Microstructural grey matter parcellation and its relevance for connectome analyses

The human brain connectome is closely linked to the anatomical framework provided by the structural segregation of the cortex into distinct cortical areas. Therefore, a thorough anatomical reference for the analysis and interpretation of connectome data is indispensable to understand the structure and function of different regions of the cortex, the white matter fibre architecture connecting them, and the interplay between these different entities. This article focuses on parcellation schemes of the cerebral grey matter and their relevance for connectome analyses. In particular, benefits and limitations of using different available atlases and parcellation schemes are reviewed. It is furthermore discussed how atlas information is currently used in connectivity analyses with major focus on seed-based and seed-target analyses, connectivity-based parcellation results, and the robust anatomical interpretation of connectivity data. Particularly this last aspect opens the possibility of integrating connectivity information into given anatomical frameworks, paving the way to multi-modal atlases of the human brain for a thorough understanding of structure-function relationships.

Activation shift in elderly subjects across functional systems: an fMRI study

The functional specificity of brain areas is diminished with age and accompanied by the recruitment of additional brain regions in healthy older adults. This process has repeatedly been demonstrated within distinct functional domains, in particular the visual system. However, it is yet unclear, whether this phenomenon in healthy aging, i.e., a reduced activation of task-associated areas and increased activation of additional regions, is also present across different functional systems. In the present functional imaging study, comprising 102 healthy subjects, we therefore assessed two distinct tasks engaging the sensory-motor system and the visual attention system, respectively. We found a significant interaction between age and task in the parietal operculum bilaterally. This area as a part of the sensory-motor system showed an age-related decrease in its BOLD-response to the motor task and an age-related increase of neural activity in response to the visual attention task. The opposite response pattern, i.e., reduced visual attention activation and increased response to the motor task, was observed for regions associated with the visual task: the superior parietal area 7A and the dorsal pre-motor cortex. Importantly, task performance was not correlated with age in either task. This age-by-task interaction indicates that a reduction of functional specificity in the aging brain may be counteracted by the increased recruitment of additional regions not only within, but also across functional domains. Our results thus emphasize the need for comparisons across different functional domains to gain a better understanding of age-related effects on the specificity of functional systems.

Development of cortical folding during evolution and ontogeny

Cortical folding is a hallmark of many, but not all, mammalian brains. The degree of folding increases with brain size across mammals, but at different scales between orders and families. In this review we summarize recent studies that have shed light on cortical folding and discuss new models that arise from these data. Genetic analyses argue for an independent development of brain volume and gyrification, but more recent data on the cellular development of the cortex and its connectivity highlight the role of these processes in cortical folding (grey matter hypothesis). This, and the widely discussed tension hypothesis, further tested by analyzing the mechanical properties of maturing nerve fibers, synapses, and dendrites, can provide the basis for a future integrative view on cortical folding.

Experimental induction of reading difficulties in normal readers provides novel insights into the neurofunctional mechanisms of visual word recognition

Phonological and visual dysfunctions may result in reading deficits like those encountered in developmental dyslexia. Here, we use a novel approach to induce similar reading difficulties in normal readers in an event-related fMRI study, thus systematically investigating which brain regions relate to different pathways relating to orthographic-phonological (e.g. grapheme-to-phoneme conversion, GPC) vs. visual processing. Based upon a previous behavioural study (Tholen et al. 2011), the retrieval of phonemes from graphemes was manipulated by lowering the identifiability of letters in familiar vs. unfamiliar shapes. Visual word and letter processing was impeded by presenting the letters of a word in a moving, non-stationary manner. FMRI revealed that the visual condition activated cytoarchitectonically defined area hOC5 in the magnocellular pathway and area 7A in the right mesial parietal cortex. In contrast, the grapheme manipulation revealed different effects localised predominantly in bilateral inferior frontal gyrus (left cytoarchitectonic area 44; right area 45) and inferior parietal lobule (including areas PF/PFm), regions that have been demonstrated to show abnormal activation in dyslexic as compared to normal readers. This pattern of activation bears close resemblance to recent findings in dyslexic samples both behaviourally and with respect to the neurofunctional activation patterns. The novel paradigm may thus prove useful in future studies to understand reading problems related to distinct pathways, potentially providing a link also to the understanding of real reading impairments in dyslexia.

