Layered White Matter Organization of the Superior Longitudinal Fasciculus–Arcuate Fasciculus Complex: Insular Topography and Surgical Implications from Combined Fiber Dissection and Tractography

Layered White Matter Organization of the Superior Longitudinal Fasciculus–Arcuate Fasciculus Complex: Insular Topography and Surgical Implications from Combined Fiber Dissection and Tractography | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Layered White Matter Organization of the Superior Longitudinal Fasciculus–Arcuate Fasciculus Complex: Insular Topography and Surgical Implications from Combined Fiber Dissection and Tractography Ufuk Temtek, Muhammet Elveren, Eray Serhat Aktan, Serhat Gündoğdu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9163139/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The superior longitudinal fasciculus (SLF) and arcuate fasciculus (AF) complex forms a critical white matter network involved in language and higher cognitive functions within the perisylvian region. However, its three-dimensional topographic relationship with the insular region and its layered organization relative to deeper projection fibers remain insufficiently characterized. This study aimed to investigate the anatomical relationship of the SLF–AF complex with the insula and its layered organization with respect to the corona radiata using combined white matter dissection and diffusion tractography. Microsurgical white matter dissections were performed on 10 postmortem human brains (20 hemispheres) prepared using the Klingler technique. Specimens were fixed in formaldehyde, frozen, and subsequently thawed to facilitate fiber separation. Dissections were carried out under an operating microscope using a stepwise lateral-to-medial approach to progressively expose superficial and deep white matter layers. Dissection findings were correlated with deterministic diffusion tractography analyses performed using DSI Studio, based on diffusion tensor imaging data obtained from the Human Connectome Project. Reconstruction of the SLF, AF, and corona radiata was performed using a region-of-interest (ROI)-based approach guided by anatomical landmarks. Dissection findings demonstrated that the SLF–AF complex forms a continuous superficial association fiber layer located near the cortical surface within the perisylvian region. Following insular decortication, deeper white matter structures became progressively visible, revealing the corona radiata as a more medial and deeper projection system. A consistent hierarchical organization was observed across all specimens, consisting of superficial association fibers (SLF–AF complex), intermediate projection fibers (corona radiata), and deeper subcortical structures. Tractographic reconstructions confirmed these findings and demonstrated a reproducible spatial relationship between the SLF–AF complex, insula, and corona radiata. The SLF–AF complex represents a superficial association fiber layer in the insular region, whereas the corona radiata constitutes a deeper projection system. This layered organization provides a practical and clinically applicable anatomical framework for preserving functional white matter tracts and defining safe resection boundaries during insular and perisylvian neurosurgical procedures. superior longitudinal fasciculus arcuate fasciculus insula tractography white matter dissection neurosurgery Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The superior longitudinal fasciculus (SLF) and arcuate fasciculus (AF) are major long-range association fiber systems connecting frontal, parietal, and temporal cortical regions and constitute a fundamental component of perisylvian connectivity [ 1 , 2 ]. While the AF has classically been described as a curved pathway linking the posterior temporal cortex to the inferior frontal gyrus, the SLF has been considered a broader association system connecting frontal, parietal, and occipital regions [ 3 , 4 ]. Contemporary studies using white matter dissection and diffusion imaging have demonstrated that these structures are not homogeneous but consist of multiple subcomponents with distinct connectivity profiles [ 1 , 5 ]. Functionally, the SLF–AF complex plays a central role in the dorsal language network and higher-order cognitive processes. In the dominant hemisphere, the AF is critically involved in phonological processing and speech production, whereas the SLF contributes to attention, working memory, and sensorimotor integration [ 6 , 7 ]. In the non-dominant hemisphere, these pathways are associated with spatial attention and neglect syndromes [ 8 ]. From a neurosurgical perspective, preservation of the SLF–AF complex is essential during surgical procedures involving the insular, opercular, and temporoparietal regions. Damage to these pathways may result in significant language and cognitive deficits [ 2 , 9 ]. Although diffusion tractography has become a valuable tool in preoperative planning, its limitations necessitate validation through anatomical dissection [ 10 – 12 ]. Despite extensive research on segmentation and connectivity, the layered topographic relationship between the SLF–AF complex and deeper white matter structures—particularly in relation to the insula—remains insufficiently explored. Therefore, the present