Papers by Claude J Bajada
NeuroImage, 2019
There has been a recent increased interest in the structural connectivity of the cortex. However,... more There has been a recent increased interest in the structural connectivity of the cortex. However, an important feature of connectivity remains relatively unexplored; tract length. In this article, we develop an approach to characterize fiber length distributions across the human cerebral cortex. We used data from 76 participants of the Adult {WU}-Minn Human Connectome Project using probabilistic tractography. We found that connections of different lengths are not evenly distributed across the cortex. They form patterns where certain areas have a high density of fibers of a specific length while other areas have very low density. To assess the relevance of these new maps in relation to established characteristics, we compared them to structural indices such as cortical myelin content and cortical thickness. Additionally, we assessed their relation to resting state network organization. We noted that areas with very short fibers have relatively more myelin and lower cortical thickness while the pattern is inverted for longer fibers. Furthermore, the cortical fiber length distributions produce specific correlation patterns with functional resting state networks. Specifically, we find evidence that as resting state networks increase in complexity, their length profiles change. The functionally more complex networks correlate with maps of varying lengths while primary networks have more restricted correlations. We posit that these maps are a novel way of differentiating between 'local modules' that have restricted connections to 'neighboring' areas and 'functional integrators' that have more far reaching connectivity. Copyright \copyright 2018. Published by Elsevier Inc.
The temporal lobe has been associated with various cognitive functions which include memory, audi... more The temporal lobe has been associated with various cognitive functions which include memory, auditory cognition and semantics. However, at a higher level of conceptualisation, all of the functions associated with the temporal lobe can be considered as lying along one major axis; from modality-specific to modality-general processing. This paper used a spectral reordering technique on resting-state and task-based functional data to extract the major organisational axis of the temporal lobe in a bottom-up, data-driven fashion. Independent parcellations were performed on resting-state scans from 71 participants and active semantic task scans from 23 participants acquired using dual echo gradient echo planar imaging in order to preserve signal in inferior temporal cortex. The resulting organisational axis was consistent (over dataset and hemisphere) and progressed from superior temporal gyrus and posterior inferior temporal cortex to ventrolateral anterior temporal cortex. A hard parcellation separated a posterior (superior temporal and posterior fusiform and inferior temporal gyri) and an anterior cluster (ventrolateral anterior temporal lobe). The functional connectivity of the hard clusters supported the hypothesis that the connectivity gradient separated modality-specific and modality-general regions. This hypothesis was then directly tested by performing a VOI analysis upon an independent semantic task-based data set including auditory and visually presented stimuli. This confirmed that the ventrolateral anterior aspects of the temporal lobe are associated with modality-general processes whilst posterior and superior aspects are specific to certain modalities, with the posterior inferior subregions involved in visual processes and superior regions involved in audition.
NeuroImage, Apr 2017
The temporal lobe has been implicated in multiple cognitive domains through lesion studies as wel... more The temporal lobe has been implicated in multiple cognitive domains through lesion studies as well as cognitive neuroimaging research. There has been a recent increased interest in the structural and connective architecture that underlies these functions. However there has not yet been a comprehensive exploration of the patterns of connectivity that appear across the temporal lobe. This article uses a data driven, spectral reordering approach in order to understand the general axes of structural connectivity within the temporal lobe.
Two important findings emerge from the study. Firstly, the temporal lobe's overarching patterns of connectivity are organised along two key structural axes: medial to lateral and anteroventral to posterodorsal, mirroring findings in the functional literature. Secondly, the connective organisation of the temporal lobe is graded and transitional; this is reminiscent of the original work of 19th Century neuroanatomists, who posited the existence of some regions which transitioned between one another in a graded fashion. While regions with unique connectivity exist, the boundaries between these are not always sharp. Instead there are zones of graded connectivity reflecting the influence and overlap of shared connectivity.