Superficially located white matter structures commonly seen in the human and the macaque brain with diffusion tensor imaging

The white matter of the brain consists of fiber tracts that connect different regions of the brain. Among these tracts, the intrahemispheric cortico-cortical connections are called association fibers. The U-fibers are short association fibers that connect adjacent gyri. These fibers were thought to work as part of the cortico-cortical networks to execute associative brain functions. However, their anatomy and functions have not been documented in detail for the human brain. In past studies, U-fibers have been characterized in the human brain with diffusion tensor imaging (DTI). However, the validity of such findings remains unclear. In this study, DTI of the macaque brain was performed, and the anatomy of U-fibers was compared with that of the human brain reported in a previous study. The macaque brain was chosen because it is the most commonly used animal model for exploring cognitive functions and the U-fibers of the macaque brain have been already identified by axonal tracing studies, which makes it an ideal system for confirming the DTI findings. Ten U-fibers found in the macaque brain were also identified in the human brain, with a similar organization and topology. The delineation of these species-conserved white matter structures may provide new options for understanding brain anatomy and function.

Cytoarchitectonic mapping of the human dorsal extrastriate cortex

The dorsal visual stream consists of several functionally specialized areas, but most of their cytoarchitectonic correlates have not yet been identified in the human brain. The cortex adjacent to Brodmann area 18/V2 was therefore analyzed in serial sections of ten human post-mortem brains using morphometrical and multivariate statistical analyses for the definition of areal borders. Two previously unknown cytoarchitectonic areas (hOc3d, hOc4d) were detected. They occupy the medial and, to a smaller extent, lateral surface of the occipital lobe. The larger area, hOc3d, is located dorso-lateral to area V2 in the region of superior and transverse occipital, as well as parieto-occipital sulci. Area hOc4d was identified rostral to hOc3d; it differed from the latter by larger pyramidal cells in lower layer III, thinner layers V and VI, and a sharp cortex-white-matter borderline. The delineated areas were superimposed in the anatomical MNI space, and probabilistic maps were calculated. They show a relatively high intersubject variability in volume and position. Based on their location and neighborhood relationship, areas hOc3d and hOc4d are putative anatomical substrates of functionally defined areas V3d and V3a, a hypothesis that can now be tested by comparing probabilistic cytoarchitectonic maps and activation studies of the living human brain.

The Role of Human Parietal Area 7A as a Link between Sequencing in Hand Actions and in Overt Speech Production

Research on the evolutionary basis of the human language faculty has proposed the mirror neuron system as a link between motor processing and speech development. Consequently, most work has focused on the left inferior frontal cortex, in particular Broca's region, and the left inferior parietal cortex. However, the direct link between planning of hand motor and speech actions has yet to be elucidated. Thus, the present study investigated whether motor sequencing of hand vs. speech actions has a common neural denominator. For the hand motor task, 25 subjects performed single, repeated, or sequenced button presses with either the left or right hand. The speech task was in analogy; the same subjects produced the syllable "po" once or repeatedly, or a sequence of different syllables ("po-pi-po"). Speech motor vs. hand motor effectors resulted in increased perisylvian activation including Broca's region (left area 44 and areas medially adjacent to left area 45). In contrast, common activation for sequenced vs. repeated production of button presses and syllables revealed the effector-independent involvement of left area 7A in the superior parietal lobule (SPL) in sequencing. These data demonstrate that sequencing of vocal gestures, an important precondition for ordered utterances and ultimately human speech, shares area 7A, rather than inferior parietal regions, as a common cortical module with hand motor sequencing. Interestingly, area 7A has previously also been shown to be involved in the observation of hand and non-hand actions. In combination with the literature, the present data thus suggest a distinction between area 44, which is specifically recruited for (cognitive aspects of) speech, and SPL area 7A for general aspects of motor sequencing. In sum, the study demonstrates a previously underspecified role of the SPL in the origins of speech, and may be discussed in the light of embodiment of speech and language in the motor system.

Longitudinal deformation-based morphometry reveals spatio-temporal dynamics of brain volume changes in patients with corticobasal syndrome

Background/Objective

Corticobasal syndrome (CBS) is a rare neurodegenerative disorder characterized by a progressive and asymmetric manifestation of cortical and basal-ganglia symptoms of different origin. The spatio-temporal dynamics of cerebral atrophy in CBS is barely known. This study aimed to longitudinally quantify the individual dynamics of brain volume changes in patients with CBS as compared to healthy controls.

