Direct Inside-Out Observation of Superficial White Matter Fasciculi in the Human Brain

Background: Recent methodological advances in the study of the cerebral white matter have left short association fibers relatively underexplored due to their compact and juxtacortical nature, which represent significant challenges for both post-mortem post-cortex removal dissection and magnetic resonance-based diffusion imaging. Objective: To introduce a novel inside-out post-mortem fiber dissection technique to assess short association fiber anatomy. Methods: Six cerebral specimens were obtained from a body donation program and underwent fixation in formalin. Following two freezing and thawing cycles, a standardized protocol involving peeling fibers from deep structures towards the cortex was developed. Results: The inside-out technique effectively exposed the superficial white matter. The procedure revealed distinguishable intergyral fibers, demonstrating their dissectability and enabling the identification of their orientation. The assessment of layer thickness was possible through direct observation and ex vivo morphological magnetic resonance imaging. Conclusion: The inside-out fiber technique effectively demonstrates intergyral association fibers in the post-mortem human brain. It adds to the neuroscience armamentarium, overcoming methodological obstacles and offering an anatomical substrate essential for neural circuit modeling and the evaluation of neuroimaging congruence. Impact statement The inside-out fiber dissection technique enables a totally new perception of cerebral connectivity as the observer navigates inside the parenchyma and looks toward the cerebral surface with the subcortical white matter and the cortical mantle in place. This approach has proven very effective for exposing intergyral association fibers, which have shown to be much more distinguishable from an inner perspective. It gave rise to unprecedented images of the human superficial white matter and allowed, for the first time, direct observation of this vast mantle of fascicles on entire cerebral hemisphere aspects.

Management of Intraventricular Meningiomas

Intraventricular meningiomas (IVM) are intracranial tumors that originate from collections of arachnoid cells within the choroid plexus. The incidence of meningiomas is estimated to be about 97.5 per 100,000 individuals in the United States with IVMs constituting 0.7% to 3%. Positive outcomes have been observed with surgical treatment of intraventricular meningiomas. This review explores elements of surgical care and management of patients with IVM, highlighting nuances in surgical approaches, their indications, and considerations.

Right hemisphere and metaphor comprehension: A connectionist perspective

Metaphor comprehension is a cognitively complex task, with evidence pointing to the engagement of multiple cerebral areas. In addition, the involvement of the right hemisphere appears to vary with cognitive effort. Therefore, the interconnecting pathways of such distributed cortical centers should be taken into account when studying this topic. Despite this, the potential contribution of white matter fasciculi has received very little attention in the literature to date and is not mentioned in most metaphor comprehension studies. To highlight the probable implications of the right inferior fronto-occipital fasciculus, right superior longitudinal system, and callosal radiations, we bring together findings from different research fields. The aim is to describe important insights enabled by the cross-fertilization of functional neuroimaging, clinical findings, and structural connectivity.

The vertical superior longitudinal fascicle and the vertical occipital fascicle

Association fibers of the human brain have long been considered to exclusively follow an anterior-posterior direction. Using magnetic resonance imaging techniques that allow in-vivo fiber dissection, vertically oriented association fibers have been rediscovered or newly described. Aside from the frontal aslant tract (FAT) in the frontal lobe, the vertical occipital fascicle (VOF) and the vertical portion of the superior longitudinal fascicle system (vSLF) have been studied in recent years. The aim of this review was to give an overview on the current knowledge regarding these two fiber tracts. A review of the available literature in the Medline database was conducted to gather all available publications dealing with either the VOF or the vSLF. One thousand two hundred seventy-three articles were obtained from the literature search of which a total of 71 articles met the final inclusion criteria of this review. We describe the history of the discovery of the respective fiber tract, its anatomical course and its boundaries integrating blunt fiber dissection studies and functional MRI/tractography studies. We discuss the functional properties of the respective fiber tract and its relevance in neurosurgery. The VOF is a fiber tract that has been discovered in the late XIX century and long been forgotten before being rediscovered in the 1970's. It lies lateral to the fibers of the sagittal stratum and mainly connects the superior and inferior occipital lobe. It plays a major role in reading and visual word and language comprehension and is said to be the main link between dorsal and ventral visual streams. The vSLF has many synonyms and is part of the superior longitudinal fascicle system. Recent studies were able to provide more insight into this set of fiber tracts showing distinct connections running from the superior and inferior parietal lobule to the posterior part of the temporal lobe. Its functional role is still not completely cleared. It is said to play a role in visual and auditory semantic language comprehension. It lies directly lateral to the arcuate fascicle. The VOF and the vSLF are vertically oriented fiber tracts connecting the temporo-parieto-occipital region and play a major role in the communication of dorsal and ventral visual streams (VOF), reading (VOF, vSLF) and visual and auditory semantic language comprehension (vSLF). They can consistently be identified using ex vivo blunt dissection techniques and in-vivo fiber tractography. Because of their localization and orientation these two fiber tracts can be combined to a fiber bundle system called posterior transverse system (PTS).