study aims to define the layered anatomical organization of the SLF–AF complex in relation to the insular region and to explore its surgical implications. Materials and Methods Specimens and Dissection Procedure Ten postmortem human brains (20 hemispheres) without intracranial pathology were included. Specimens were prepared according to the Klingler technique [18], involving fixation in formaldehyde, freezing at −15°C, and subsequent thawing to facilitate white matter fiber separation. White matter dissections were performed under an operating microscope (×6–×10 magnification) using a systematic lateral-to-medial approach. Cortical structures were progressively removed to expose successive layers of white matter organization, beginning with short U-fibers and extending to long association and projection fibers. Dissections were documented using high-resolution photography. Diffusion MRI and Tractography Diffusion tractography was performed using Human Connectome Project data. Deterministic fiber tracking was conducted using DSI Studio with a step size of 1 mm, angular threshold of 60°, and minimum fiber length of 20 mm. The SLF, AF, and corona radiata were reconstructed using ROI-based methods guided by anatomical landmarks identified during dissection. Dissection–Tractography Correlation Qualitative comparisons were made between dissection findings and tractographic reconstructions, focusing on spatial relationships between the SLF–AF complex, insula, and corona radiata. The analysis was primarily descriptive, focusing on anatomical consistency across specimens. Results Following cortical removal, short U-fibers were identified as the most superficial connections between adjacent gyri. Beneath this layer, long association fibers forming the SLF–AF complex were consistently observed as a continuous superficial layer within the perisylvian region. Stepwise dissection revealed that the SLF–AF complex maintains a close relationship with the insular cortex and becomes more prominent following removal of opercular structures. The AF demonstrated a characteristic curved trajectory in the posterior perisylvian region, while the SLF extended more broadly toward frontal regions. Following insular decortication, deeper white matter structures became progressively visible. The corona radiata was consistently identified beneath the SLF–AF complex, occupying a deeper and more medial plane. A hierarchical organization was observed, consisting of superficial association fibers (SLF–AF complex), projection fibers (corona radiata), and deep gray matter structures. Tractographic reconstructions confirmed these findings, demonstrating a consistent spatial relationship between the SLF–AF complex, insula, and corona radiata. A consistent correlation between anatomical dissection and tractography was observed. Discussion This study demonstrates that the superior longitudinal fasciculus–arcuate fasciculus (SLF–AF) complex forms a superficial association fiber layer in the perisylvian region, whereas the corona radiata represents a deeper projection system. Following insular decortication, the SLF–AF complex was consistently observed in a superficial plane, while the corona radiata occupied a deeper and more medial position. These findings indicate that perisylvian white matter organization follows a hierarchical layered arrangement extending from superficial association fibers to deeper projection systems. Classical neuroanatomical descriptions have traditionally defined the SLF–AF complex as a single connection pathway linking frontal and temporal regions. However, contemporary studies using white matter dissection and diffusion imaging have demonstrated that this system exhibits a multi-segmented and heterogeneous organization [1,5]. The segmentation models proposed by Makris et al. and Catani et al. have provided important insights into their structural and functional diversity [14,15]. The findings of the present study are consistent with these models but further extend the existing literature by demonstrating the layered topographic relationship between the SLF–AF complex and deeper projection systems in the insular region. From a functional perspective, the SLF–AF complex is a critical component of the dorsal language network and higher cognitive control systems. In the dominant hemisphere, the arcuate fasciculus plays a key role in phonological processing and speech production, while the superior longitudinal fasciculus contributes to attention, working memory, and sensorimotor integration [6,7]. The layered organization demonstrated in this study may represent an anatomical substrate for this functional differentiation. From a neurosurgical perspective, these findings have significant clinical implications. The superficial location of the SLF–AF complex indicates that it represents an early encountered functional boundary during surgical approaches involving the insular, opercular, and temporoparietal regions. Preservation of this layer is critical for maintaining language and cognitive functions [2,9]. In contrast, deeper dissection exposes the corona radiata, where injury may lead to motor and subcortical deficits. Therefore, this layered organization provides a practical anatomical framework for defining safe resection boundaries and