TARGET AUDIENCE Researchers interested in diffusion-weighted imaging and tractography. INTRODUCTI... more TARGET AUDIENCE Researchers interested in diffusion-weighted imaging and tractography. INTRODUCTION Water protons in cerebral-spinal fluid (CSF) exhibit isotropic diffusion. If CSF is not correctly masked out in magnetic resonance high angular-resolution diffusion images (HARDI) then tractography can generate spurious and random anatomical connections outside of the brain's parenchyma, via voxels containing CSF. This is a particular problem in the cerebral cortex where inefficient masking out of CSF can allow tracks to "jump" across gyri and sulci erroneously. Usually a high-resolution T1 volume is acquired along with HARDI data. The T1 is segmented into different tissue types and the CSF segmentation is co-registered to the diffusion data as a mask. However, the differences in resolution between the T1 and diffusion images and the associated partial volume effects, lead to inaccuracies in masking out CSF in diffusion space and can even cause parts of the brain parenchyma to be masked out. Here, we present a method based on double inversion recovery (DIR) 1,2 to segment and mask out CSF efficiently and improve cortical tractography. METHODS Imaging: HARDI, Proton-density (PD) and DIR data were acquired in a healthy control (M, 28 years of age), on a 3 T Philips Achieva scanner (Philips Healthcare, Best, Netherlands) using a 32-channel head coil. HARDI: PGSE EPI, single shot, TR = 15 s, TE = 59 ms, cardiac gated, halfscan factor = 0.679, 128 × 101 matrix reconstructed to 128 × 128, reconstructed resolution 1.875 × 1.875 mm 2 , slice thickness 2.1 mm, 60 contiguous slices, 43 non-collinear diffusion sensitization directions at b = 1200 s/mm 2 (Δ = 32.0 ms, δ = 12.5 ms), 1 at b = 0, SENSE factor = 2.5, total time = 13 mins, corrected for susceptibility and eddy current-induced distortion 3 . PD: 3D SE, turbo spin echo factor 75, multishot, TR = 8 s, TE = 9 ms, FA = 90°, 128 × 104 matrix reconstructed to 128 × 128, reconstructed resolution 1.875 × 1.875 × 2.1 mm 3 , 60 contiguous slices, SENSE factor = 2.5, total time = 6.7 mins. DIR-CSF: 3D IR, turbo spin echo factor 75, multishot, TR = 8 s, TE = 9 ms, TI = 1850 and 2250 ms to annul signal from grey and white matter, 128 × 104 matrix reconstructed to 128 × 128, reconstructed resolution 1.875 × 1.875 × 2.1 mm 3 , 60 contiguous slices, SENSE factor = 2.5, total time = 6.7 mins. CSF Mapping: PD and DIR-CSF were acquired with the same imaging geometry and were both used to create a normalized probability map of intravoxel CSF, thresholded to produce a binary mask. Co-Alignment: The PD volume was co-aligned with the b = 0 images by implementing 3-parameter rigid body registration in FSL's (http://fsl.fmrib.ox.ac.uk/fsl) 8 FLIRT 4,5 . The transformation matrix obtained from this was applied to the DIR-CSF. Constrained Spherical Deconvolution and Model-Based Residual Bootstrapping: As described in 6,7 with the exception that the dominant diffusion orientation distribution function was generated with 28 spherical harmonics (lmax=6) 8 here. Probabilistic Tractography: The fiber orientations estimated over 32 MBR bootstrap iterations in every voxel of the brain formed a probability density function (PDF) for probabilistic tractography using PICo 9,10,11 , with 10,000 Monte Carlo streamlines. We used a single principle direction with high uncertainty in a voxel if the number of fiber orientations was greater than 3, on any bootstrap iteration. Probabilistic fiber tracking was seeded from a small region on the medial cerebral cortex in the right hemisphere. RESULTS shows orthogonal views of the binarised CSF mask from DIR-CSF. Orthogonal views of the b = 0 volume in greyscale overlaid by the binarised CSF mask from DIR-CSF is shown in , and overlaid with colour-rendered output of probabilistic tractography are shown in . The colour map is scaled from 0.1% to 10% and greater (of 10,000 streamlines). are coaligned. Probabilistic connections generated without a CSF mask is displayed in and with the CSF mask from DIR-CSF ( ) is displayed in . It is clear to observe how spurious and random tracks can easily pass through CSF, giving anatomically implausible connections ( ). However, accurately masking out CSF using DIR-CSF ensures that tractography is constrained within the brain and produce anatomically acceptable connections . DISCUSSION/CONCLUSION The DIR imaging method allows signal to be preserved from just one tissue type in the brain. DIR data are acquired at the same resolution and geometry as HARDI data, and therefore makes DIR an ideal technique to mask out CSF in HARDI data as they are in the same space and the voxels have the same partial volume effects. It is vital to remove CSF-filled voxels from HARDI images before performing tractography in order to reduce false positive connections while enhancing true positive connections. Segmenting tissues (including CSF) on high-resolution T1 volumes of highly-atrophied brains based on a "healthy" template can be problematic. The DIR method would be best for masking out CSF in such brains for subsequent tractography. DIR is able to preserve signal from CSF most successfully as CSF has a relatively consistent T1 across the brain, whereas the T1 values of grey and white matter have greater variation.