What's your name again? A review of the superior longitudinal and arcuate fasciculus evolving nomenclature

Studies of the superior longitudinal fasciculus (SLF) have multiplied in recent decades owing to methodological advances, but the absence of a convention for nomenclature remains a source of confusion. Here, we have reviewed existing nomenclatures in the context of the research studies that generated them and we have identified their agreements and disagreements. A literature search was conducted using PubMed/MEDLINE, Web-of-Science, Embase, and a review of seminal publications, without restrictions regarding publication date. Our search revealed that diffusion imaging, autoradiography, and fiber dissection have been the main methods contributing to tract designation. The first two have been particularly influential in systematizing the horizontal elements distant from the lateral sulcus. Twelve approaches to naming were identified, eight of them differing considerably from each other. The terms SLF and arcuate fasciculus (AF) were often used as synonyms until the second half of the 20th century. During the last 15 years, this has ceased to be the case in a growing number of publications. The term AF has been used to refer to the assembly of three different segments, or exclusively to long frontotemporal fibers. Similarly, the term SLF has been employed to denote the whole superior longitudinal associative system, or only the horizontal frontoparietal parts. As only partial correspondence can be identified among the available nomenclatures, and in the absence of an official designation of all anatomical structures that can be encountered in clinical practice, a high level of vigilance regarding the effectiveness of every oral or written act of communication is mandatory.

Hodology of the superior longitudinal system of the human brain: a historical perspective, the current controversies, and a proposal

The description of human white matter pathways experienced a tremendous improvement, thanks to the advancement of neuroimaging and dissection techniques. The downside of this progress is the production of redundant and conflicting literature, bound by specific studies' methods and aims. The Superior Longitudinal System (SLS), encompassing the arcuate (AF) and the superior longitudinal fasciculi (SLF), becomes an illustrative example of this fundamental issue, being one of the most studied white matter association pathways of the brain. Herein, we provide a complete illustration of this white matter fiber system's current definition, from its early descriptions in the nineteenth century to its most recent characterizations. We propose a review of both in vivo diffusion magnetic resonance imaging-based tractography and anatomical dissection studies, enclosing all the information available up to date. Based on these findings, we reconstruct the wiring diagram of the SLS, highlighting a substantial variability in the description of its cortical sites of termination and the taxonomy and partonomy that characterize the system. We aim to level up discrepancies in the literature by proposing a parallel across the various nomenclature. Consistent with the topographical arrangement already documented for commissural and projection pathways, we suggest approaching the SLS organization as an orderly and continuous wiring diagram, respecting a medio-lateral palisading topography between the different frontal, parietal, occipital, and temporal gyri rather than in terms of individualized fascicles. A better and complete description of the fine organization of white matter association pathways' connectivity is fundamental for a better understanding of brain function and their clinical and neurosurgical applications.

The Evolution of Cerebral Language Localization: Historical Analysis and Current Trends

Language localization has been an evolving concept over the past 150 years, with the emergence of several important yet conflicting ideologies. The classical theory, starting from the phrenologic work of Gall to the identification of specific regions of language function by Broca, Wernicke, and others, proposed that discrete subcomponents of language were organized into separate anatomic structural regions. The holism theory was postulated in an attempt to disclose that language function was instead attributed to a larger region of the cortex, in which cerebral regions may have the capability of assuming the function of damaged areas. However, this theory was largely abandoned in favor of discrete structural localizationist viewpoints. The subsequent cortical stimulatory work of Penfield led to the development of maps of localization, assigning an eloquent designation to specific regions. The expanding knowledge of cortical and subcortical anatomy allowed for the development of anatomically and functionally integrative language models. In particular, the dual stream model revisited the concept of regional interconnectivity and expanded the concept of eloquence. Advancements in cortical-subcortical stimulation, neurophysiologic monitoring, magnetic resonance diffusion tensor imaging/functional magnetic resonance imaging, awake neurosurgical technique, and knowledge gained by white matter tract anatomy and the Human Connectome Project, shed new light on the dynamic interconnectivity of the cerebrum. New studies are progressively opening doors to this paradigm, showing the dynamic and interdependent nature of language function. In this review, the evolution of language toward the evolving paradigm of dynamic language function and interconnectivity and its impact on shaping the neurosurgical paradigm are outlined.