optimizing functional preservation during surgery. This layered anatomical understanding may significantly reduce the risk of functional deficits and improve surgical outcomes in insular and perisylvian tumor resections. The consistency between tractography and dissection findings supports the role of diffusion-based imaging as a complementary tool for evaluating white matter anatomy [10,13]. However, limitations related to fiber crossing, algorithmic constraints, and the use of group-averaged data should be acknowledged [10–12]. Additionally, the use of ex vivo specimens and tractography-based reconstructions may not fully represent in vivo functional dynamics. This study provides a novel anatomical framework by clearly demonstrating the layered organization of the SLF–AF complex in relation to the insular region and its spatial relationship with deeper projection systems. Unlike previous studies focusing primarily on segmentation and connectivity, this work introduces a clinically applicable topographic model that directly translates into surgical practice. Limitations Deterministic tractography has limitations in resolving crossing fibers, and group-averaged datasets may not reflect individual anatomical variability. These limitations were mitigated by correlating tractographic findings with anatomical dissection data. Additionally, the use of ex vivo specimens and tractography-based reconstructions may not fully represent in vivo functional dynamics. Conclusion The SLF–AF complex constitutes a superficial association fiber layer, whereas the corona radiata forms a deeper projection system. This layered organization provides important insights into white matter architecture and offers a practical and clinically applicable anatomical framework for safer neurosurgical interventions in the insular and perisylvian regions. Declarations Author Contribution U.T. and M.E. wrote the main manuscript text and U.T. , E.S.A, M.H.Ş and S.G. prepared figures 1 and 2. U.T. , M.E. and M.H.Ş prepared figures 3 and 4. All authors reviewed the manuscript." Ethics Approval This study was conducted in accordance with institutional and international ethical standards for the use of postmortem human specimens. Conflict of Interest The authors declare no conflict of interest. Funding No funding was received for this study. References Janelle F, et al. Superior longitudinal fasciculus: a review of the anatomical descriptions with functional correlates. Front Neurol. 2022;13:864754. https://doi.org/10.3389/fneur.2022.864754 Vavassori L, et al. The arcuate fasciculus: combining structure and function into surgical considerations. Brain Behav. 2023;13:e3009. https://doi.org/10.1002/brb3.3009 de Oliveira JVMP, et al. What’s your name again? A review of the superior longitudinal fasciculus nomenclature. Clin Anat. 2021;34(8):1161–1173. https://doi.org/10.1002/ca.23764 Dick AS, Tremblay P. Beyond the arcuate fasciculus: consensus and controversy in the study of language pathways. Brain. 2012;135(12):3529–3550. https://doi.org/10.1093/brain/aws222 Basile GA, et al. Functional anatomy and topographical organization of the arcuate fasciculus. Commun Biol. 2024;7:198. https://doi.org/10.1038/s42003-024-05876-2 Ivanova MV, et al. Functional contributions of the arcuate fasciculus to language processing. Front Hum Neurosci. 2021;15:672665. https://doi.org/10.3389/fnhum.2021.672665 Zong F, et al. Language function of the superior longitudinal fasciculus in the human brain. Front Neurosci. 2023;17:1189363. https://doi.org/10.3389/fnins.2023.1189363 Andreoli M, et al. Association of disruption of the right posterior arcuate fasciculus with spatial attention deficits. J Neurosurg. 2024;141(2):468–478. https://doi.org/10.3171/2023.5.JNS23456 Panesar SS, et al. Tractography for surgical neuro-oncology planning. Neurotherapeutics. 2019;16(2):429–446. https://doi.org/10.1007/s13311-018-00693-8 Kamagata K, et al. Advancements in diffusion MRI tractography for preoperative planning. Invest Radiol. 2024;59(3):150–162. https://doi.org/10.1097/RLI.0000000000001000 Voets NL, et al. Consensus recommendations for clinical functional MRI applied to language mapping. Aperture Neuro. 2025;4:100045. https://doi.org/10.52294/AN.2025.100045 Sanchez S, et al. Language neuroscience in the operating room. Front Oncol. 2025;15:1298765. https://doi.org/10.3389/fonc.2025.1298765 Yeh FC. Shape analysis of the human association pathways. Neuroimage. 2020;223:117329. https://doi.org/10.1016/j.neuroimage.2020.117329 Makris N, Kennedy DN, McInerney S, et al. Segmentation of subcomponents within the superior longitudinal fascicle in humans. Cereb Cortex. 2005;15(6):854–869. https://doi.org/10.1093/cercor/bhh186 Catani M, Jones DK, ffytche DH. Perisylvian language networks of the human brain. Ann Neurol. 2005;57(1):8–16. https://doi.org/10.1002/ana.20319 Burdach KF. Vom Baue und Leben des Gehirns. Leipzig: Dyk’sche Buchhandlung; 1822. Dejerine J. Anatomie des Centres Nerveux. Paris: Rueff; 1895. Klingler J. Erleichterung der makroskopischen Präparation des Gehirns durch den Gefrierprozess. Schweiz Arch Neurol Psychiatr. 1935;36:247–256. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9163139","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":632078778,"identity":"e1621360-4a74-49b4-83a9-ffafaf18e63d","order_by":0,"name":"Ufuk Temtek","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYDAC5gPMEAYP8wEgKSFDWAtbAjMDUK0EAw9bAkgLDylaeAzAlhHUYXCMx9j4Q8W2On6eM59f3aix4GFgP3x0AyEtCQfO3JaQ7O3dZp1zDOgwnrS0G3i13O8xPnCw7baEwXnebcY5bEAtEjxm+LUAbTlw8N9tCfvzPM+Mc/4RqSXhYAPQFt4e5se5bURokTzGVmxw5thtyRlnjpkx5/ZJ8LAR8gvfMebNEhU1t/n5e5Iff875VifHz374GF4tyIBNAkwSqxwEmD+QonoUjIJRMApGDgAAHtJIGYBWkccAAAAASUVORK5CYII=","orcid":"","institution":"Erzurum Regional Training and Research Hospital","correspondingAuthor":true,"prefix":"","firstName":"Ufuk","middleName":"","lastName":"Temtek","suffix":""},{"id":632078782,"identity":"719221cb-c05c-4206-aed2-47c2171b5c1c","order_by":1,"name":"Muhammet Elveren","email":"","orcid":"","institution":"Erzurum Regional Training and Research Hospital","correspondingAuthor":false,"prefix":"","firstName":"Muhammet","middleName":"","lastName":"Elveren","suffix":""},{"id":632078783,"identity":"56004b69-1a42-45d0-b24d-5970d5724460","order_by":2,"name":"Eray Serhat Aktan","email":"","orcid":"","institution":"Erzurum Regional Training and Research Hospital","correspondingAuthor":false,"prefix":"","firstName":"Eray","middleName":"Serhat","lastName":"Aktan","suffix":""},{"id":632078786,"identity":"e467282c-1faa-49e7-824e-f58fe2194c06","order_by":3,"name":"Serhat Gündoğdu","email":"","orcid":"","institution":"Atatürk University","correspondingAuthor":false,"prefix":"","firstName":"Serhat","middleName":"","lastName":"Gündoğdu","suffix":""},{"id":632078787,"identity":"6b98bafc-4e2d-465c-a352-c6481b598f45","order_by":4,"name":"Mehmet Hakan Şahin","email":"","orcid":"","institution":"Atatürk University","correspondingAuthor":false,"prefix":"","firstName":"Mehmet","middleName":"Hakan","lastName":"Şahin","suffix":""}],"badges":[],"createdAt":"2026-03-18 21:53:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9163139/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9163139/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108407152,"identity":"63357747-856d-4701-a79a-99f40c8819b8","added_by":"auto","created_at":"2026-05-04 09:48:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2643789,"visible":true,"origin":"","legend":"\u003cp\u003eStepwise cortical and subcortical dissection demonstrating the superficial association fiber layer of the SLF–AF complex.\u003c/p\u003e\n\u003cp\u003e(A) Lateral view of the right hemisphere following removal of the arachnoid membrane, pia mater, and vascular structures, illustrating major cortical landmarks.\u003c/p\u003e\n\u003cp\u003e(B) Exposure of short U-fibers after partial cortical decortication, with preservation of selected frontal and parietal regions.\u003c/p\u003e\n\u003cp\u003e(C) Progressive removal and shortening of the supramarginal and angular gyri revealing the SLF–AF complex as a superficial association layer.\u003c/p\u003e\n\u003cp\u003e(D) Complete removal of cortical walls further exposing the SLF–AF complex and arcuate fasciculus.\u003c/p\u003e\n\u003cp\u003e(E) Exposure of the insular cortex following removal of temporal and parietal opercula, demonstrating insular gyri and the frontal extension of the SLF–AF complex, along with the frontal aslant tract.\u003c/p\u003e\n\u003cp\u003eAbbreviations: AF, arcuate fasciculus; AnG, angular gyrus; alg, anterior long gyrus; asg, anterior short gyrus; FAT, frontal aslant tract; IFGOr, inferior frontal gyrus pars orbitalis; IFGOp, inferior frontal gyrus pars opercularis; IFGTr, inferior frontal gyrus pars triangularis; IPL, inferior parietal lobule; MFG, middle frontal gyrus; msg, medial short gyrus; MTG, middle temporal gyrus; plg, posterior long gyrus; PoG, postcentral gyrus; PrG, precentral gyrus; psg, posterior short gyrus; SFG, superior frontal gyrus; SLF, superior longitudinal fasciculus; sls, superior limiting sulcus; SMG, supramarginal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; TOp, temporal operculum; uf, short U-fibers.\u003c/p\u003e","description":"","filename":"fig.1.png","url":"https://assets-eu.researchsquare.com/files/rs-9163139/v1/5ea717aa694f2098724c6493.png"},{"id":108407153,"identity":"1c37c166-fb92-4d9a-a049-356f2090a2cd","added_by":"auto","created_at":"2026-05-04 09:48:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2921798,"visible":true,"origin":"","legend":"\u003cp\u003eLayered organization of white matter following insular decortication.\u003c/p\u003e\n\u003cp\u003e(A) Exposure of deeper structures following insular decortication demonstrating the relationship between the SLF–AF complex and underlying projection fibers, including the corona radiata and globus pallidus.\u003c/p\u003e\n\u003cp\u003e(B) Posterior extension of corona radiata fibers after partial removal of the arcuate fasciculus.\u003c/p\u003e\n\u003cp\u003eAbbreviations: AF, arcuate fasciculus; cr, corona radiata; Gp, globus pallidus; SLF, superior longitudinal fasciculus; UF, uncinate fasciculus.\u003c/p\u003e","description":"","filename":"fig.2.png","url":"https://assets-eu.researchsquare.com/files/rs-9163139/v1/e0a9082e6788bbc6e84c3df8.png"},{"id":108407154,"identity":"c9a04e01-d91a-42d1-8afd-e847403e9ca1","added_by":"auto","created_at":"2026-05-04 09:48:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1028521,"visible":true,"origin":"","legend":"\u003cp\u003eTractographic reconstruction of the SLF–AF complex and its cortical connectivity.