Methods

We used deformation-field-based morphometry (DFM) to study volumetric changes of each individual brain in short intervals of a few months. DFM enabled the quantitative analysis of local volume changes without predefining regions of interest in MR images of 6 patients with CBS and 11 matched healthy controls. A total of 64 whole brain 3D-MR-scans were acquired two to eight times over periods of 14 to 26 months. Based on repeated registrations of MR observations to the initial scan, maps of local volume ratio changes were computed.

Results

Compared to controls patients showed significant and increasing volume loss over time in premotor and primary-motor-cortices, somatosensory area 3a, superior parietal areas BA 5/7, and corticospinal tract. Furthermore, significant and asymmetric atrophy was identified in the caudate nucleus head, putamen, pallidum, motor-thalamus and substantia nigra. Temporal lobe was affected in those patients who presented progressive cognitive impairment.

Conclusions

The analysis revealed localized, pathological changes in brains of patients with CBS, which differed significantly from those occurring during aging in healthy controls. As compared to age- and sex-matched controls, brains of CBS patients showed a common degenerating neural network comprising the motor circuit with basal ganglia and motor thalamic nuclei as well as the premotor and primary-motor-cortex.

Architecture and organizational principles of Broca's region

Identifying cortical areas for language and speech processing is a prerequisite for cognitive neuroscience and clinical research. Although Broca's region is one of the essential nodes in the language network, its anatomical constituents are ill-defined and multiple definitions of Broca's region exist. Sanides' concept of microstructural gradations interpreted Broca's region as developing from neighboring motor, dorsolateral-prefrontal, and insular cortices. Recent mapping approaches based on cytoarchitecture, transmitter receptor distribution, and connectivity revealed a highly differentiated segregation of this region far beyond Brodmann's classical scheme. This novel segregational concept of structural and functional architecture more adequately reflects the various functions of Broca's region in cognitive and/or linguistic processes.

Cytoarchitectonical analysis and probabilistic mapping of two extrastriate areas of the human posterior fusiform gyrus

The human extrastriate visual cortex comprises numerous functionally defined areas, which are not identified in the widely used cytoarchitectonical map of Brodmann. The ventral part of the extrastriate cortex is particularly devoted to the identification of visual objects, faces and word forms. We analyzed the region immediately antero-lateral to hOc4v in serially sectioned (20 μm) and cell body-stained human brains using a quantitative observer-independent cytoarchitectonical approach to further identify the anatomical organization of the extrastriate cortex. Two novel cytoarchitectonical areas, FG1 and FG2, were identified on the posterior fusiform gyrus. The results of ten postmortem brains were then registered to their MRI volumes (acquired before histological processing), 3D reconstructed, and spatially normalized to the Montreal Neurological Institute reference brain. Finally, probabilistic maps were generated for each cytoarchitectonical area by superimposing the areas of the individual brains in the reference space. Comparison with recent functional imaging studies yielded that both areas are located within the object-related visual cortex. FG1 fills the gap between the retinotopically mapped area VO-1 and a posterior fusiform face patch. FG2 is probably the correlate of this face patch.

The time course of neurolinguistic and neuropsychological symptoms in three cases of logopenic primary progressive aphasia

Primary progressive aphasia (PPA) is a rare clinical dementia syndrome affecting predominantly language abilities. Word-finding difficulties and comprehension deficits despite relatively preserved cognitive functions are characteristic symptoms during the first two years, and distinguish PPA from other dementia types like Alzheimer's disease. However, the dynamics of changes in language and non-linguistic abilities are not well understood. Most studies on progression used cross-sectional designs, which provide only limited insight into the course of the disease. Here we report the results of a longitudinal study in three cases of logopenic PPA over a period of 18 months, with exemplary longitudinal data from one patient even over 46 months. A comprehensive battery of neurolinguistic and neuropsychological tests was applied four times at intervals of six months. Over this period, deterioration of verbal abilities such as picture naming, story retelling, and semantic word recall was found, and the individual decline was quantified and compared between the three patients. Furthermore, decrease in non-verbal skills such as divided attention and increasing apraxia was observed in all three patients. In addition, inter-subject variability in the progression with different focuses was observed, with one patient developing a non-fluent PPA variant. The longitudinal, multivariate investigation of logopenic PPA thus provides novel insights into the progressive deterioration of verbal as well as non-verbal abilities. These deficits may further interact and thus form a multi-causal basis for the patients' problems in every-day life which need to be considered when planning individually targeted intervention in PPA.

