Current studies focus on enhancing the sensitivity of functional magnetic resonance imaging in studying the auditory cortex and subcortical pathways in response to tonal stimulation, to evaluate the integrity of the auditory cortex before cochlear implantation, and as a screening tool for hearing impairment in the young child.

SUMMARY

Recent work has been encouraging: silent functional magnetic resonance imaging techniques allow for better evaluation of the auditory cortex with less confounding scanner noises. Functional magnetic resonance imaging can be safely and reproducibly performed in hearing-impaired children and in the preoperative evaluation of candidates for cochlear implantation.