Auditory tracts identified with combined fMRI and diffusion tractography.

The auditory tracts in the human brain connect the inferior colliculus (IC) and medial geniculate body (MGB) to various components of the auditory cortex (AC). While in non-human primates and in humans, the auditory system is differentiated in core, belt and parabelt areas, the correspondence between these areas and anatomical landmarks on the human superior temporal gyri is not straightforward, and at present not completely understood. However it is not controversial that there is a hierarchical organization of auditory stimuli processing in the auditory system. The aim of this study was to demonstrate that it is possible to non-invasively and robustly identify auditory projections between the auditory thalamus/brainstem and different functional levels of auditory analysis in the cortex of human subjects in vivo combining functional magnetic resonance imaging (fMRI) with diffusion MRI, and to investigate the possibility of differentiating between different components of the auditory pathways (e.g. projections to areas responsible for sound, pitch and melody processing). We hypothesized that the major limitation in the identification of the auditory pathways is the known problem of crossing fibers and addressed this issue acquiring DTI with b-values higher than commonly used and adopting a multi-fibre ball-and-stick analysis model combined with probabilistic tractography. Fourteen healthy subjects were studied. Auditory areas were localized functionally using an established hierarchical pitch processing fMRI paradigm. Together fMRI and diffusion MRI allowed the successful identification of tracts connecting IC with AC in 64 to 86% of hemispheres and left sound areas with homologous areas in the right hemisphere in 86% of hemispheres. The identified tracts corresponded closely with a three-dimensional stereotaxic atlas based on postmortem data. The findings have both neuroscientific and clinical implications for delineation of the human auditory system in vivo

Functional sex differences in human primary auditory cortex

Background We used PET to study cortical activation during auditory stimulation and found sex differences in the human primary auditory cortex (PAC). Regional cerebral blood flow (rCBF) was measured in 10 male and 10 female volunteers while listening to sounds (music or white noise) and during a baseline (no auditory stimulation). Results and discussion We found a sex difference in activation of the left and right PAC when comparing music to noise. The PAC was more activated by music than by noise in both men and women. But this difference between the two stimuli was significantly higher in men than in women. To investigate whether this difference could be attributed to either music or noise, we compared both stimuli with the baseline and revealed that noise gave a significantly higher activation in the female PAC than in the male PAC. Moreover, the male group showed a deactivation in the right prefrontal cortex when comparing noise to the baseline, which was not present in the female group. Interestingly, the auditory and prefrontal regions are anatomically and functionally linked and the prefrontal cortex is known to be engaged in auditory tasks that involve sustained or selective auditory attention. Thus we hypothesize that differences in attention result in a different deactivation of the right prefrontal cortex, which in turn modulates the activation of the PAC and thus explains the sex differences found in the activation of the PAC. Conclusion Our results suggest that sex is an important factor in auditory brain studies

Cortical Folding Patterns and Predicting Cytoarchitecture

The human cerebral cortex is made up of a mosaic of structural areas, frequently referred to as Brodmann areas (BAs). Despite the widespread use of cortical folding patterns to perform ad hoc estimations of the locations of the BAs, little is understood regarding 1) how variable the position of a given BA is with respect to the folds, 2) whether the location of some BAs is more variable than others, and 3) whether the variability is related to the level of a BA in a putative cortical hierarchy. We use whole-brain histology of 10 postmortem human brains and surface-based analysis to test how well the folds predict the locations of the BAs. We show that higher order cortical areas exhibit more variability than primary and secondary areas and that the folds are much better predictors of the BAs than had been previously thought. These results further highlight the significance of cortical folding patterns and suggest a common mechanism for the development of the folds and the cytoarchitectonic fields.National Center for Research Resources (U.S.) (P41-RR14075)National Center for Research Resources (U.S.) (R01-RR16594-01A1)National Center for Research Resources (U.S.) (NCRR BIRN Morphometric Project BIRN002, U24 RR021382)National Institute of Biomedical Imaging and Bioengineering (U.S.) (R01 EB001550)National Institute of Biomedical Imaging and Bioengineering (U.S.) (R01 EB006758)National Institute of Neurological Disorders and Stroke (U.S.) (R01 NS052585-01)Mental Illness and Neuroscience Discovery (MIND) InstituteNational Institutes of Health (U.S.) (NIH Roadmap for Medical Research (grant U54 EB005149))Hermann von Helmholtz-Gemeinschaft Deutscher ForschungszentrenDeutsche Forschungsgemeinschaft (DFG)National Institutes of Health. National Institute for Biomedical Imaging and BioengineeringNational Institute of Neurological Disorders and Stroke (U.S.)National Institute of Mental Health (U.S.

Growing up with one ear : central auditory structure and function in unilateral ear canal atresia