Minimally invasive trans-sulcal parafascicular surgical resection of cerebral tumors: translating anatomy to early clinical experience

The minimally invasive port-based trans-sulcal parafascicular surgical corridor (TPSC) has incrementally evolved to provide a safe, feasible, and effective alternative to access subcortical and intraventricular pathologies. A detailed anatomical foundation is important in mitigating cortical and white matter tract injury with this corridor. Thus, the aims of this study are (1) to provide a detailed anatomical construct and overview of TPSCs and (2) to translate an anatomical framework to early clinical experience. Based on regional anatomical constraints, suitable parafascicular entry points were identified and described. Fiber tracts at both minimal and increased risks for each corridor were analyzed. TPSC-managed cases for metastatic or primary brain tumors were retrospectively reviewed. Adult patients 18 years or older with Karnofsky Performance Status (KPS) ≥ 70 were included. Subcortical brain metastases between 2 and 6 cm or primary brain tumors between 2 and 5 cm were included. Patient-specific corridors and trajectories were determined using MRI-tractography. Anatomy: The following TPSCs were described and translated to clinical practice: superior frontal, inferior frontal, inferior temporal, intraparietal, and postcentral sulci. Clinical: Eleven patients (5 males, 6 females) were included (mean age = 52 years). Seven tumors were metastatic, and 4 were primary. Gross total, near total, and subtotal resection was achieved in 7, 3, and 1 patient(s), respectively. Three patients developed intraoperative complications; all recovered from their intraoperative deficits and returned to baseline in 30 days. A detailed TPSC anatomical framework is critical in conducting safe and effective port-based surgical access. This review may represent one of the few early translational TPSC studies bridging anatomical data to clinical subcortical and intraventricular surgical practice.

White Matter-Governed Superior Frontal Sulcus Surgical Paradigm: A Radioanatomic Microsurgical Study—Part I

BACKGROUND

Frontal subcortical and intraventricular pathologies are traditionally accessed via transcortical or interhemispheric-transcallosal corridors.

OBJECTIVE

To describe the microsurgical subcortical anatomy of the superior frontal sulcus (SFS) corridor.

METHODS

Cadaveric dissections were undertaken and correlated with magnetic resonance imaging/diffusion-tensor imaging-Tractography. Surgical cases demonstrated clinical applicability.

RESULTS

SFS was divided into the following divisions: proximal, precentral sulcus to coronal suture; middle, 3-cm anterior to coronal suture; and distal, middle division to the orbital crest. Anatomy was organized as layered circumferential rings projecting radially towards the ventricles: (1) outer ring: at the level of the SFS, the following lengths were measured: (A) precentral sulcus to coronal suture = 2.29 cm, (B) frontal bone projection of superior sagittal sinus (SSS) to SFS = 2.37 cm, (C) superior temporal line to SFS = 3.0 cm, and (D) orbital crest to distal part of SFS = 2.32 cm; and (2) inner ring: (a) medial to SFS, U-fibers, frontal aslant tract (FAT), superior longitudinal fasciculus I (SLF-I), and cingulum bundle, (b) lateral to SFS, U-fibers, (SLF-II), claustrocortical fibers (CCF), and inferior fronto-occipital fasciculus, and (c) intervening fibers, FAT, corona radiata, and CCF. The preferred SFS parafascicular entry point (SFSP-EP) also referred to as the Kassam-Monroy entry point (KM-EP) bisects the distance between the midpupillary line and the SSS and has the following coordinates: x = 2.3 cm (lateral to SSS), y ≥ 3.5 cm (anterior to CS), and z = parallel corona radiata and anterior limb of the internal capsule.