The language-number interface in the brain: a complex parametric study of quantifiers and quantities

The neural bases for numerosity and language are of perennial interest. In monkeys, neural separation of numerical Estimation and numerical Comparison has been demonstrated. As linguistic and numerical knowledge can only be compared in humans, we used a new fMRI paradigm in an attempt to dissociate Estimation from Comparison, and at the same time uncover the neural relation between numerosity and language. We used complex stimuli: images depicting a proportion between quantities of blue and yellow circles were coupled with sentences containing quantifiers that described them (e.g., “most/few of the circles are yellow”). Participants verified sentences against images. Both Estimation and Comparison recruited adjacent, partially overlapping bi-hemispheric fronto-parietal regions. Additional semantic analysis of positive vs. negative quantifiers involving the interpretation of quantity and numerosity specifically recruited left area 45. The anatomical proximity between numerosity regions and those involved in semantic analysis points to subtle links between the number system and language. Results fortify the homology of Estimation and Comparison between humans and monkeys.

Processing noncanonical sentences in broca's region: reflections of movement distance and type

Various noncanonical sentence constructions are derived from basic sentence structures by a phrase displacement called Movement. The moved phrase (filler) leaves a silent copy at the extracted position (gap) and is reactivated when the hearer/reader passes over the gap. Consequently, memory operations are assumed to occur to establish the filler-gap link. For languages that have a relatively free word order like German, a distinct linguistic operation called Scrambling is proposed. Although Movement and Scrambling are assumed to be different linguistic operations, they both involve memory prone filler-gap processes. To clarify whether filler-gap memory processes in Scrambling and Movement differ neuroanatomically, we designed a functional magnetic resonance imaging study and compared the effect of memory load parameterized by filler-gap distance in the 2 sentence types. Here, we show that processing of the 2 sentence types commonly relies on a left hemispheric network consisting of the inferior frontal gyrus, middle part of the middle temporal gyrus, and intraparietal sulcus. However, we found differences for the 2 sentence types in the linearity of filler-gap distance effect. Thus, the present results suggest that the same neural substrate supports the memory processes of sentences constructed by Movement and Scrambling, although differentially modulated by memory load.

Organization of the human inferior parietal lobule based on receptor architectonics

Human inferior parietal lobule (IPL) plays a key role in various cognitive functions. Its functional diversity, including attention, language, and action processing, is reflected by its structural segregation into 7 cytoarchitectonically distinct areas, each with characteristic connectivity patterns. We hypothesized that commonalities of the cytoarchitectonic, connectional, and functional diversity of the IPL should be reflected by a correlated transmitter receptor-based organization. Since the function of a cortical area requires a well-tuned receptor balance, the densities of 15 different receptors were measured in each IPL area. A hierarchical cluster analysis of the receptor balance revealed a tripartite segregation of the IPL into a rostral, middle, and caudal group. Comparison with other cortical areas showed strong similarities with Broca's region for all 3 groups, with the superior parietal cortex for the middle, and with extrastriate visual areas for the caudal group. Notably, caudal-most area PGp has a receptor fingerprint very similar to that of ventral extrastriate visual cortex. We therefore propose a new organizational model of the human IPL, consisting of 3 clusters, which corresponds to its known cytoarchitectonic, connectional, and functional diversity at the molecular level. This might reflect a general organizational principle of human IPL, beyond specific functional domains.

