Inter-hemispheric asymmetries in STS and AF and the structural relationship between the two

Superior temporal sulcus (STS), a grey matter structure, and arcuate fasciculus (AF), a white matter tract, have several characteristics in common. They are structurally asymmetric across brain hemispheres, they seem to hold overlapping cognitive functions, and they are located in close proximity to each other. The purpose of the current study is to explore the structural relationship between STS and AF. Two hypotheses are stated. First, it is hypothesised that the depth asymmetry in STS will correlate negatively with AF volume tract asymmetry, as they are assumed to compete for the same limited space in the brain. The second hypothesis is based on evidence suggesting that STS and AF are involved in language processing. It is hypothesised that a positive correlation between STS asymmetry and AF asymmetry will be found for variables assumed relevant for language processing. To address this, T1-weigted anatomical MR images and diffusion-weighted MR images were acquired from twenty participants. The current findings do not support the hypotheses. Negative correlations were found between left AF volume and left STS depth and between left AF volume and left STS length. In the right hemisphere, positive correlations were found between right AF volume and right STS and between right AF volume and right STS length. In light of this, two new hypotheses are suggested. In the right hemisphere, STS and AF may share social cognitive functionality. In the left hemisphere, a language related compensation mechanism may underlie the correlations.Superior temporal sulucs (STS), ein gråsubstans-struktur, og arcuate fasciculus (AF), ein kvitsubstans-fiberbunt, har fleire eigenskapar til felles. Dei er strukturelt asymmetriske på tvers av hjernehemisfærane, dei synest å ha overlappande kognitive funksjonar, og dei ligg i nærleiken av kvarandre. Føremålet med denne studien er å utforske det strukturelle forholdet mellom STS og AF. To hypotesar er føreslått. Først, ein antar at den djupneasymmetrien i STS korrelerer negativt med volumasymmetrien i AF fordi desse tenkast å konkurrere om overlappande områder i hjernen. Den andre hypotesen er basert på forsking som indikerer at både STS og AF er involvert i språkprosessering. Ein antar å finne ein positiv korrelasjon mellom STS-asymmetri og AF-asymmetri for variablar antas å vere relevante for språkprosessering. For å adressere dette vart det innhenta T1-vekta anatomiske MR-bilete og diffusjonsvekta MR-bilete frå tjue deltakarar. Resultatet av studien gir ikkje støtte til hypotesane. Ein finn negative korrelasjonar mellom venstre AF volum og venstre STS djupn og venstre AF volum og STS lengde. I høgre hemisfære finner ein at høgre AF volum korrelerer positive med STS djupn og høgre AF volum korrelerer positivt med høgre STS lengde. I lys av dette, foreslås to nye hypotesar. I høgre hemisfære antas STS og AF å dele sosialkognitiv funksjonalitet. I venstre hemisfære antas ein språkrelatert kompenasjonsmekanisme å ligge til grunn for korrelasjonen.PROPSY317PRPSY

The infancy of the human brain

The human infant brain is the only known machine able to master a natural language and develop explicit, symbolic, and communicable systems of knowledge that deliver rich representations of the external world. With the emergence of non-invasive brain imaging, we now have access to the unique neural machinery underlying these early accomplishments. After describing early cognitive capacities in the domains of language and number, we review recent findings that underline the strong continuity between human infants’ and adults’ neural architecture, with notably early hemispheric asymmetries and involvement of frontal areas. Studies of the strengths and limitations of early learning, and of brain dynamics in relation to regional maturational stages, promise to yield a better understanding of the sources of human cognitive achievements.This work was supported by the Center for Brains, Minds and Machines (CBMM), funded by NSF STC award CCF – 1231216

