Abnormal White Matter Blood-Oxygen-Level-Dependent Signals in Chronic Mild Traumatic Brain Injury

Whole-brain, gray, and white matter time-locked functional signal changes with simple tasks and model-free analysis

Recent studies have revealed the production of time-locked blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signals throughout the entire brain in response to tasks, challenging the existence of sparse and localized brain functions and highlighting the pervasiveness of potential false negative fMRI findings. "Whole-brain" actually refers to gray matter, the only tissue traditionally studied with fMRI. However, several reports have demonstrated reliable detection of BOLD signals in white matter, which have previously been largely ignored. Using simple tasks and analyses, we demonstrate BOLD signal changes across the whole brain, in both white and gray matters, in similar manner to previous reports of whole brain studies. We investigated whether white matter displays time-locked BOLD signals across multiple structural pathways in response to a stimulus in a similar manner to the cortex. We find that both white and gray matter show time-locked activations across the whole brain, with a majority of both tissue types showing statistically significant signal changes for all task stimuli investigated. We observed a wide range of signal responses to tasks, with different regions showing different BOLD signal changes to the same task. Moreover, we find that each region may display different BOLD responses to different stimuli. Overall, we present compelling evidence that, just like all gray matter, essentially all white matter in the brain shows time-locked BOLD signal changes in response to multiple stimuli, challenging the idea of sparse functional localization and the prevailing wisdom of treating white matter BOLD signals as artifacts to be removed.

Precision functional MRI mapping reveals distinct connectivity patterns for depression associated with traumatic brain injury

Depression associated with traumatic brain injury (TBI) is believed to be clinically distinct from primary major depressive disorder (MDD) and may be less responsive to conventional treatments. Brain connectivity differences between the dorsal attention network (DAN), default mode network (DMN), and subgenual cingulate have been implicated in TBI and MDD. To characterize these distinctions, we applied precision functional mapping of brain network connectivity to resting-state functional magnetic resonance imaging data from five published patient cohorts, four discovery cohorts (n = 93), and one replication cohort (n = 180). We identified a distinct brain connectivity profile in TBI-associated depression that was independent of TBI, MDD, posttraumatic stress disorder (PTSD), depression severity, and cohort. TBI-associated depression was independently associated with decreased DAN-subgenual cingulate connectivity, increased DAN-DMN connectivity, and the combined effect of both. This effect was stronger when using precision functional mapping relative to group-level network maps. Our results support the possibility of a physiologically distinct "TBI affective syndrome," which may benefit from individualized neuromodulation approaches to target its distinct neural circuitry.

Visuomotor Synchronization: Military Normative Performance

INTRODUCTION

Cognitive processes such as perception and reasoning are preceded and dependent on attention. Because of the close overlap between neural circuits of attention and eye movement, attention may be objectively quantified with recording of eye movements during an attention-dependent task. Our previous work demonstrated that performance scores on a circular visual tracking task that requires dynamic synchronization of the gaze with the target motion can be impacted by concussion, sleep deprivation, and attention deficit/hyperactivity disorder. The current study examined the characteristics of performance on a standardized predictive visual tracking task in a large sample from a U.S. Military population to provide military normative data.

MATERIALS AND METHODS

The sample consisted of 1,594 active duty military service members of either sex aged 18-29 years old who were stationed at Fort Hood Army Base. The protocol was reviewed and approved by the U.S. Army Medical Research and Materiel Command Institutional Review Board. Demographic, medical, and military history data were collected using questionnaires, and performance-based data were collected using a circular visual tracking test and Trail Making Test. Differences in visual tracking performance by demographic characteristics were examined with a multivariate analysis of variance, as well as a Kolmogorov-Smirnov test and a rank-sum test. Associations with other measures were examined with a rank-sum test or Spearman correlations.

RESULTS

Robust sex differences in visual tracking performance were found across the various statistical models, as well as age differences in several isolated comparisons. Accordingly, norms of performance scores, described in terms of percentile standings, were developed adjusting for age and sex. The effects of other measures on visual tracking performance were small or statistically non-significant. An examination of the score distributions of various metrics suggested that strategies preferred by men and women may optimize different aspects of visual tracking performance.

CONCLUSION

This large-scale quantification of attention, using dynamic visuomotor synchronization performance, provides rigorously characterized age- and sex-based military population norms. This study establishes analytics for assessing normal and impaired attention and detecting changes within individuals over time. Practical applications for combat readiness and surveillance of attention impairment from sleep insufficiency, concussion, medication, or attention disorders will be enhanced with portable, easily accessible, fast, and reliable dynamic eye-tracking technologies.