Sexual dimorphism and laterality in the evolution of the primate prefrontal cortex

Social selective pressures are commonly considered as the main driving force of primate brain evolution. Primate social behaviour is, however, known to be sexually dimorphic, and no previous study has made a direct comparison between male and female brain structures across species. We quantify sex-specific evolutionary trends in the prefrontal cortex of anthropoid primates (including humans) to investigate how sexual selection has shaped brain evolution in primates. The prefrontal cortex is of particular importance to the investigation of sexual dimorphism in primate brain evolution because of its association to those cognitive capacities central to primate (and human) evolution: sociality and higher-order cognitive processing. Our results demonstrate sex-by-hemisphere differences in the evolution of the prefrontal cortex in humans and non-human anthropoid primates congruent with the principal selective pressures considered to underlie anthropoid behavioural evolution. Our findings further show how sexual selection can shape brain adaptation in primates and provide an evolutionary framework for interpreting sex and sex-by-hemisphere differences in cortical organization in humans and non-human primates.

High-resolution fiber tract reconstruction in the human brain by means of three-dimensional polarized light imaging

Functional interactions between different brain regions require connecting fiber tracts, the structural basis of the human connectome. To assemble a comprehensive structural understanding of neural network elements from the microscopic to the macroscopic dimensions, a multimodal and multiscale approach has to be envisaged. However, the integration of results from complementary neuroimaging techniques poses a particular challenge. In this paper, we describe a steadily evolving neuroimaging technique referred to as three-dimensional polarized light imaging (3D-PLI). It is based on the birefringence of the myelin sheaths surrounding axons, and enables the high-resolution analysis of myelinated axons constituting the fiber tracts. 3D-PLI provides the mapping of spatial fiber architecture in the postmortem human brain at a sub-millimeter resolution, i.e., at the mesoscale. The fundamental data structure gained by 3D-PLI is a comprehensive 3D vector field description of fibers and fiber tract orientations – the basis for subsequent tractography. To demonstrate how 3D-PLI can contribute to unravel and assemble the human connectome, a multiscale approach with the same technology was pursued. Two complementary state-of-the-art polarimeters providing different sampling grids (pixel sizes of 100 and 1.6 μm) were used. To exemplarily highlight the potential of this approach, fiber orientation maps and 3D fiber models were reconstructed in selected regions of the brain (e.g., Corpus callosum, Internal capsule, Pons). The results demonstrate that 3D-PLI is an ideal tool to serve as an interface between the microscopic and macroscopic levels of organization of the human connectome.

Left cytoarchitectonic BA 44 processes syntactic gender violations in determiner phrases

Recent neuroimaging studies make contradictory predictions about the involvement of left Brodmann's area (BA) 44 in processing local syntactic violations in determiner phrases (DPs). Some studies suggest a role for BA 44 in detecting local syntactic violations, whereas others attribute this function to the left premotor cortex. Therefore, the present event-related functional magnetic resonance imaging (fMRI) study investigated whether left-cytoarchitectonic BA 44 was activated when German DPs involving syntactic gender violations were compared with correct DPs (correct: 'der Baum'-the[masculine] tree[masculine]; violated: 'das Baum'--the[neuter] tree[masculine]). Grammaticality judgements were made for both visual and auditory DPs to be able to generalize the results across modalities. Grammaticality judgements involved, among others, left BA 44 and left BA 6 in the premotor cortex for visual and auditory stimuli. Most importantly, activation in left BA 44 was consistently higher for violated than for correct DPs. This finding was behaviourally corroborated by longer reaction times for violated versus correct DPs. Additional brain regions, showing the same effect, included left premotor cortex, supplementary motor area, right middle and superior frontal cortex, and left cerebellum. Based on earlier findings from the literature, the results indicate the involvement of left BA 44 in processing local syntactic violations when these include morphological features, whereas left premotor cortex seems crucial for the detection of local word category violations.

Anatomical global spatial normalization

Anatomical global spatial normalization (aGSN) is presented as a method to scale high-resolution brain images to control for variability in brain size without altering the mean size of other brain structures. Two types of mean preserving scaling methods were investigated, “shape preserving” and “shape standardizing”. aGSN was tested by examining 56 brain structures from an adult brain atlas of 40 individuals (LPBA40) before and after normalization, with detailed analyses of cerebral hemispheres, all gyri collectively, cerebellum, brainstem, and left and right caudate, putamen, and hippocampus. Mean sizes of brain structures as measured by volume, distance, and area were preserved and variance reduced for both types of scale factors. An interesting finding was that scale factors derived from each of the ten brain structures were also mean preserving. However, variance was best reduced using whole brain hemispheres as the reference structure, and this reduction was related to its high average correlation with other brain structures. The fractional reduction in variance of structure volumes was directly related to ρ², the square of the reference-to-structure correlation coefficient. The average reduction in variance in volumes by aGSN with whole brain hemispheres as the reference structure was approximately 32%. An analytical method was provided to directly convert between conventional and aGSN scale factors to support adaptation of aGSN to popular spatial normalization software packages.

