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A. Visual white matter tracts identified by tractography in representative subjects (left, Healthy5; right, LHON1; magenta, optic tract; green, optic radiation). Tracts are shown above an axial slice of T1-weighted image. See Supplementary Fig. 2 for examples of other subjects. B. Fractional Anisotropy (FA) and quantitative T1 (qT1) map in a representative healthy subject (Healthy5).
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In patients with retinal ganglion cell diseases, recent diffusion tensor imaging (DTI) studies have revealed structural abnormalities in visual white matter tracts such as the optic tract, and optic radiation. However, the microstructural origin of these diffusivity changes is unknown as DTI metrics involve multiple biological factors and do not co...
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The mitochondrial genetic disorder, Leber’s hereditary optic neuropathy (LHON), is caused by a mutation in MT-ND4 gene, encoding NADH dehydrogenase subunit 4. It leads to the progressive death of retinal ganglion cells (RGCs) and causes visual impairment or even blindness. However, the precise mechanisms of LHON disease penetrance and progression a...
Citations
... 117,118 The LGN ROIs estimated by this method roughly match the position of the LGN in anatomical data. 119 Kammen and colleagues (2016) 120 proposed a conceptually similar, but more sophisticated automated approach to identify the LGN ROI from tractography. Specifically, they perform a tracking from the optic chiasm and primary visual cortex (V1) first and then estimate the LGN ROI based on information about where streamlines from the optic chiasm intersect with the thalamus, as well as shape information relative to other structures. ...
... 124,125 Nevertheless, once researchers can identify the optic chiasm and LGN as seed voxels, the identification of the OT from dMRI in a standard resolution is achievable and widely tested. 118,126 Tractometry studies on this tract A variety of studies used dMRI-based tractometry to evaluate tissue properties of the OT in disorders damaging RGCs and the ON, such as glaucoma, [127][128][129][130][131] Leber's hereditary optic neuropathy (LHON) 118,119 (Fig. 4B), and optic neuritis. 132 These studies show that dMRI-based tractometry has the sensitivity to identify the OT tissue abnormalities that are caused by these disorders. ...
... [174][175][176] to estimate eccentricity representation in V1 and then classify OR streamlines into foveal, mid-periphery, and far-periphery, based on streamline endpoints near V1. 72,119 These studies found that tissue changes caused by the LHON and macular degeneration are most prominently observed in the OR terminating near foveal V1, consistent with a prediction from the spatial pattern of visual field loss caused by these disorders. Kruper and colleagues (2023) 177 used the same approach to analyze the OR in the data from a large number of healthy participants in the UK Biobank and identified that age dependency in white matter is different among different subcomponents of the OR divided by eccentricity representation in the V1. ...
Diffusion-weighted MRI (dMRI) provides a unique non-invasive view of human brain tissue properties. The present review article focuses on tractometry analysis methods that use dMRI to assess the properties of brain tissue within the long-range connections comprising brain networks. We focus specifically on the major white matter tracts that convey visual information. These connections are particularly important because vision provides rich information from the environment that supports a large range of daily life activities. Many of the diseases of the visual system are associated with advanced aging, and tractometry of the visual system is particularly important in the modern aging society. We provide an overview of the tractometry analysis pipeline, which includes a primer on dMRI data acquisition, voxelwise model fitting, tractography, recognition of white matter tracts, and calculation of tract tissue property profiles. We then review dMRI-based methods for analyzing visual white matter tracts: the optic nerve, optic tract, optic radiation, forceps major, and vertical occipital fasciculus. For each tract, we review background anatomical knowledge together with recent findings in tractometry studies on these tracts and their properties in relation to visual function and disease. Overall, we find that measurements of the brain's visual white matter are sensitive to a range of disorders and correlate with perceptual abilities. We highlight new and promising analysis methods, as well as some of the current barriers to progress toward integration of these methods into clinical practice. These barriers, such as variability in measurements between protocols and instruments, are targets for future development.
... This method has opened up avenues to evaluate the tissue properties of white matter tracts in individuals and compare them with distributions in a healthy population [8]. This approach has been confirmed to be useful for understanding the effects of retinal disorders on white matter [9][10][11][12], the relationship between language-related white matter tracts and reading performance [13][14][15][16], the relationship between white matter tracts in the limbic system and psychiatric disorders [17], age dependency of white matter tissue properties [18][19][20], and lateralization of white matter tracts [21][22][23]. ...