Temporal lobe networks are associated with multiple cognitive domains. Despite an upsurge of inte... more Temporal lobe networks are associated with multiple cognitive domains. Despite an upsurge of interest in connectional neuroanatomy, the terminations of the main fibre tracts in the human brain are yet to be mapped. This information is essential given that neurological, neuroanatomical and computational accounts expect neural functions to be strongly shaped by the pattern of white-matter connections. This paper uses a probabilistic tractography approach to identify the main cortical areas that contribute to the major temporal lobe tracts. In order to associate the tract terminations to known functional domains of the temporal lobe, eight automated meta-analyses were performed using the Neurosynth database. Overlaps between the functional regions highlighted by the meta-analyses and the termination maps were identified in order to investigate the functional importance of the tracts of the temporal lobe. The termination maps are made available in the Supplementary Materials of this article for use by researchers in the field.
Cortex, Aug 2015
It is now ten years since a ‘ventral language pathway’ was demonstrated in vivo in the human brai... more It is now ten years since a ‘ventral language pathway’ was demonstrated in vivo in the human brain. In the intervening decade, this result has been replicated and expanded to include multiple possible pathways and functions. Despite this considerable level of research interest, age-old debates regarding the origin, course, termination and, indeed, the very existence of the tracts identified still remain. The current review examines four major tracts associated with the ventral ‘semantic’ language network, with the aim of elucidating and clarifying their structural and functional roles. Historical and modern conceptualisations of the tracts’ neuroanatomical origins and terminations will be discussed, and key discrepancies and debates examined. It is argued that much of the controversy regarding the language pathways has resulted from inconsistencies in terminology, and the lack of a white matter ‘lingua franca’.
The optimism bias is a well-established psychological phenomenon. Its study has implications that... more The optimism bias is a well-established psychological phenomenon. Its study has implications that are far reaching in fields as diverse as mental health and economic theory. With the emerging field of cognitive neuroscience and the advent of advanced neuroimaging techniques, it has been possible to investigate the neural basis of the optimism bias and to understand in which neurological conditions this natural bias fails. This review first defines the optimism bias, discusses its implications and reviews the literature that investigates its neural basis. Finally some potential pitfalls in experimental design are discussed.
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Papers by Claude J Bajada
Two important findings emerge from the study. Firstly, the temporal lobe's overarching patterns of connectivity are organised along two key structural axes: medial to lateral and anteroventral to posterodorsal, mirroring findings in the functional literature. Secondly, the connective organisation of the temporal lobe is graded and transitional; this is reminiscent of the original work of 19th Century neuroanatomists, who posited the existence of some regions which transitioned between one another in a graded fashion. While regions with unique connectivity exist, the boundaries between these are not always sharp. Instead there are zones of graded connectivity reflecting the influence and overlap of shared connectivity.
Two important findings emerge from the study. Firstly, the temporal lobe's overarching patterns of connectivity are organised along two key structural axes: medial to lateral and anteroventral to posterodorsal, mirroring findings in the functional literature. Secondly, the connective organisation of the temporal lobe is graded and transitional; this is reminiscent of the original work of 19th Century neuroanatomists, who posited the existence of some regions which transitioned between one another in a graded fashion. While regions with unique connectivity exist, the boundaries between these are not always sharp. Instead there are zones of graded connectivity reflecting the influence and overlap of shared connectivity.