Diffusion-Weighted Imaging in Mild Traumatic Brain Injury: A Systematic Review of the Literature

There is evidence that diffusion-weighted imaging (DWI) is able to detect tissue alterations following mild traumatic brain injury (mTBI) that may not be observed on conventional neuroimaging; however, findings are often inconsistent between studies. This systematic review assesses patterns of differences in DWI metrics between those with and without a history of mTBI. A PubMed literature search was performed using relevant indexing terms for articles published prior to May 14, 2020. Findings were limited to human studies using DWI in mTBI. Articles were excluded if they were not full-length, did not contain original data, if they were case studies, pertained to military populations, had inadequate injury severity classification, or did not report post-injury interval. Findings were reported independently for four subgroups: acute/subacute pediatric mTBI, acute/subacute adult mTBI, chronic adult mTBI, and sport-related concussion, and all DWI acquisition and analysis methods used were included. Patterns of findings between studies were reported, along with strengths and weaknesses of the current state of the literature. Although heterogeneity of sample characteristics and study methods limited the consistency of findings, alterations in DWI metrics were most commonly reported in the corpus callosum, corona radiata, internal capsule, and long association pathways. Many acute/subacute pediatric studies reported higher FA and lower ADC or MD in various regions. In contrast, acute/subacute adult studies most commonly indicate lower FA within the context of higher MD and RD. In the chronic phase of recovery, FA may remain low, possibly indicating overall demyelination or Wallerian degeneration over time. Longitudinal studies, though limited, generally indicate at least a partial normalization of DWI metrics over time, which is often associated with functional improvement. We conclude that DWI is able to detect structural mTBI-related abnormalities that may persist over time, although future DWI research will benefit from larger samples, improved data analysis methods, standardized reporting, and increasing transparency.

Volumetric MRI Findings in Mild Traumatic Brain Injury (mTBI) and Neuropsychological Outcome

Region of interest (ROI) volumetric assessment has become a standard technique in quantitative neuroimaging. ROI volume is thought to represent a coarse proxy for making inferences about the structural integrity of a brain region when compared to normative values representative of a healthy sample, adjusted for age and various demographic factors. This review focuses on structural volumetric analyses that have been performed in the study of neuropathological effects from mild traumatic brain injury (mTBI) in relation to neuropsychological outcome. From a ROI perspective, the probable candidate structures that are most likely affected in mTBI represent the target regions covered in this review. These include the corpus callosum, cingulate, thalamus, pituitary-hypothalamic area, basal ganglia, amygdala, and hippocampus and associated structures including the fornix and mammillary bodies, as well as whole brain and cerebral cortex along with the cerebellum. Ventricular volumetrics are also reviewed as an indirect assessment of parenchymal change in response to injury. This review demonstrates the potential role and limitations of examining structural changes in the ROIs mentioned above in relation to neuropsychological outcome. There is also discussion and review of the role that post-traumatic stress disorder (PTSD) may play in structural outcome in mTBI. As emphasized in the conclusions, structural volumetric findings in mTBI are likely just a single facet of what should be a multimodality approach to image analysis in mTBI, with an emphasis on how the injury damages or disrupts neural network integrity. The review provides an historical context to quantitative neuroimaging in neuropsychology along with commentary about future directions for volumetric neuroimaging research in mTBI.

Eye Tracking Parameters Correlate with the Level of Cerebral Oxygen Saturation in Mild Traumatic Brain Injury: A Preliminary Study

AIM

The aim of this study was to assess the relationship between oculomotor synergies and brain oxygen status at mild traumatic brain injury (mTBI) using simultaneous comparison of eye-tracking (ET) parameters and cerebral oxygen saturation.

MATERIAL AND METHODS

This non-randomised single-centre prospective study included 77 patients with mTBI (mean age was 36.3 ± 4.8 years, 48 men, 29 women, median GCS 13.7 ± 0.7). Cerebral oximetry was used to detect oxygen saturation level (SctO₂) in the frontal lobe pole (FLP) region. Eye movements were measured simultaneously using the EyeTracker. Calculated parameters were: vertical and horizontal angular eyeball velocity (AV); left vertical speed (LVS); right vertical speed (RVS); left horizontal speed (LHS); and right horizontal speed (RHS). The indices of vertical and horizontal eye version (version index, Vx) were calculated as the Pearson correlation coefficient between the corresponding AV of the right and left eyes. Significance was pre-set to p < 0.05.

RESULTS

SctO₂ in the FLP varied from 62% to 79%. The average SctO₂ values were 69.26 ± 6.96% over the left FLP and 70.25 ± 7.58% over the right FLP (p = 0.40). The total analysis of the eye-tracking data revealed the following values of gaze parameters: LVS - 0.327 ± 0.263 rad/sec; LHS - 0.201 ± 0.164 rad/sec; RVS - 0.361 ± 0.269 rad/sec; and RHS - 0.197 ± 0.124 rad/sec. The calculated vertical version index (VVx) was 0.80 ± 0.12. The calculated horizontal version index (HVx) was 0.82 ± 0.11. The VVx and HVx were correlated with SctO₂ levels in the FLP (p = 0.038; r = 0.235; p = 0.048; r = 0.218, respectively p = 0.035; r = 0.241; p = 0.039; r = 0.235, respectively).

CONCLUSIONS

VVx and HVx correlate with the SctO₂ level in the FLP (p < 0.01) in mTBI. No significant correlation was detected between the level of the SctO₂ level and vertical and horizontal AV of the eyeballs. Eye tracking can help quantify the severity of ocular conjugation impairments after mTBI, as well as explore the contribution that cerebral oxygen status disorders make to this process.