... We defined ROIs for tractography on the T1-weighted images acquired from each subject, which were defined using the methods already established in previous studies [8][9][10][11]26]. ...
... The lateral geniculate nucleus (LGN) was identified using methods described in previous publications [9][10][11]. In brief, we performed deterministic tractography by defining the optic chiasm as a seed to determine streamline termination near the LGN. ...
Diffusion-weighted magnetic resonance imaging (dMRI) is the only available method to measure the tissue properties of white matter tracts in living human brains and has opened avenues for neuroscientific and clinical studies on human white matter. However, dMRI using conventional simultaneous multi-slice (SMS) single-shot echo planar imaging (ssEPI) still presents challenges in the analyses of some specific white matter tracts, such as the optic nerve, which are heavily affected by susceptibility-induced artifacts. In this study, we evaluated dMRI data acquired by using SMS readout-segmented EPI (rsEPI), which aims to reduce susceptibility-induced artifacts by dividing the acquisition space into multiple segments along the readout direction to reduce echo spacing. To this end, we acquired dMRI data from 11 healthy volunteers by using SMS ssEPI and SMS rsEPI, and then compared the dMRI data of the human optic nerve between the SMS ssEPI and SMS rsEPI datasets by visual inspection of the datasets and statistical comparisons of fractional anisotropy (FA) values. In comparison with the SMS ssEPI data, the SMS rsEPI data showed smaller susceptibility-induced distortion and exhibited a significantly higher FA along the optic nerve. In summary, this study demonstrates that despite its prolonged acquisition time, SMS rsEPI is a promising approach for measuring the tissue properties of the optic nerve in living humans and will be useful for future neuroscientific and clinical investigations of this pathway.
... The MTV data were acquired from all subjects according to a previously described protocol ( Mezer et al., 2013 ;Oishi et al., 2018 ;Takemura et al., 2019 ;Minami et al., 2020 ). In brief, four fast low-angle shot (FLASH) images were measured with flip angles of 4°, 10°, 20°, and 30°(TR, 12 ms; TE, 2.43 ms) with 1 mm isotropic voxels. ...
... Finally, the MTV map is aligned to the T1w MRI data to enable further comparisons with other images in the same coordinate space. The full analysis pipeline and validation results for the MTV method can be found in previous publications ( Mezer et al., 2013( Mezer et al., , 2016Oishi et al., 2018 ;Takemura et al., 2019 ;Minami et al., 2020 ). The code for calculating MTV is publicly available ( https://github.com/mezera/mrQ ...
... Recent developments in quantitative MRI have enabled the quantification of MRI parameters, which allows the comparison of brain tissue properties between human subjects ( Mezer et al., 2013 ;Weiskopf et al., 2015 ;Forstmann et al., 2016 ;Keuken et al., 2017 ;Cercignani et al., 2018 ). These quantitative MRI measurements have provided valuable insights into the tissue properties of cortical areas ( Sereno et al., 2013 ;Lutti et al., 2014 ;Carey et al., 2018 ) and white matter ( Stüber et al., 2014 ;Schurr et al., 2018 ;Takemura et al., 2019 ). Mezer et al. (2013) proposed MTV methods and demonstrated consistency of MTV measurements with lipid volume fractions in a phantom, high test-retest reproducibility, and sensitivity for white matter tissue changes in patients with multiple sclerosis. ...
The lateral geniculate nucleus (LGN) is a key thalamic nucleus in the visual system, which has an important function in relaying retinal visual input to the visual cortex. The human LGN is composed mainly of magnocellular (M) and parvocellular (P) subdivisions, each of which has different stimulus selectivity in neural response properties. Previous studies have discussed the potential relationship between LGN subdivisions and visual disorders based on psychophysical data on specific types of visual stimuli. However, these relationships remain speculative because non-invasive measurements of these subdivisions are difficult due to the small size of the LGN. Here we propose a method to identify these subdivisions by combining two structural MR measures: high-resolution proton-density weighted images and macromolecular tissue volume (MTV) maps. We defined the M and P subdivisions based on MTV fraction data and tested the validity of the definition by (1) comparing the data with that from human histological studies, (2) comparing the data with functional magnetic resonance imaging measurements on stimulus selectivity, and (3) analyzing the test-retest reliability. The findings demonstrated that the spatial organization of the M and P subdivisions was consistent across subjects and in line with LGN subdivisions observed in human histological data. Moreover, the difference in stimulus selectivity between the subdivisions identified using MTV was consistent with previous physiology literature. The definition of the subdivisions based on MTV was shown to be robust over measurements taken on different days. These results suggest that MTV mapping is a promising approach for evaluating the tissue properties of LGN subdivisions in living humans. This method potentially will enable neuroscientific and clinical hypotheses about the human LGN subdivisions to be tested.
