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Direct Visualization and Characterization of The bioRxiv preprint doi: https://doi.org/10.1101/2020.03.25.008318; this version posted March 27, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Direct visualization and characterization of the 2 human zona incerta and surrounding structures 3 Running Title: Jonathan C. Lau1,2,3,4,*, Yiming Xiao2,3, Roy A.M. Haast2,3, Greydon Gilmore1,4, Direct visualization Kamil Uludag7,8,9, Keith W. MacDougall1, Ravi S. Menon2,3,6, of the zona incerta 1 1,2,3,4,6,& 1,2,3,4,5,6,& Andrew G. Parrent , Terry M. Peters , Ali R. Khan Keywords: brain; atlas; human; neuroanatomy; zona incerta; 7T; deep brain stimulation; T1; quantitative MRI 4 5 1Department of Clinical Neurological Sciences, Division of Neurosurgery, Western University, London, Ontario, Canada 6 2Imaging Research Laboratories, Robarts Research Institute Canada, Western University, London, Ontario, Canada 7 3Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, London, Ontario, Canada 8 4School of Biomedical Engineering, Western University, London, Ontario, Canada 9 5Brain and Mind Institute, Western University, London, Ontario, Canada 10 6Department of Medical Biophysics, Western University, London, Ontario, Canada 11 7IBS Center for Neuroscience Imaging Research, Sungkyunkwan University, Seobu-ro, 2066, Jangan-gu, Suwon, South Korea 12 8Department of Biomedical Engineering, N Center, Sungkyunkwan University, Seobu-ro, 2066, Jangan-gu, Suwon, South Korea 13 9Techna Institute and Koerner Scientist in MR Imaging, University Health Network, 100 College St, Toronto, ON, Canada 14 *Corresponding author 15 &Joint senior authors 16 17 Abstract: The zona incerta (ZI) is a small gray matter region of the deep brain first identified in the 19th 18 century, yet direct in vivo visualization and characterization has remained elusive. Noninvasive detection of 19 the ZI and surrounding region could be critical to further our understanding of this widely connected but 20 poorly understood deep brain region and could contribute to the development and optimization of 21 neuromodulatory therapies. We demonstrate that high resolution (submillimetric) longitudinal (T1) 22 relaxometry measurements at high magnetic field strength (7 Tesla) can be used to delineate the ZI from 23 surrounding white matter structures, specifically the fasciculus cerebellothalamicus, fields of Forel 24 (fasciculus lenticularis, fasciculus thalamicus, field H), and medial lemniscus. Using this approach, we 25 successfully derived in vivo estimates of the size, shape, location, and tissue characteristics of 26 substructures in the ZI region, confirming observations only previously possible through histological 27 evaluation that this region is not just a space between structures but contains distinct morphological entities 28 that should be considered separately. Our findings pave the way for increasingly detailed in vivo study and 29 provide a structural foundation for precise functional and neuromodulatory investigation. bioRxiv preprint doi: https://doi.org/10.1101/2020.03.25.008318; this version posted March 27, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 30 Introduction 31 The zona incerta (ZI) is a small but diffuse structure in the deep brain first identified by Auguste Forel in 32 1877, famously described as "an immensely confusing area about which nothing can be said" (Forel, 1877). 33 Forel appreciated that the ZI consisted of gray matter located between the external medullary lamina of the 34 thalamus and the corpus Luysi (subthalamic nucleus; STN) of otherwise "indefinite" description. It is telling 35 that Forel found the ZI so difficult to describe given his crucial role in the delineation of surrounding white 36 matter tracts still referred to eponymously as the fields of Forel (Gallay et al., 2008). Since its original 37 description, much has been learned about the ZI and surrounding structures although robust in vivo 38 visualization has remained elusive. 39 40 The anatomical boundaries of the ZI have generally been described in the context of its more discrete 41 neighbors rather than based on any consistent feature of the region itself. Packed in a small area between 42 the ventral thalamus, STN, and red nucleus (RN), the ZI is situated at a complex junction of major white 43 matter pathways including the cerebellothalamic, pallidothalamic, medial lemniscal, and corticospinal tracts. 