\u003c/p\u003e\n\u003cp\u003e(A) Three-dimensional reconstruction of cortical regions and associated SLF–AF and AF fiber pathways, with color-coded cortical areas.\u003c/p\u003e\n\u003cp\u003e(B) Subcortical visualization of SLF and AF following removal of cortical structures.\u003c/p\u003e\n\u003cp\u003e(C) Isolated reconstruction of the SLF–AF complex and arcuate fasciculus.\u003c/p\u003e\n\u003cp\u003e(D) Spatial relationship between the SLF–AF complex and the insula.\u003c/p\u003e\n\u003cp\u003e(Color code: light yellow, SFG; dark yellow, PrG; orange, PoG; dark red, IFGOp; purple, IFGTr; pink, IFGOr; dark green, STG; turquoise, MTG; red, AF; blue, insula; yellow, corona radiata; green, SLF.)\u003c/p\u003e\n\u003cp\u003eAbbreviations: AF, arcuate fasciculus; IFGOp, inferior frontal gyrus pars opercularis; IFGOr, inferior frontal gyrus pars orbitalis; IFGTr, inferior frontal gyrus pars triangularis; MTG, middle temporal gyrus; PoG, postcentral gyrus; PrG, precentral gyrus; SFG, superior frontal gyrus; SLF, superior longitudinal fasciculus; STG, superior temporal gyrus.\u003c/p\u003e","description":"","filename":"fig.3.png","url":"https://assets-eu.researchsquare.com/files/rs-9163139/v1/59cf0acb08ffaed0b21dfa8f.png"},{"id":108493073,"identity":"ef39f1b8-fb63-482b-b489-2a12f2b00a4b","added_by":"auto","created_at":"2026-05-05 09:59:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1253774,"visible":true,"origin":"","legend":"\u003cp\u003eTractographic demonstration of the layered relationship between association and projection fiber systems.\u003c/p\u003e\n\u003cp\u003e(A) Combined visualization of the SLF–AF complex, arcuate fasciculus, corona radiata, and insula.\u003c/p\u003e\n\u003cp\u003e(B) Reconstruction following removal of the insula, demonstrating the deeper projection pathways of the corona radiata.\u003c/p\u003e\n\u003cp\u003e(C) Posterior projection and spatial extension of the corona radiata following removal of superficial association fibers, consistent with anatomical dissection findings.\u003c/p\u003e\n\u003cp\u003e(Color code: red, AF; blue, insula; yellow, corona radiata; green, SLF.)\u003c/p\u003e\n\u003cp\u003eAbbreviations: AF, arcuate fasciculus; cr, corona radiata; ins, insula; SLF, superior longitudinal fasciculus.\u003c/p\u003e","description":"","filename":"fig.4.png","url":"https://assets-eu.researchsquare.com/files/rs-9163139/v1/e3a1276b7e7f66a03ba27f17.png"},{"id":108804405,"identity":"db65415c-cfcd-4df9-af41-dec8c4451adb","added_by":"auto","created_at":"2026-05-08 15:20:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9508374,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9163139/v1/dd4b566b-cd9b-4ff4-be37-ca66b3d45b56.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Layered White Matter Organization of the Superior Longitudinal Fasciculus–Arcuate Fasciculus Complex: Insular Topography and Surgical Implications from Combined Fiber Dissection and Tractography","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe superior longitudinal fasciculus (SLF) and arcuate fasciculus (AF) are major long-range association fiber systems connecting frontal, parietal, and temporal cortical regions and constitute a fundamental component of perisylvian connectivity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. While the AF has classically been described as a curved pathway linking the posterior temporal cortex to the inferior frontal gyrus, the SLF has been considered a broader association system connecting frontal, parietal, and occipital regions [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Contemporary studies using white matter dissection and diffusion imaging have demonstrated that these structures are not homogeneous but consist of multiple subcomponents with distinct connectivity profiles [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFunctionally, the SLF\u0026ndash;AF complex plays a central role in the dorsal language network and higher-order cognitive processes. In the dominant hemisphere, the AF is critically involved in phonological processing and speech production, whereas the SLF contributes to attention, working memory, and sensorimotor integration [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In the non-dominant hemisphere, these pathways are associated with spatial attention and neglect syndromes [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFrom a neurosurgical perspective, preservation of the SLF\u0026ndash;AF complex is essential during surgical procedures involving the insular, opercular, and temporoparietal regions. Damage to these pathways may result in significant language and cognitive deficits [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Although diffusion tractography has become a valuable tool in preoperative planning, its limitations necessitate validation through anatomical dissection [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite extensive research on segmentation and connectivity, the layered topographic relationship between the SLF\u0026ndash;AF complex and deeper white matter structures\u0026mdash;particularly in relation to the insula\u0026mdash;remains insufficiently explored. Therefore, the present study aims to define the layered anatomical organization of the SLF\u0026ndash;AF complex in relation to the insular region and to explore its surgical implications.