Eliciting dyslexic symptoms in proficient readers by simulating deficits in grapheme-to-phoneme conversion and visuo-magnocellular processing

Among the cognitive causes of dyslexia, phonological and magnocellular deficits have attracted a substantial amount of research. Their role and their exact impact on reading ability are still a matter of debate, partly also because large samples of dyslexics are hard to recruit. Here, we report a new technique to simulate dyslexic symptoms in normal readers in two ways. Although difficulties in grapheme-to-phoneme conversion were elicited by manipulating the identifiability of written letters, visual-magnocellular processing deficits were generated by presenting letters moving dynamically on the screen. Both factors were embedded into a lexical word-pseudoword decision task with proficient German readers. Although both experimental variations systematically increased lexical decision times, they did not interact. Subjects successfully performed word-pseudoword distinctions at all levels of simulation, with consistently longer reaction times for pseudowords than for words. Interestingly, detecting a pseudoword was more difficult in the grapheme-to-phoneme conversion simulation as indicated by a significant interaction of word type and letter shape. These behavioural effects are consistent with those observed in 'real' dyslexics in the literature. The paradigm is thus a potential means of generating novel hypotheses about dyslexia, which can easily be tested with normal readers before screening and recruiting real dyslexics.

Word frequency effects in the left IFG in dyslexic and normally reading children during picture naming and reading

Word frequency effects have been reported in numerous neuroimaging studies with typically reading adults, emphasising the role of the left inferior frontal gyrus (LIFG). Within LIFG, different cytoarchitectonic modules (areas 44 and 45) have been related to phonological vs. lexico-semantic processing, respectively. This fMRI study investigated the differential impact of word frequency on LIFG activation in reading and picture naming in primary school children with and without developmental dyslexia. All children showed the typical LIFG frequency effect in both tasks. The effect was comparable in a fronto-orbital region anterior-inferior adjacent to area 45. During reading but not picture naming, a second effect was observed in area 44. Here, the fMRI effect for lexical frequency was stronger for the dyslexic than the normal readers. These findings demonstrate the neural underpinnings of a selective deficit in dyslexic children in the graphemic input lexicon, whereas abstract lexical representations appear to be processed equally well in dyslexic and normally reading children. To conclude, the present fMRI study demonstrated differential impact of word frequency on LIFG activation in primary school children during reading but not picture naming. Apart from extending previous knowledge from studies with adults to childhood, the study sheds further light on a potential neural mechanism for deficient grapheme-to-phoneme conversion in dyslexic children.

Moral concepts set decision strategies to abstract values

Persons have different value preferences. Neuroimaging studies where value-based decisions in actual conflict situations were investigated suggest an important role of prefrontal and cingulate brain regions. General preferences, however, reflect a superordinate moral concept independent of actual situations as proposed in psychological and socioeconomic research. Here, the specific brain response would be influenced by abstract value systems and moral concepts. The neurobiological mechanisms underlying such responses are largely unknown. Using functional magnetic resonance imaging (fMRI) with a forced-choice paradigm on word pairs representing abstract values, we show that the brain handles such decisions depending on the person's superordinate moral concept. Persons with a predominant collectivistic (altruistic) value system applied a “balancing and weighing” strategy, recruiting brain regions of rostral inferior and intraparietal, and midcingulate and frontal cortex. Conversely, subjects with mainly individualistic (egocentric) value preferences applied a “fight-and-flight” strategy by recruiting the left amygdala. Finally, if subjects experience a value conflict when rejecting an alternative congruent to their own predominant value preference, comparable brain regions are activated as found in actual moral dilemma situations, i.e., midcingulate and dorsolateral prefrontal cortex. Our results demonstrate that superordinate moral concepts influence the strategy and the neural mechanisms in decision processes, independent of actual situations, showing that decisions are based on general neural principles. These findings provide a novel perspective to future sociological and economic research as well as to the analysis of social relations by focusing on abstract value systems as triggers of specific brain responses.