Cognitive and Neurophysiological Models of Brain Asymmetry

Asymmetry is an inherent characteristic of brain organization in both humans and other vertebrate species, and is evident at the behavioral, neurophysiological, and structural levels. Brain asymmetry underlies the organization of several cognitive systems, such as emotion, communication, and spatial processing. Despite this ubiquity of asymmetries in the vertebrate brain, we are only beginning to understand the complex neuronal mechanisms underlying the interaction between hemispheric asymmetries and cognitive systems. Unfortunately, despite the vast number of empirical studies on brain asymmetries, theoretical models that aim to provide mechanistic explanations of hemispheric asymmetries are sparse in the field. Therefore, this Special Issue aims to highlight empirically based mechanistic models of brain asymmetry. Overall, six theoretical and four empirical articles were published in the Special Issue, covering a wide range of topics, from human handedness to auditory laterality in bats. Two key challenges for theoretical models of brain asymmetry are the integration of increasingly complex molecular data into testable models, and the creation of theoretical models that are robust and testable across different species

Cortical Surface Area and Cortical Thickness Demonstrate Differential Structural Asymmetry in Auditory-Related Areas of the Human Cortex

This investigation provides an analysis of structural asymmetries in 5 anatomically defined regions (Heschl's gyrus, HG; Heschl's sulcus, HS; planum temporale, PT; planum polare, PP; superior temporal gyrus, STG) within the human auditory-related cortex. Volumetric 3-dimensional T1-weighted magnetic resonance imaging scans were collected from 104 participants (52 males). Cortical volume (CV), cortical thickness (CT), and cortical surface area (CSA) were calculated based on individual scans of these anatomical traits. This investigation demonstrates a leftward asymmetry for CV and CSA that is observed in the HG, STG, and PT regions. As regards CT, we note a rightward asymmetry in the HG and HS. A correlation analysis of asymmetry indices between measurements for distinct regions of interest (ROIs) yields significant correlations between CT and CV in 4 of 5 ROIs (HG, HS, PT, and STG). Significant correlation values between CSA and CV are observed for all 5 ROIs. The findings suggest that auditory-related cortical areas demonstrate larger leftward asymmetry with respect to the CSA, while a clear rightward asymmetry with respect to CT is salient in both the primary and the secondary auditory cortex only. In addition, we propose that CV is not an ideal neuromarker for anatomical measurements. CT and CSA should be considered independent traits of anatomical asymmetries in the auditory-related corte

Asymmetry of the Frontal Aslant Tract is Associated with Lexical Decision

The frontal aslant tract (FAT) is a recently documented white matter tract that connects the inferior and superior frontal gyri with a tendency to be more pronounced in the left hemisphere. This tract has been found to play a role in language functions, particularly verbal fluency. However, it is not entirely clear to what extent FAT asymmetry is related to performance benefits in language-related tasks. In the present study, we aimed to fill this gap by examining the correlations between asymmetric micro- and macro-structural properties of the FAT and performance on verbal fluency and lexical decision tasks. The results showed no correlation between the FAT and verbal fluency; however, lexical decision was correlated with FAT laterality. Specifically, greater left lateralization in both micro- and macro-structural properties was related to faster lexical decision response times. The results were not due merely to motor or decision-making processes, as responses in a simple discrimination task showed no correlation with laterality. These data are the first to suggest a role for the FAT in mediating processes underlying lexical decision