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eSpecimens and Dissection Procedure\u003c/p\u003e\n\u003cp\u003eTen postmortem human brains (20 hemispheres) without intracranial pathology were included. Specimens were prepared according to the Klingler technique [18], involving fixation in formaldehyde, freezing at \u0026minus;15\u0026deg;C, and subsequent thawing to facilitate white matter fiber separation.\u003c/p\u003e\n\u003cp\u003eWhite matter dissections were performed under an operating microscope (\u0026times;6\u0026ndash;\u0026times;10 magnification) using a systematic lateral-to-medial approach. Cortical structures were progressively removed to expose successive layers of white matter organization, beginning with short U-fibers and extending to long association and projection fibers. Dissections were documented using high-resolution photography.\u003c/p\u003e\n\u003cp\u003eDiffusion MRI and Tractography\u003c/p\u003e\n\u003cp\u003eDiffusion tractography was performed using Human Connectome Project data. Deterministic fiber tracking was conducted using DSI Studio with a step size of 1 mm, angular threshold of 60\u0026deg;, and minimum fiber length of 20 mm. The SLF, AF, and corona radiata were reconstructed using ROI-based methods guided by anatomical landmarks identified during dissection.\u003c/p\u003e\n\u003cp\u003eDissection\u0026ndash;Tractography Correlation\u003c/p\u003e\n\u003cp\u003eQualitative comparisons were made between dissection findings and tractographic reconstructions, focusing on spatial relationships between the SLF\u0026ndash;AF complex, insula, and corona radiata. The analysis was primarily descriptive, focusing on anatomical consistency across specimens.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eFollowing cortical removal, short U-fibers were identified as the most superficial connections between adjacent gyri. Beneath this layer, long association fibers forming the SLF\u0026ndash;AF complex were consistently observed as a continuous superficial layer within the perisylvian region.\u003c/p\u003e\n\u003cp\u003eStepwise dissection revealed that the SLF\u0026ndash;AF complex maintains a close relationship with the insular cortex and becomes more prominent following removal of opercular structures. The AF demonstrated a characteristic curved trajectory in the posterior perisylvian region, while the SLF extended more broadly toward frontal regions.\u003c/p\u003e\n\u003cp\u003eFollowing insular decortication, deeper white matter structures became progressively visible. The corona radiata was consistently identified beneath the SLF\u0026ndash;AF complex, occupying a deeper and more medial plane. A hierarchical organization was observed, consisting of superficial association fibers (SLF\u0026ndash;AF complex), projection fibers (corona radiata), and deep gray matter structures.\u003c/p\u003e\n\u003cp\u003eTractographic reconstructions confirmed these findings, demonstrating a consistent spatial relationship between the SLF\u0026ndash;AF complex, insula, and corona radiata. A consistent correlation between anatomical dissection and tractography was observed.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study demonstrates that the superior longitudinal fasciculus\u0026ndash;arcuate fasciculus (SLF\u0026ndash;AF) complex forms a superficial association fiber layer in the perisylvian region, whereas the corona radiata represents a deeper projection system. Following insular decortication, the SLF\u0026ndash;AF complex was consistently observed in a superficial plane, while the corona radiata occupied a deeper and more medial position. These findings indicate that perisylvian white matter organization follows a hierarchical layered arrangement extending from superficial association fibers to deeper projection systems.\u003c/p\u003e\n\u003cp\u003eClassical neuroanatomical descriptions have traditionally defined the SLF\u0026ndash;AF complex as a single connection pathway linking frontal and temporal regions. However, contemporary studies using white matter dissection and diffusion imaging have demonstrated that this system exhibits a multi-segmented and heterogeneous organization [1,5]. The segmentation models proposed by Makris et al. and Catani et al. have provided important insights into their structural and functional diversity [14,15]. The findings of the present study are consistent with these models but further extend the existing literature by demonstrating the layered topographic relationship between the SLF\u0026ndash;AF complex and deeper projection systems in the insular region.\u003c/p\u003e\n\u003cp\u003eFrom a functional perspective, the SLF\u0026ndash;AF complex is a critical component of the dorsal language network and higher cognitive control systems. In the dominant hemisphere, the arcuate fasciculus plays a key role in phonological processing and speech production, while the superior longitudinal fasciculus contributes to attention, working memory, and sensorimotor integration [6,7]. The layered organization demonstrated in this study may represent an anatomical substrate for this functional differentiation.