Hippocampal subfields revealed through unfolding and unsupervised clustering of laminar and morphological features in 3D BigBrain

© 2019 Elsevier Inc. The internal structure of the human hippocampus is challenging to map using histology or neuroimaging due to its complex archicortical folding. Here, we aimed to overcome this challenge using a unique combination of three methods. First, we leveraged a histological dataset with unprecedented 3D coverage, BigBrain. Second, we imposed a computational unfolding framework that respects the topological continuity of hippocampal subfields, which are traditionally defined by laminar composition. Third, we adapted neocortical parcellation techniques to map the hippocampus with respect to not only laminar but also morphological features. Unsupervised clustering of these features revealed subdivisions that closely resemble gold standard manual subfield segmentations. Critically, we also show that morphological features alone are sufficient to derive most hippocampal subfield boundaries. Moreover, some features showed differences within subfields along the hippocampal longitudinal axis. Our findings highlight new characteristics of internal hippocampal structure, and offer new avenues for its characterization with in-vivo neuroimaging

Convergence of cortical types and functional motifs in the human mesiotemporal lobe

The mesiotemporal lobe (MTL) is implicated in many cognitive processes, is compromised in numerous brain disorders, and exhibits a gradual cytoarchitectural transition from six-layered parahippocampal isocortex to three-layered hippocampal allocortex. Leveraging an ultra-high-resolution histological reconstruction of a human brain, our study showed that the dominant axis of MTL cytoarchitectural differentiation follows the iso-to-allocortical transition and depth-specific variations in neuronal density. Projecting the histology-derived MTL model to in-vivo functional MRI, we furthermore determined how its cytoarchitecture underpins its intrinsic effective connectivity and association to large-scale networks. Here, the cytoarchitectural gradient was found to underpin intrinsic effective connectivity of the MTL, but patterns differed along the anterior-posterior axis. Moreover, while the iso-to-allocortical gradient parametrically represented the multiple-demand relative to task-negative networks, anterior-posterior gradients represented transmodal versus unimodal networks. Our findings establish that the combination of micro- and macrostructural features allow the MTL to represent dominant motifs of whole-brain functional organisation

Progress update from the hippocampal subfields group

Introduction: Heterogeneity of segmentation protocols for medial temporal lobe regions and hippocampal subfields on in vivo magnetic resonance imaging hinders the ability to integrate findings across studies. We aim to develop a harmonized protocol based on expert consensus and histological evidence. Methods: Our international working group, funded by the EU Joint Programme–Neurodegenerative Disease Research (JPND), is working toward the production of a reliable, validated, harmonized protocol for segmentation of medial temporal lobe regions. The working group uses a novel postmortem data set and online consensus procedures to ensure validity and facilitate adoption. Results: This progress report describes the initial results and milestones that we have achieved to date, including the development of a draft protocol and results from the initial reliability tests and consensus procedures. Discussion: A harmonized protocol will enable the standardization of segmentation methods across laboratories interested in medial temporal lobe research worldwid

Scaling for Big Data: An Enhanced Surface Reconstruction of the Hippocampus Leveraging Delaunay Triangulation for High-Resolution Mapping from Unfolded to Native Spaces

The ability to process and analyze large and complex neuroimaging datasets is crucial to develop computer simulations of the brain. A focus of significant interest is to model the hippocampus, a brain structure integral to memory formation and emotional regulation. This study introduces a state-of-the-art algorithmic pipeline for big data scaling and enhanced surface reconstruction of the hippocampus, utilizing Delaunay Triangulation for high-resolution surface generation.Initially, the pipeline addresses the challenge of upscaling low-resolution surface models by leveraging Delaunay Triangulation. This algorithmic approach not only maintains but enhances anatomical detail, allowing for a high-resolution representation of the hippocampus' complex topology. This process is particularly advantageous for large datasets, making it scalable and big data-compatible.Following the surface enhancement, the pipeline employs a specialized warp field algorithm to transform the high-resolution surface from an unfolded space to a native space. This ensures compatibility with existing volumetric datasets and enhances the granularity and precision of subsequent analyses. An extensive set of validation checks ensures the warp field's fidelity, safeguarding the integrity of the transformed model.Finally, the warped high-resolution surface is integrated with BigBrain hippocampal volumetric data through a robust volume-to-surface mapping algorithm. This step harmonizes the dual approaches of surface-based and volume-based analyses, allowing for comprehensive, nuanced exploration of hippocampal structure and function.The pipeline is implemented in widely-accepted neuroimaging formats like NIFTI and GIFTI, ensuring seamless integration with existing analytical tools and datasets. Preliminary results indicate a significant advancement in the accuracy and depth of hippocampal analyses. This scalable approach is versatile and holds promise for a myriad of applications, from basic neuroscience research to advanced investigations into neurodegenerative diseases and cognitive disorders. Overall, the pipeline sets a new precedent for high-resolution, big data-compatible computational analysis of complex brain structures