Speech-brain synchronization: a possible cause for developmental dyslexia

152 p.Dyslexia is a neurological learning disability characterized by the difficulty in an individual¿s ability to read despite adequate intelligence and normal opportunities. The majority of dyslexic readers present phonological difficulties. The phonological difficulty most often associated with dyslexia is a deficit in phonological awareness, that is, the ability to hear and manipulate the sound structure of language. Some appealing theories of dyslexia attribute a causal role to auditory atypical oscillatory neural activity, suggesting it generates some of the phonological problems in dyslexia. These theories propose that auditory cortical oscillations of dyslexic individuals entrain less accurately to the spectral properties of auditory stimuli at distinct frequency bands (delta, theta and gamma) that are important for speech processing. Nevertheless, there are diverging hypotheses concerning the specific bands that would be disrupted in dyslexia, and which are the consequences of such difficulties on speech processing. The goal of the present PhD thesis was to portray the neural oscillatory basis underlying phonological difficulties in developmental dyslexia. We evaluated whether phonological deficits in developmental dyslexia are associated with impaired auditory entrainment to a specific frequency band. In that aim, we measured auditory neural synchronization to linguistic and non-linguistic auditory signals at different frequencies corresponding to key phonological units of speech (prosodic, syllabic and phonemic information). We found that dyslexic readers presented atypical neural entrainment to delta, theta and gamma frequency bands. Importantly, we showed that atypical entrainment to theta and gamma modulations in dyslexia could compromise perceptual computations during speech processing, while reduced delta entrainment in dyslexia could affect perceptual and attentional operations during speech processing. In addition, we characterized the links between the anatomy of the auditory cortex and its oscillatory responses, taking into account previous studies which have observed structural alterations in dyslexia. We observed that the cortical pruning in auditory regions was linked to a stronger sensitivity to gamma oscillation in skilled readers, but to stronger theta band sensitivity in dyslexic readers. Thus, we concluded that the left auditory regions might be specialized for processing phonological information at different time scales (phoneme vs. syllable) in skilled and dyslexic readers. Lastly, by assessing both children and adults on similar tasks, we provided the first evaluation of developmental modulations of typical and atypical auditory sampling (and their structural underpinnings). We found that atypical neural entrainment to delta, theta and gamma are present in dyslexia throughout the lifespan and is not modulated by reading experience

Cortical Representation of Lateralized Grasping in Chimpanzees (Pan troglodytes): A Combined MRI and PET Study

Functional imaging studies in humans have localized the motor-hand region to a neuroanatomical landmark call the KNOB within the precentral gyrus. It has also been reported that the KNOB is larger in the hemisphere contralateral to an individual's preferred hand, and therefore may represent the neural substrate for handedness. The KNOB has also been neuronatomically described in chimpanzees and other great apes and is similarly associated with handedness. However, whether the chimpanzee KNOB represents the hand region is unclear from the extant literature. Here, we used PET to quantify neural metabolic activity in chimpanzees when engaged in unilateral reach-and-grasping responses and found significantly lateralized activation of the KNOB region in the hemisphere contralateral to the hand used by the chimpanzees. We subsequently constructed a probabilistic map of the KNOB region in chimpanzees in order to assess the overlap in consistency in the anatomical landmarks of the KNOB with the functional maps generated from the PET analysis. We found significant overlap in the anatomical and functional voxels comprising the KNOB region, suggesting that the KNOB does correspond to the hand region in chimpanzees. Lastly, from the probabilistic maps, we compared right- and left-handed chimpanzees on lateralization in grey and white matter within the KNOB region and found that asymmetries in white matter of the KNOB region were larger in the hemisphere contralateral to the preferred hand. These results suggest that neuroanatomical asymmetries in the KNOB likely reflect changes in connectivity in primary motor cortex that are experience dependent in chimpanzees and possibly humans

Arcuate Fasciculus Abnormalities and Their Relationship with Psychotic Symptoms in Schizophrenia

Disruption of fronto-temporal connections involving the arcuate fasciculus (AF) may underlie language processing anomalies and psychotic features such as auditory hallucinations in schizophrenia. No study to date has specifically investigated abnormalities of white matter integrity at particular loci along the AF as well as its regional lateralization in schizophrenia. We examined white matter changes (fractional anisotropy (FA), axial diffusivity (AD), asymmetry indices) along the whole extent of the AF and their relationship with psychotic symptoms in 32 males with schizophrenia and 44 healthy males. Large deformation diffeomorphic metric mapping and Fiber Assignment Continuous Tracking were employed to characterize FA and AD along the geometric curve of the AF. Our results showed that patients with schizophrenia had lower FA in the frontal aspects of the left AF compared with healthy controls. Greater left FA and AD lateralization in the temporal segment of AF were associated with more severe positive psychotic symptoms such as delusions and hallucinations in patients with schizophrenia. Disruption of white matter integrity of the left frontal AF and accentuation of normal left greater than right asymmetry of FA/AD in the temporal AF further support the notion of aberrant fronto-temporal connectivity in schizophrenia. AF pathology can affect corollary discharge of neural signals from frontal speech/motor initiation areas to suppress activity of auditory cortex that may influence psychotic phenomena such as auditory hallucinations and facilitate elaboration of delusional content