... In particular, MRI makes it possible to localise brain regions, structures, and processes linked to specific types of visual field loss. Recent structural and functional MRI studies of LHON have identified widespread pathological changes in different stages of the disease process, and across the visual system, from the optic tracts to the cortex (Barcella et al., 2010;Manners et al., 2015;Takemura et al., 2019;Long et al., 2019;Jonak et al., 2020b). These studies employ various informative MRI methodologies, which can answer important open questions about LHON. ...
... None of these studies found correlations of volume with disease duration, visual acuity, or visual field defects. When compared to age-match normal-sighted controls, reduced FA was also found in the optic tracts of adults with chronic LHON (Milesi et al., 2012;Ogawa et al., 2014;Takemura et al., 2019), which paired with a reduction in visual acuity (Milesi et al., 2012) supporting the functional relevance of DTI measures. Using quantitative MRI, Takemura et al. (2019) showed reduced FA and increased qT1 values in the optic tracts of 7 adults with chronic LHON, compared to 20 adult normal-sighted controls. ...
... When compared to age-match normal-sighted controls, reduced FA was also found in the optic tracts of adults with chronic LHON (Milesi et al., 2012;Ogawa et al., 2014;Takemura et al., 2019), which paired with a reduction in visual acuity (Milesi et al., 2012) supporting the functional relevance of DTI measures. Using quantitative MRI, Takemura et al. (2019) showed reduced FA and increased qT1 values in the optic tracts of 7 adults with chronic LHON, compared to 20 adult normal-sighted controls. This is further evidence that loss of macromolecular content (i.e., from axonal or myelin loss) is pervasive across the retinogeniculate pathway. ...
Leber Hereditary Optic Neuropathy (LHON) is an inherited mitochondrial retinal disease that causes the degeneration of retinal ganglion cells and leads to drastic loss of visual function. In the last decades, there has been a growing interest in using Magnetic Resonance Imaging (MRI) to better understand mechanisms of LHON beyond the retina. This is partially due to the emergence of gene-therapies for retinal diseases, and the accompanying expanded need for reliably quantifying and monitoring visual processing and treatment efficiency in patient populations. This paper aims to draw a current picture of key findings in this field so far, the challenges of using neuroimaging methods in patients with LHON, and important open questions that MRI can help address about LHON disease mechanisms and prognoses, including how downstream visual brain regions are affected by the disease and treatment and why, and how scope for neural plasticity in these pathways may limit or facilitate recovery.
... We then defined the lateral geniculate nucleus (LGN) ROIs manually by following streamlines found using deterministic tractography from a seed-region in the optic chiasm to their termination points. The LGN ROI is defined as a 4 mm radius sphere covering these endpoints (Takemura et al., 2019). We also identified the V1 ROI for tractography using the Brodmann area atlas implemented in FreeSurfer. ...
... In addition to the OT, we also identified the OR. For the OR, we used ConTrack to generate streamlines connecting the LGN and V1 ROIs and to reject outlier streamlines (Takemura et al., 2019). ...
... Evaluating tissue properties of the OT. We evaluated tissue properties of the OT using the AFQ MATLAB toolbox (Yeatman et al., 2012;Duan et al., 2015;Takemura et al., 2019). Briefly, we resampled each streamline to 100 equidistant nodes. ...
