Katrin Amunts

Motor cortex and hand motor skills: Structural compliance in the human brain

Recent studies in humans and nonhuman primates have shown that the functional organization of the human sensorimotor cortex changes following sensory stimulation or following the acquisition of motor skills. It is unknown whether functional plasticity in response to the acquisition of new motor skills and the continued performance of complicated bimanual movements for years is associated with structural changes in the organization of the motor cortex. Professional musicians, especially keyboard and string players, are a prototypical group for investigating these changes in the human brain. Using magnetic resonance images, we measured the length of the posterior wall of the precentral gyrus bordering the central sulcus (intrasulcal length of the precentral gyrus, ILPG) in horizontal sections through both hemispheres of right‐handed keyboard players and of an age‐ and handedness‐matched control group. Lacking a direct in vivo measurement of the primary motor cortex in humans, we assumed that the ILPG is a measure of the size of the primary motor cortex. Left‐right asymmetry in the ILPG was analyzed and compared between both groups. Whereas controls exhibited a pronounced left‐larger‐than‐right asymmetry, keyboard players had more symmetrical ILPG. The most pronounced differences in ILPG between keyboard players and controls were seen in the most dorsal part of the presumed cortical hand representation of both hemispheres. This was especially true in the nondominant right hemispheres. The size of the ILPG was negatively correlated with age of commencement of musical training in keyboard players, supporting our hypothesis that the human motor cortex can exhibit functionally induced and long‐lasting structural adaptations. Hum. Brain Mapping 5:206–215, 1997.

21 – Quantitative Analysis of Cyto- and Receptor Architecture of the Human Brain

This chapter discusses the principles of classical architectonic mapping in the context of recent imaging techniques. It presents an observer-independent approach for a quantitative analysis of cortical areas and their borders, which is based on a multivariate statistical analysis of the cytoarchitecture and illustrates the application of this approach for the cytoarchitectonic mapping of the human visual cortex. Important criteria for cytoarchitectonic mapping are the absolute thickness of cortical layers, the proportionate thickness of a layer relative to the other cortical layers and to the total cortical depth, the presence of clearly recognizable laminar borders and vertical columns, the packing density and size of neuronal cell bodies, the homogeneous or clustered distribution of cell bodies throughout the layers, and the presence of special cell types such as Betz cells. Understanding the regional distributions of neurotransmitter receptors is likely to provide a crucial intermediary level of description between function and structure, since different cytoarchitectonic and functional areas have different mean receptor densities as well as distinct laminar distribution patterns. Another way for a better understanding of brain function and the underlying anatomy is to compare architectonic maps obtained in postmortem brains with activation maps obtained in functional imaging studies in a common spatial reference system. Since these two kinds of maps stem from different subsets of brains, such a comparison must be performed on a probabilistic basis.

A stereological approach to human cortical architecture: Identification and delineation of cortical areas

Stereology offers a variety of procedures to analyze quantitatively the regional and laminar organization in cytoarchitectonically defined areas of the human cerebral cortex. Conventional anatomical atlases are of little help in localizing specific cortical areas, since most of them are based on a single brain and use highly observer-dependent criteria for the delineation of cortical areas. In consequence, numerous cortical maps exist which greatly differ with respect to number, position, size and extent of cortical areas. We describe a novel algorithm-based procedure for the delineation of cortical areas, which exploits the automated estimation of volume densities of cortical cell bodies. Spatial sampling of the laminar pattern is performed with density profiles, followed by multivariate analysis of the profiles shape, which locates the cytoarchitectonic borders between neighboring cortical areas at sites where the laminar pattern changes significantly. The borders are then mapped to a human brain atlas system comprising tools for three dimensional reconstruction, visualization and morphometric analysis. A sample of brains with labeled cortical areas is warped into the reference brain of the atlas system in order to generate a population map of the cortical areas, which describes the intersubject variability in spatial conformation of cortical areas. These population maps provide a novel tool for the interpretation of images obtained with functional imaging techniques.

Architecture of the Cerebral Cortex

This chapter focuses on the structural organization of the cerebral cortex and its segregation into cortical areas. Various classical as well as forgotten cortical maps based on cyto- and myeloarchitectonic criteria are discussed in detail and compared with functional imaging data. Cytoarchitecture describes the spatial distribution of neuronal cell bodies demonstrated by cell body staining. It considers cortical features such as the presence or absence of particular cell types, their arrangement in layers and columns, differences in cortical thickness and other microscopical features. Myeloarchitecture informs about the regional and laminar distribution of myelinated fibers. We provide an overview of different cortical types from evolutionary (paleocortex and archicortex, neocortex) and microscopical/histological perspectives (allocortex versus isocortex). The role of intersubject variability is emphasized, and examples of interhemispheric asymmetry are given. Finally, the chapter summarizes how the different cortical parcellations of the maps relate to each other.