\u003c/p\u003e\n\u003cp\u003eFrom a neurosurgical perspective, these findings have significant clinical implications. The superficial location of the SLF\u0026ndash;AF complex indicates that it represents an early encountered functional boundary during surgical approaches involving the insular, opercular, and temporoparietal regions. Preservation of this layer is critical for maintaining language and cognitive functions [2,9]. In contrast, deeper dissection exposes the corona radiata, where injury may lead to motor and subcortical deficits. Therefore, this layered organization provides a practical anatomical framework for defining safe resection boundaries and optimizing functional preservation during surgery. This layered anatomical understanding may significantly reduce the risk of functional deficits and improve surgical outcomes in insular and perisylvian tumor resections.\u003c/p\u003e\n\u003cp\u003eThe consistency between tractography and dissection findings supports the role of diffusion-based imaging as a complementary tool for evaluating white matter anatomy [10,13]. However, limitations related to fiber crossing, algorithmic constraints, and the use of group-averaged data should be acknowledged [10\u0026ndash;12]. Additionally, the use of ex vivo specimens and tractography-based reconstructions may not fully represent in vivo functional dynamics.\u003c/p\u003e\n\u003cp\u003eThis study provides a novel anatomical framework by clearly demonstrating the layered organization of the SLF\u0026ndash;AF complex in relation to the insular region and its spatial relationship with deeper projection systems. Unlike previous studies focusing primarily on segmentation and connectivity, this work introduces a clinically applicable topographic model that directly translates into surgical practice.\u003c/p\u003e\n\u003cp\u003eLimitations\u003c/p\u003e\n\u003cp\u003eDeterministic tractography has limitations in resolving crossing fibers, and group-averaged datasets may not reflect individual anatomical variability. These limitations were mitigated by correlating tractographic findings with anatomical dissection data. Additionally, the use of ex vivo specimens and tractography-based reconstructions may not fully represent in vivo functional dynamics.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe SLF\u0026ndash;AF complex constitutes a superficial association fiber layer, whereas the corona radiata forms a deeper projection system. This layered organization provides important insights into white matter architecture and offers a practical and clinically applicable anatomical framework for safer neurosurgical interventions in the insular and perisylvian regions.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eU.T. and M.E. wrote the main manuscript text and U.T. , E.S.A, M.H.Ş and S.G. prepared figures 1 and 2. U.T. , M.E. and M.H.Ş prepared figures 3 and 4. All authors reviewed the manuscript.\u0026quot;\u003c/p\u003e\n\u003ch2\u003eEthics Approval\u003c/h2\u003e\n\u003cp\u003eThis study was conducted in accordance with institutional and international ethical standards for the use of postmortem human specimens.\u003c/p\u003e\n\u003ch2\u003eConflict of Interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eNo funding was received for this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJanelle F, et al. Superior longitudinal fasciculus: a review of the anatomical descriptions with functional correlates. Front Neurol. 2022;13:864754. https://doi.org/10.3389/fneur.2022.864754\u003c/li\u003e\n\u003cli\u003eVavassori L, et al. The arcuate fasciculus: combining structure and function into surgical considerations. Brain Behav. 2023;13:e3009. https://doi.org/10.1002/brb3.3009\u003c/li\u003e\n\u003cli\u003ede Oliveira JVMP, et al. What\u0026rsquo;s your name again? A review of the superior longitudinal fasciculus nomenclature. Clin Anat. 2021;34(8):1161\u0026ndash;1173. https://doi.org/10.1002/ca.23764\u003c/li\u003e\n\u003cli\u003eDick AS, Tremblay P. Beyond the arcuate fasciculus: consensus and controversy in the study of language pathways. Brain. 2012;135(12):3529\u0026ndash;3550. https://doi.org/10.1093/brain/aws222\u003c/li\u003e\n\u003cli\u003eBasile GA, et al. Functional anatomy and topographical organization of the arcuate fasciculus. Commun Biol. 2024;7:198. https://doi.org/10.1038/s42003-024-05876-2\u003c/li\u003e\n\u003cli\u003eIvanova MV, et al. Functional contributions of the arcuate fasciculus to language processing. Front Hum Neurosci. 2021;15:672665. https://doi.org/10.3389/fnhum.2021.672665\u003c/li\u003e\n\u003cli\u003eZong F, et al. Language function of the superior longitudinal fasciculus in the human brain. Front Neurosci. 2023;17:1189363. https://doi.org/10.3389/fnins.2023.1189363\u003c/li\u003e\n\u003cli\u003eAndreoli M, et al. Association of disruption of the right posterior arcuate fasciculus with spatial attention deficits. J Neurosurg. 2024;141(2):468\u0026ndash;478. https://doi.org/10.3171/2023.5.JNS23456\u003c/li\u003e\n\u003cli\u003ePanesar SS, et al. Tractography for surgical neuro-oncology planning. Neurotherapeutics. 2019;16(2):429\u0026ndash;446. https://doi.org/10.1007/s13311-018-00693-8\u003c/li\u003e\n\u003cli\u003eKamagata K, et al. Advancements in diffusion MRI tractography for preoperative planning. Invest Radiol. 