44 Along its dorsal, ventral, and medial borders, the ZI is surrounded by the fasciculus thalamicus (ft; also 45 known as the H1 field of Forel), the fasciculus lenticularis (fl; also known as the H2 field of Forel), and the 46 H field, which is a convergence of the fl and the ansa lenticularis (al), respectively (Gallay et al., 2008; 47 Nieuwenhuys et al., 2007). The rostral ZI (rZI) is continuous with the reticular nucleus of the thalamus 48 laterally and with the lateral hypothalamus anteromedially. The caudal ZI (cZI) is laterally bounded by the 49 STN and posterior limb of the internal capsule. To date, most of the details regarding the region are the 50 result of meticulous study of post-mortem specimens (Gallay et al., 2008; Morel, 2007; Schaltenbrand and 51 Wahren, 1977). 52 53 Cytoarchitectonic and myeloarchitectonic studies have identified the ZI as a gray matter complex consisting 54 of loosely arranged neurons of heterogeneous morphology with a diverse immunohistochemical profile 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.03.25.008318; this version posted March 27, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 55 (Nieuwenhuys et al., 2007). In Golgi preparations of the ZI, two main neuronal classes have been identified: 56 principal cells and interneurons (Ma et al., 1997). Gene expression studies have revealed a common 57 embryological origin along with the reticular nucleus of the thalamus and pregeniculate nucleus of the 58 ventral diencephalon, specifically the prethalamic segment, which predominantly contains GABAergic 59 neurons (Puelles et al., 2012; Watson et al., 2014). Through immunohistochemical analysis in experimental 60 animals, a general pattern of at least four component ZI sectors has emerged in the rostral, dorsal, ventral, 61 and caudal directions (Mitrofanis, 2005). Tract-tracing studies have identified extensive and often bilateral 62 connections between the ZI and the cortex, subcortex, and spinal cord (Mitrofanis, 2005; Watson et al., 63 2014). At least five functional subsectors within the ZI have been suggested: auditory, limbic, motor, 64 somatosensory, and visual. However, unlike other nearby structures like the STN, no robust 65 immunohistochemical biomarker has been described for the ZI proper. 66 67 The diversity of chemical expression and widespread connections suggest an important modulatory role of 68 the ZI in regulating brain function. The ZI forms extensive inhibitory connections with spinothalamic relay 69 nuclei in experimental animals, and thus may play an important role in modulating neuropathic pain and the 70 somatosensory system (Masri et al., 2009; Truini et al., 2013). In a perhaps related manner, the rostral ZI 71 is believed to provide inhibitory control over the thalamus during sleep (Llinás and Jahnsen, 1982; Watson 72 et al., 2014), which may also relate to its perceived role in modulating consciousness (Giacino et al., 2014; 73 Mitrofanis, 2005; Power and Mitrofanis, 2001). Finally, recent evidence suggests an important role for the 74 ZI in modulating fear generalization (Venkataraman et al., 2019) and appetite (Zhao et al., 2019). 75 76 In humans, the most well-studied role of the ZI is as a putative target for neuromodulatory therapy 77 transmitted either within the cZI or its vicinity, which has been observed to be highly effective for the 78 treatment of essential tremor (Hariz and Blomstedt, 2017). These investigations began in the 1960s with 79 selective ablation (Bertrand et al., 1969; Mundinger, 1965; Spiegel et al., 1962, 1964; Spiegel and Wycis, 80 1954; Velasco et al., 1975; Wertheimer et al., 1960), but as technologies improved, various groups 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.03.25.008318; this version posted March 27, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 81 (Blomstedt et al., 2010; Mohadjer et al., 1990; Nowacki et al., 2018a; Plaha et al., 2006; Velasco et al., 82 2001) demonstrated that electrical stimulation to these regions was also effective. Yet because of poor 83 direct visualization, controversy has remained as to whether the therapeutic effect is derived from 84 modulation of the cell bodies in the cZI, wayward connections such as the fasciculus cerebellothalamicus 85 (fct; also known as the prelemniscal radiations or raprl) (Castro et al., 2015; Velasco et al., 1972),
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