Chapter 18 – High-Resolution Fiber and Fiber Tract Imaging Using Polarized Light Microscopy in the Human, Monkey, Rat, and Mouse Brain

Polarized light imaging (PLI) enables ultra-high resolution visualization of nerve fibers in postmortem brains by the birefringent property of myelin. Using the Jones calculus, 3D orientation of fibers can be determined in serial sections throughout the brain. We present examples of fiber architecture with an unprecedented spatial resolution both in the white matter and within the cerebral cortex of human, vervet monkey, and rodent brains. Crossing of fibers can be directly visualized without any assumptions or modeling, and fibers can be followed from the white matter into the most superficial layers of the cortex. Even extremely small fiber tracts are visible. Furthermore, the fiber architecture within the cerebral cortex provides a new approach for its parcellation into areas with distinct distribution, orientation, and density patterns of nerve fibers. PLI also provides the anatomical ground truth for the evaluation of results derived from diffusion weighted imaging.

Posterior Parietal Cortex: Multimodal Association Cortex

This chapter provides an overview of the organization of the posterior parietal cortex which consists of the inferior and superior parietal lobules and the cortex within the intraparietal sulcus. The posterior parietal cortex is a multimodal association cortex, integrating functions from different input modalities. Such complexity is also revealed by its architecture, providing a mosaic of several different architectonical areas within the posterior parietal cortex, which cannot reliably be predicted by macroanatomical landmarks. With respect to disorders which also involve the posterior parietal cortex and its underlying white matter, e.g. apraxia or neglect, differences in connections from the posterior parietal cortex are an essential issue.

Cytoarchitecture and Maps of the Human Cerebral Cortex

This article focuses on the cytoarchitectonic organization of the cerebral cortex and its segregation into cortical areas. Cytoarchitecture describes the spatial distribution of neuronal cell types, their arrangement in horizontal layers and vertical columns, as well as cortical thickness. Based on cytoarchitectonic characteristics, the cortex is divided into iso- and allocortex, with the mesocortex as a transition region between both. Some emphasis is put on the presentation of nearly forgotten but still relevant classical cytoarchitectonic maps which are discussed in the light of recent quantitative microscopical analyses and studies of intersubject variability (probability maps) as well as functional imaging data.

Basal Forebrain Anatomical Systems in MRI Space

The basal forebrain comprises heterogeneous structures located close to the medial and ventral surfaces of the cerebral hemispheres. This region contains a number of interdigitating anatomical structures, including the basal nucleus of Meynert, the ventral striatum (nucleus accumbens), and the cell groups underneath the globus pallidus in the substantia innominata that bridge the centromedial amygdala to the bed nucleus of the stria terminalis (‘extended amygdala’). This region is involved in cortical activation, attention, learning, memory, reward, cortical plasticity, and also disease states: cholinergic corticopetal projection neurons degenerate in Alzheimers and related disorders. Recent studies using postmortem probabilistic maps and resting-state functional connectivity analysis have begun to shed light on distinct and shared functions across the complex anatomical landscape of the ventral forebrain.

Simulation-based validation of the physical model in 3D Polarized Light Imaging

3D Polarized Light Imaging provides a high-resolution reconstruction of nerve fiber pathways in human postmortem brains. In this study, the currently used model for the nerve fiber reconstruction has been validated using numerical simulations.

3D Polarized Light Imaging Portrayed: Visualization of Fiber Architecture Derived from 3D-PLI

3D polarized light imaging (3D-PLI) is a neuroimaging technique that has recently opened up new avenues to study the complex architecture of nerve fibers in postmortem brains at microscopic scales. In a specific voxel-based analysis, each voxel is assigned a single 3D fiber orientation vector. This leads to comprehensive 3D vector fields. In order to inspect and analyze such high-resolution fiber orientation vector field, also in combination with complementary microscopy measurements, appropriate visualization techniques are essential to overcome several challenges, such as the massive data sizes, the large amount of both unique and redundant information at different scales, or the occlusion issues of inner structures by outer layers. Here, we introduce a comprehensive software tool that is able to visualize all information of a typical 3D-PLI dataset in an adequate and sophisticated manner. This includes the visualization of (i) anatomic structural and fiber architectonic data in one representation, (ii) a large-scale fiber orientation vector field, and (iii) a clustered version of the field. Alignment of a 3D-PLI dataset to an appropriate brain atlas provides expert-based delineation, segmentation, and, ultimately, visualization of selected anatomical structures. By means of these techniques, a detailed analysis of the complex fiber architecture in 3D is feasible.