Individual differences among human brains exist at many scales, spanning gene expression, white matter tissue properties, and the size and shape of cortical areas. One notable example is an approximately 3-fold range in the size of human primary visual cortex (V1), a much larger range than is found in overall brain size. A previous study (Andrews et al., 1997) reported a correlation between optic tract (OT) cross-section area and V1 size in postmortem human brains, suggesting that there may be a common developmental mechanism for multiple components of the visual pathways. We evaluated the relationship between properties of the OT and V1 in a much larger sample of living human brains by analyzing the Human Connectome Project (HCP) 7 Tesla Retinotopy Dataset (including 107 females and 71 males). This dataset includes retinotopic maps measured with functional MRI (fMRI) and fiber tract data measured with diffusion MRI (dMRI). We found a negative correlation between OT fractional anisotropy (FA) and V1 surface area (r = −0.19). This correlation, although small, was consistent across multiple dMRI datasets differing in acquisition parameters. Further, we found that both V1 surface area and OT properties were correlated among twins, with higher correlations for monozygotic (MZ) than dizygotic (DZ) twins, indicating a high degree of heritability for both properties. Together, these results demonstrate covariation across individuals in properties of the retina (OT) and cortex (V1) and show that each is influenced by genetic factors.
... com/ vista lab/ contr ack). This tool was used to identify visual pathways in previous studies 23,24 . A set of 1000 candidate pathways for ON was generated, and the top 10% of the most likely fibers were selected by the ConTrack scoring algorithm. ...
The aim of this study is to quantitatively investigate the microstructural properties of the optic nerve (ON) in vivo using diffusion tensor imaging (DTI) in patients with unilateral optic atrophy (OA) and to determine their association with retinal nerve fiber layer (RNFL) thickness of the optic nerve head (ONH). Six patients with unilateral OA and 11 control subjects underwent DTI. ONs from ONH to the orbital apex were tracked. Fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were computed in both ONs and their correlation with RNFL thickness measured using optical coherence tomography was also analyzed. FA of atrophic ON was lower than that of non-affected and control ONs (atrophic [A], 0.136 ± 0.059; non-affected [N], 0.384 ± 0.048; control [C], 0.389 ± 0.053). MD and RD of atrophic ONs were higher than those of non-affected and control ONs (MD, A, 0.988 ± 0.247; N, 0.658 ± 0.058; C, 0.687 ± 0.079; RD, A, 0.920 ± 0.247; N, 0.510 ± 0.054; C, 0.532 ± 0.078). All DTI measures of atrophic ON except for AD showed a significant correlation with RNFL thickness of ONH; FA showed the strongest correlation, followed by RD and MD (FA, R² = 0.936, P < 0.001; RD, R² = 0.795, P < 0.001; MD, R² = 0.655, P = 0.001). This study reports quantitative analysis of the ON using DTI and differences in DTI measures between atrophic and normal ONs. The significant correlation between DTI measures and RNFL thickness suggests the applicability of DTI as a clinical tool to evaluate the ON.
... Probabilistic tractography extends this analysis to create maps that are derived from the probability that two regions are connected, in this case the terminal portion of the optic nerve and the lateral geniculate nucleus (LGN) [14]. No previous study has applied tractography to the evaluation of LHON patients over the whole anterior optic pathway including the optic nerve [15,16]. ...
... Previous studies employing diffusion-weighted imaging to investigate white matter changes in LHON patients have demonstrated reduced FA in the optic radiation, as well as in other white matter tracts to a lesser extent [16,32,33]. Studies in the anterior optic pathway typically disclose reduced FA along at least part of the tract [15,16], although study cohorts are typically in the subacute to chronic phase of the disease. ...
... Previous studies employing diffusion-weighted imaging to investigate white matter changes in LHON patients have demonstrated reduced FA in the optic radiation, as well as in other white matter tracts to a lesser extent [16,32,33]. Studies in the anterior optic pathway typically disclose reduced FA along at least part of the tract [15,16], although study cohorts are typically in the subacute to chronic phase of the disease. As yet, no results have been reported in the nerve itself, and diffusion parameters have been limited to those obtainable from the diffusion tensor model, based on a single diffusion weighting value. ...