2024;59(3):150\u0026ndash;162. https://doi.org/10.1097/RLI.0000000000001000\u003c/li\u003e\n\u003cli\u003eVoets NL, et al. Consensus recommendations for clinical functional MRI applied to language mapping. Aperture Neuro. 2025;4:100045. https://doi.org/10.52294/AN.2025.100045\u003c/li\u003e\n\u003cli\u003eSanchez S, et al. Language neuroscience in the operating room. Front Oncol. 2025;15:1298765. https://doi.org/10.3389/fonc.2025.1298765\u003c/li\u003e\n\u003cli\u003eYeh FC. Shape analysis of the human association pathways. Neuroimage. 2020;223:117329. https://doi.org/10.1016/j.neuroimage.2020.117329\u003c/li\u003e\n\u003cli\u003eMakris N, Kennedy DN, McInerney S, et al. Segmentation of subcomponents within the superior longitudinal fascicle in humans. Cereb Cortex. 2005;15(6):854\u0026ndash;869. https://doi.org/10.1093/cercor/bhh186\u003c/li\u003e\n\u003cli\u003eCatani M, Jones DK, ffytche DH. Perisylvian language networks of the human brain. Ann Neurol. 2005;57(1):8\u0026ndash;16. https://doi.org/10.1002/ana.20319\u003c/li\u003e\n\u003cli\u003eBurdach KF. Vom Baue und Leben des Gehirns. Leipzig: Dyk\u0026rsquo;sche Buchhandlung; 1822.\u003c/li\u003e\n\u003cli\u003eDejerine J. Anatomie des Centres Nerveux. Paris: Rueff; 1895.\u003c/li\u003e\n\u003cli\u003eKlingler J. Erleichterung der makroskopischen Pr\u0026auml;paration des Gehirns durch den Gefrierprozess. Schweiz Arch Neurol Psychiatr. 1935;36:247\u0026ndash;256.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"superior longitudinal fasciculus, arcuate fasciculus, insula, tractography, white matter dissection, neurosurgery","lastPublishedDoi":"10.21203/rs.3.rs-9163139/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9163139/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe superior longitudinal fasciculus (SLF) and arcuate fasciculus (AF) complex forms a critical white matter network involved in language and higher cognitive functions within the perisylvian region. However, its three-dimensional topographic relationship with the insular region and its layered organization relative to deeper projection fibers remain insufficiently characterized. This study aimed to investigate the anatomical relationship of the SLF\u0026ndash;AF complex with the insula and its layered organization with respect to the corona radiata using combined white matter dissection and diffusion tractography. Microsurgical white matter dissections were performed on 10 postmortem human brains (20 hemispheres) prepared using the Klingler technique. Specimens were fixed in formaldehyde, frozen, and subsequently thawed to facilitate fiber separation. Dissections were carried out under an operating microscope using a stepwise lateral-to-medial approach to progressively expose superficial and deep white matter layers. Dissection findings were correlated with deterministic diffusion tractography analyses performed using DSI Studio, based on diffusion tensor imaging data obtained from the Human Connectome Project. Reconstruction of the SLF, AF, and corona radiata was performed using a region-of-interest (ROI)-based approach guided by anatomical landmarks. Dissection findings demonstrated that the SLF\u0026ndash;AF complex forms a continuous superficial association fiber layer located near the cortical surface within the perisylvian region. Following insular decortication, deeper white matter structures became progressively visible, revealing the corona radiata as a more medial and deeper projection system. A consistent hierarchical organization was observed across all specimens, consisting of superficial association fibers (SLF\u0026ndash;AF complex), intermediate projection fibers (corona radiata), and deeper subcortical structures. Tractographic reconstructions confirmed these findings and demonstrated a reproducible spatial relationship between the SLF\u0026ndash;AF complex, insula, and corona radiata. The SLF\u0026ndash;AF complex represents a superficial association fiber layer in the insular region, whereas the corona radiata constitutes a deeper projection system. This layered organization provides a practical and clinically applicable anatomical framework for preserving functional white matter tracts and defining safe resection boundaries during insular and perisylvian neurosurgical procedures.\u003c/p\u003e","manuscriptTitle":"Layered White Matter Organization of the Superior Longitudinal Fasciculus–Arcuate Fasciculus Complex: Insular Topography and Surgical Implications from Combined Fiber Dissection and Tractography","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-04 09:48:16","doi":"10.21203/rs.3.rs-9163139/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"cbbd9bc4-c127-4e25-8f51-b0f0e4e96230","owner":[],"postedDate":"May 4th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Rejected","date":"2026-05-02T15:22:45+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-01T08:49:37+00:00","index":64,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-30T01:35:24+00:00","index":62,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-04T09:48:19+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-04 09:48:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9163139","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9163139","identity":"rs-9163139","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}