The following thesis aims to give more insight into the functional and structural response of the central auditory system to congenital unilateral ear canal atresia (UCA) and the accompanying asymmetric hearing with conductive hearing loss on the atretic side. There is clear evidence that unilateral hearing loss, including UCA, has a negative impact on sound localization ability and perception of speech in noise. There is a spread in performance within the group, and the reason for this is not well known. In paper I of this thesis we examined sound localization with eye tracking and perception of speech in a cocktail party setting, in participants with congenital unilateral ear canal atresia, who had no hearing aids before age 12 (n=12) and compared to normal hearing references. Results show that the level of hearing loss on the atretic ear was associated with sound localization ability but not to speech perception. In the second study, participants with UCA (n=17) underwent MRI-scanning of the brain with diffusion weighted imaging (DWI). A method is described how to segment the white matter bundle between the medial geniculate body of the thalamus and the primary auditory cortex, the acoustic radiation (AR). Methods to define the AR are previously described in high resolution diffusion weighted imaging (DWI) scans but is very time consuming or has problems with including more structures around the primary auditory cortex (PAC). An algorithm was trained to quickly segment the core of the AR in individual clinical scans. The white matter tract was also assessed with measurements of fractional anisotropy (FA), but no differences were found between UCA and normal hearing (NH) controls. The third study describes the measurements of the grey matter of the primary auditory cortex of the Heschl’s gyrus in the same participants as in paper II. Thickness and volume of the Heschl’s gyrus were compared within the groups of UCA and controls, and between the groups. A difference in thickness was found between the left and right side (right thicker than left, corrected p=0.0012) in UCA, whereas controls had symmetric thickness. Volumes and total thickness were not different compared to controls. Rat brains from 12 months old rats with a surgically constructed left-sided ear canal atresia were examined in study IV. DWI was acquired in a research camera for rodents, 9.4 T magnetic field and a prolonged scanning time. Tractography and FA measurements were obtained both from whole brains and from tracts between auditory regions of interest (ROIs) using two different software. FA was generally higher in UCA rats than in controls. The AR was asymmetric in FA (left<right) in UCA, whereas FA was symmetric in controls. The FA was found to be lower at the left connection (same side as hearing loss) cochlear nucleus -inferior colliculus compared to the right side in UCA, while it was symmetric in controls. This finding (CN-IC) aligns with previous histology findings in ferrets with unilateral conductive hearing loss

A Missing Connection: A Review of the Macrostructural Anatomy and Tractography of the Acoustic Radiation

The auditory system of mammals is dedicated to encoding, elaborating and transporting acoustic information from the auditory nerve to the auditory cortex. The acoustic radiation (AR) constitutes the thalamo-cortical projection of this system, conveying the auditory signals from the medial geniculate nucleus (MGN) of the thalamus to the transverse temporal gyrus on the superior temporal lobe. While representing one of the major sensory pathways of the primate brain, the currently available anatomical information of this white matter bundle is quite limited in humans, thus constituting a notable omission in clinical and general studies on auditory processing and language perception. Tracing procedures in humans have restricted applications, and the in vivo reconstruction of this bundle using diffusion tractography techniques remains challenging. Hence, a more accurate and reliable reconstruction of the AR is necessary for understanding the neurobiological substrates supporting audition and language processing mechanisms in both health and disease. This review aims to unite available information on the macroscopic anatomy and topography of the AR in humans and non-human primates. Particular attention is brought to the anatomical characteristics that make this bundle difficult to reconstruct using non-invasive techniques, such as diffusion-based tractography. Open questions in the field and possible future research directions are discussed

Reduced structural connectivity between left auditory thalamus and the motion-sensitive planum temporale in developmental dyslexia

Developmental dyslexia is characterized by the inability to acquire typical reading and writing skills. Dyslexia has been frequently linked to cerebral cortex alterations; however recent evidence also points towards sensory thalamus dysfunctions: dyslexics showed reduced responses in the left auditory thalamus (medial geniculate body, MGB) during speech processing in contrast to neurotypical readers. In addition, in the visual modality, dyslexics have reduced structural connectivity between the left visual thalamus (lateral geniculate nucleus, LGN) and V5/MT, a cerebral cortex region involved in visual movement processing. Higher LGN-V5/MT connectivity in dyslexics was associated with the faster rapid naming of letters and numbers (RANln), a measure that is highly correlated with reading proficiency. We here tested two hypotheses that were directly derived from these previous findings. First, we tested the hypothesis that dyslexics have reduced structural connectivity between the left MGB and the auditory motion-sensitive part of the left planum temporale (mPT). Second, we hypothesized that the amount of left mPT-MGB connectivity correlates with dyslexics RANln scores. Using diffusion tensor imaging based probabilistic tracking we show that male adults with developmental dyslexia have reduced structural connectivity between the left MGB and the left mPT, confirming the first hypothesis. Stronger left mPT-MGB connectivity was not associated with faster RANnl scores in dyslexics, but in neurotypical readers. Our findings provide first evidence that reduced cortico-thalamic connectivity in the auditory modality is a feature of developmental dyslexia, and that it may also impact on reading related cognitive abilities in neurotypical readers

Binaural specialisation in human auditory cortex: an fMRI investigation of interaural correlation sensitivity

A listener's sensitivity to the interaural correlation (IAC) of sound plays an important role in several phenomena in binaural hearing. Although IAC has been examined extensively in neurophysiological studies in animals and in psychophysical studies in humans, little is known about the neural basis of sensitivity to IAC in humans. The present study employed functional magnetic resonance imaging to measure blood oxygen level-dependent (BOLD) activity in auditory brainstem and cortical structures in human listeners during presentation of band-pass noise stimuli between which IAC was varied systematically. The stimuli evoked significant bilateral activation in the inferior colliculus, medial geniculate body, and auditory cortex. There was a significant positive relationship between BOLD activity and IAC which was confined to a distinct subregion of primary auditory cortex located bilaterally at the lateral extent of Heschl's gyrus. Comparison with published anatomical data indicated that this area may also be cytoarchitecturally distinct. Larger differences in activation were found between levels of IAC near unity than between levels near zero. This response pattern is qualitatively compatible with previous measures of psychophysical and neurophysiological sensitivity to IAC