Tractography based on multishell diffusion-weighted magnetic resonance imaging (DWI) can be used to estimate the course of myelinated white matter tracts and nerves, yielding valuable information regarding normal anatomy and variability. DWI is sensitive to the local tissue microstructure, so tractography can be used to estimate tissue properties within nerve tracts at a resolution of millimeters. This study aimed to test the applicability of the method using a disease with a well-established pattern of myelinated nerve involvement. Eight patients with LHON and 13 age-matched healthy controls underwent tractography of the anterior optic pathway. Diffusion parameters were compared between groups, and for the patient group correlated with clinical/ophthalmological parameters. Tractography established the course of the anterior optic pathway in both patients and controls. Localized changes in fractional anisotropy were observed, and related to estimates of different tissue compartments within the nerve and tract. The proportion of different compartments correlated with markers of disease severity. The method described allows both anatomical localization and tissue characterization in vivo, permitting both visualization of variation at the individual level and statistical inference at the group level. It provides a valuable adjunct to ex vivo anatomical and histological study of normal variation and disease processes.
... For example, Barcella et al. reported that the optical radiation was atrophied in the chronic LHON using voxel-based morphometry (VBM) (Barcella et al., 2010). Microstructural changes in the optical radiation in the chronic LHON, as represented by decreased fractional anisotropy (FA) and increased mean diffusivity (MD) and radial diffusivity (RD), were also confirmed by several recent studies (Milesi et al., 2012;Rizzo et al., 2012;Ogawa et al., 2014;Manners et al., 2015;Takemura et al., 2019). Besides the WM changes, VBM also detected decreased GM volume (GMV) in the primary visual cortex (Barcella et al., 2010). ...
Purpose
The aim of this study was to investigate the brain gray matter volume (GMV) and spontaneous functional connectivity (FC) changes in patients with chronic Leber's hereditary optic neuropathy (LHON), and their relations with clinical measures.
Methods
A total of 32 patients with chronic LHON and matched sighted healthy controls (HC) underwent neuro-ophthalmologic examinations and multimodel magnetic resonance imaging (MRI) scans. Voxel-based morphometry (VBM) was used to detect the GMV differences between the LHON and HC. Furthermore, resting-state FC analysis using the VBM-identified clusters as seeds was carried out to detect potential functional reorganization in the LHON. Finally, the associations between the neuroimaging and clinical measures were performed.
Results
The average peripapillary retinal nerve fiber layer (RNFL) thickness of the chronic LHON was significantly thinner (T = −16.421, p < 0.001), and the mean defect of the visual field was significantly higher (T = 11.28, p < 0.001) than the HC. VBM analysis demonstrated a significantly lower GMV of bilateral calcarine gyri (CGs) in the LHON than in the HC (p < 0.05). Moreover, in comparison with the HC, the LHON had significantly lower FC between the centroid of the identified left CG and ipsilateral superior occipital gyrus (SOG) and higher FC between this cluster and the ipsilateral posterior cingulate gyrus (p < 0.05, corrected). Finally, the GMV of the left CG was negatively correlated with the LHON duration (r = −0.535, p = 0.002), and the FC between the left CG and the ipsilateral posterior cingulate gyrus of the LHON was negatively correlated with the average peripapillary RNFL thickness (r = −0.522, p = 0.003).
Conclusion
The atrophied primary visual cortex of the chronic LHON may be caused by transneuronal degeneration following the retinal damage. Moreover, our findings suggest that the functional organization of the atrophied primary visual cortex has been reshaped in the chronic LHON.
... Thirty volunteers with normal visual function and no visual field defects also participated in this study as control subjects (14 females; mean age, 51.4 years; age range, 36-71 years) ( Table 2). With reference to a previous study, 40 this sample size was predicted to be sufficient to identify a large effect size in group difference (d = 1.10) using a two-tailed, two-sample t-test (evaluated by G*Power 3.1.9.6). ...
... For all subjects, qT1 was measured following the protocols described in previous publications. 32,40 Four fast low-angle shot (FLASH) images with flip angles of 4°, 10°, 20°, and 30°(TR, 12 ms; TE, 2.41 ms) and isotropic 2-mm voxels were acquired. Five additional spin-echo inversion-recovery (SEIR) scans with an EPI readout (TR, 3000 ms; TE, 49 ms; 2× acceleration) were acquired to remove field inhomogeneities. ...
... We identified the OT and OR by analyzing the dMRI data using the same method as that used in previous studies ( Fig. 2A). 40 Tissue Property Evaluation. We evaluated the tissue properties of OT and OR using Automated Fiber Quantification (AFQ) (https://github.com/yeatmanlab/AFQ). 54 Briefly, each streamline was resampled to 100 equidistant nodes, and tissue properties (qT1, ICVF, ODI, IsoV, FA, MD, AD, RD, and MTV) were calculated at each node of each streamline. ...