CONCLUSION

SFS corridor can be divided into lateral, medial, and intervening white matter tract segments. Based on morphometric assessment, the optimal SFSP-EP is y ≥ 3.5 cm, x = 2.3 cm, and z = parallel to corona radiata and anterior limb of the internal capsule.

Dorsal component of the superior longitudinal fasciculus revisited: novel insights from a focused fiber dissection study

OBJECTIVE

The aim of this study was to investigate the anatomical consistency, morphology, axonal connectivity, and correlative topography of the dorsal component of the superior longitudinal fasciculus (SLF-I) since the current literature is limited and ambiguous.

METHODS

Fifteen normal, adult, formalin-fixed cerebral hemispheres were studied through a medial to lateral fiber microdissection technique. In 5 specimens, the authors performed stepwise focused dissections of the lateral cerebral aspect to delineate the correlative anatomy between the SLF-I and the other two SLF subcomponents, namely the SLF-II and SLF-III.

RESULTS

The SLF-I was readily identified as a distinct fiber tract running within the cingulate or paracingulate gyrus and connecting the anterior cingulate cortex, the medial aspect of the superior frontal gyrus, the pre–supplementary motor area (pre-SMA), the SMA proper, the paracentral lobule, and the precuneus. With regard to the morphology of the SLF-I, two discrete segments were consistently recorded: an anterior and a posterior segment. A clear cleavage plane could be developed between the SLF-I and the cingulum, thus proving their structural integrity. Interestingly, no anatomical connection was revealed between the SLF-I and the SLF-II/SLF-III complex.

CONCLUSIONS

Study results provide novel and robust anatomical evidence on the topography, morphology, and subcortical architecture of the SLF-I. This fiber tract was consistently recorded as a distinct anatomical entity of the medial cerebral aspect, participating in the axonal connectivity of high-order paralimbic areas.

Subdividing the superior longitudinal fasciculus using local quantitative MRI

The association fibers of the superior longitudinal fasciculus (SLF) connect parietal and frontal cortical regions in the human brain. The SLF comprises of three distinct sub-bundles, each presenting a different anatomical trajectory, and specific functional roles. Nevertheless, in vivo studies of the SLF often consider the entire SLF complex as a single entity. In this work, we suggest a data-driven approach that relies on microstructure measurements for separating SLF-III from the rest of the SLF. We apply the SLF-III separation procedure in three independent datasets using parameters of diffusion MRI (fractional anisotropy), as well as relaxometry-based parameters (T1, T2, T2* and T2-weighted/T1-weighted). We show that the proposed procedure is reproducible across datasets and tractography algorithms. Finally, we suggest that differential crossing with different white-matter tracts is the source of the distinct MRI signatures of SLF-II and SLF-III.

Mapping the human middle longitudinal fasciculus through a focused anatomo-imaging study: shifting the paradigm of its segmentation and connectivity pattern

Τhe middle longitudinal fasciculus (MdLF) was initially identified in humans as a discrete subcortical pathway connecting the superior temporal gyrus (STG) to the angular gyrus (AG). Further anatomo-imaging studies, however, proposed more sophisticated but conflicting connectivity patterns and have created a vague perception on its functional anatomy. Our aim was, therefore, to investigate the ambiguous structural architecture of this tract through focused cadaveric dissections augmented by a tailored DTI protocol in healthy participants from the Human Connectome dataset. Three segments and connectivity patterns were consistently recorded: the MdLF-I, connecting the dorsolateral Temporal Pole (TP) and STG to the Superior Parietal Lobule/Precuneus, through the Heschl's gyrus; the MdLF-II, connecting the dorsolateral TP and the STG with the Parieto-occipital area through the posterior transverse gyri and the MdLF-III connecting the most anterior part of the TP to the posterior border of the occipital lobe through the AG. The lack of an established termination pattern to the AG and the fact that no significant leftward asymmetry is disclosed tend to shift the paradigm away from language function. Conversely, the theory of "where" and "what" auditory pathways, the essential relationship of the MdLF with the auditory cortex and the functional role of the cortical areas implicated in its connectivity tend to shift the paradigm towards auditory function. Allegedly, the MdLF-I and MdLF-II segments could underpin the perception of auditory representations; whereas, the MdLF-III could potentially subserve the integration of auditory and visual information.