Purpose:
Glaucoma is a disorder that involves visual field loss caused by retinal ganglion cell damage. Previous diffusion magnetic resonance imaging (dMRI) studies have demonstrated that retinal ganglion cell damage affects tissues in the optic tract (OT) and optic radiation (OR). However, because previous studies have used a simple diffusion tensor model to analyze dMRI data, the microstructural interpretation of white matter tissue changes remains uncertain. In this study, we used a multi-contrast MRI approach to further clarify the type of microstructural damage that occurs in patients with glaucoma.
Methods:
We collected dMRI data from 17 patients with glaucoma and 30 controls using 3-tesla (3T) MRI. Using the dMRI data, we estimated three types of tissue property metrics: intracellular volume fraction (ICVF), orientation dispersion index (ODI), and isotropic volume fraction (IsoV). Quantitative T1 (qT1) data, which may be relatively specific to myelin, were collected from all subjects.
Results:
In the OT, all four metrics showed significant differences between the glaucoma and control groups. In the OR, only the ICVF showed significant between-group differences. ICVF was significantly correlated with qT1 in the OR of the glaucoma group, although qT1 did not show any abnormality at the group level.
Conclusions:
Our results suggest that, at the group level, tissue changes in OR caused by glaucoma might be explained by axonal damage, which is reflected in the intracellular diffusion signals, rather than myelin damage. The significant correlation between ICVF and qT1 suggests that myelin damage might also occur in a smaller number of severe cases.
... Defining ROIs for tractography. We identified regions of interest (ROIs) for tractography based on T1w images in each individual subject, using the same method employed in previous work (Ogawa et al., 2014;Takemura et al., 2019). Briefly, the optic chiasm was defined based on Freesurfer's automated segmentation (Fischl, 2012). ...
... Briefly, the optic chiasm was defined based on Freesurfer's automated segmentation (Fischl, 2012). We then defined the lateral geniculate nucleus (LGN) ROIs manually using a previously-validated method (Takemura et al., 2019). This method identifies the approximate location of the LGN by following streamlines found using deterministic tractography from a seed-region in the optic chiasm to their termination points. ...
... This method identifies the approximate location of the LGN by following streamlines found using deterministic tractography from a seed-region in the optic chiasm to their termination points. The LGN ROI is defined as a 4-mm radius sphere covering these endpoints (Takemura et al., 2019). We also identified the V1 ROI for tractography using the Brodmann Area atlas implemented in Freesurfer. ...
Individual differences among human brains exist at many scales, spanning gene expression, white matter tissue properties, and the size and shape of cortical areas. One notable example is an approximately 3-fold range in the size of human primary visual cortex (V1), a much larger range than is found in overall brain size. A previous study (Andrews et al., 1997) reported a correlation between optic tract cross-section area and V1 size in post-mortem human brains, suggesting that there may be a common developmental mechanism for multiple components of the visual pathways. We evaluated the relationship between properties of the optic tract and V1 in a much larger sample of living human brains by analyzing the Human Connectome Project 7 Tesla Retinotopy Dataset. This dataset includes retinotopic maps measured with functional MRI (fMRI) and fiber tract data measured with diffusion MRI (dMRI). We found a negative correlation between optic tract fractional anisotropy and V1 surface area ( r = -0.2). This correlation, though small, was consistent across multiple dMRI datasets differing in acquisition parameters. Further, we found that both V1 size and optic tract properties were correlated among twins, with higher correlations for monozygotic than dizygotic twins, indicating a high degree of heritability for both properties. Together, these results demonstrate covariation across individuals in properties of the retina (optic tract) and cortex (V1) and show that each is influenced by genetic factors.
Significance statement
The size of human primary visual cortex (V1) has large inter-individual differences. These differences cannot be explained by differences in overall brain size. A previous post-mortem study reported a correlation between the size of the human optic tract and V1. In this study, we evaluated the relationship between the optic tract and V1 in living humans by analyzing a neuroimaging dataset that included functional and diffusion MRI data. We found a small, but robust correlation between optic tract tissue properties and V1 size, supporting the existence of structural covariance between the optic tract and V1 in living humans. The results suggest that characteristics of retinal ganglion cells, reflected in optic tract measurements, are related to individual differences in human V1.