A window into eye movement dysfunction following mTBI: A scoping review of magnetic resonance imaging and eye tracking findings

Mild traumatic brain injury (mTBI), commonly known as concussion, is a complex neurobehavioral phenomenon affecting six in 1000 people globally each year. Symptoms last between days and years as microstructural damage to axons and neurometabolic changes result in brain network disruption. There is no clinically available objective biomarker to diagnose the severity of injury or monitor recovery. However, emerging evidence suggests eye movement dysfunction (e.g., saccades and smooth pursuits) in patients with mTBI. Patients with a higher symptom burden and prolonged recovery time following injury may show higher degrees of eye movement dysfunction. Likewise, recent advances in magnetic resonance imaging (MRI) have revealed both white matter tract damage and functional network alterations in mTBI patients, which involve areas responsible for the ocular motor control. This scoping review is presented in three sections: Section 1 explores the anatomical control of eye movements to aid the reader with interpreting the discussion in subsequent sections. Section 2 examines the relationship between abnormal MRI findings and eye tracking after mTBI based on the available evidence. Finally, Section 3 communicates gaps in our knowledge about MRI and eye tracking, which should be addressed in order to substantiate this emerging field.

Neuropathological Mechanisms of Mild Traumatic Brain Injury: A Perspective From Multimodal Magnetic Resonance Imaging

Mild traumatic brain injury (mTBI) accounts for more than 80% of the total number of TBI cases. The mechanism of injury for patients with mTBI has a variety of neuropathological processes. However, the underlying neurophysiological mechanism of the mTBI is unclear, which affects the early diagnosis, treatment decision-making, and prognosis evaluation. More and more multimodal magnetic resonance imaging (MRI) techniques have been applied for the diagnosis of mTBI, such as functional magnetic resonance imaging (fMRI), arterial spin labeling (ASL) perfusion imaging, susceptibility-weighted imaging (SWI), and diffusion MRI (dMRI). Various imaging techniques require to be used in combination with neuroimaging examinations for patients with mTBI. The understanding of the neuropathological mechanism of mTBI has been improved based on different angles. In this review, we have summarized the application of these aforementioned multimodal MRI techniques in mTBI and evaluated its benefits and drawbacks.

Eye Movements in Mild Traumatic Brain Injury: Ocular Biomarkers

Mild traumatic brain injury (mTBI, or concussion), results from direct and indirect trauma to the head (i.e. a closed injury of transmitted forces), with or without loss of consciousness. The current method of diagnosis is largely based on symptom assessment and clinical history. There is an urgent need to identify an objective biomarker which can not only detect injury, but inform prognosis and recovery. Ocular motor impairment is argued to be ubiquitous across mTBI subtypes and may serve as a valuable clinical biomarker with the recent advent of more affordable and portable eye tracking technology. Many groups have positively correlated the degree of ocular motor impairment to symptom severity with a minority attempting to validate these findings with diffusion tract imaging and functional MRI. However, numerous methodological issues limit the interpretation of results, preventing any singular ocular biomarker from prevailing. This review will comprehensively describe the anatomical susceptibility, clinical measurement, and current eye tracking literature surrounding saccades, smooth pursuit, vestibulo-ocular reflex, vergence, pupillary light reflex, and accommodation in mTBI.

Brain Connectivity Changes in Post-Concussion Syndrome as the Neural Substrate of a Heterogeneous Syndrome

BACKGROUND

Post-concussion syndrome (PCS) or persistent symptoms of concussion refers to a constellation of symptoms that persist for weeks and months after a concussion. To better capture the heterogeneity of the symptoms of patients with post-concussion syndrome, we aimed to separate patients into clinical subtypes based on brain connectivity changes.

METHODS

Subject-specific structural and functional connectomes were created based on Diffusion Weighted and Resting State Functional Magnetic Resonance Imaging, respectively. Following an informed dimensionality reduction, a gaussian mixture model was used on patient specific structural and functional connectivity matrices to find potential patient clusters. For validation, the resulting patient subtypes were compared in terms of cognitive, neuropsychiatric, and post-concussive symptom differences.

RESULTS

Multimodal analyses of brain connectivity were predictive of behavioural outcomes. Our modelling revealed 2 patient subtypes; mild and severe. The severe group showed significantly higher levels of depression, anxiety, aggression, and a greater number of symptoms than the mild patient subgroup.

CONCLUSION

This study suggests that structural and functional connectivity changes together can help us better understand the symptom severity and neuropsychiatric profiles of patients with post-concussion syndrome. This work allows us to move towards precision medicine in concussions and provides a novel machine learning approach that can be applicable to other heterogeneous conditions.

Diffusion and functional MRI findings and their relationship to behaviour in postconcussion syndrome: a scoping review

Postconcussion syndrome (PCS) is a term attributed to the constellation of symptoms that fail to recover after a concussion. PCS is associated with a variety of symptoms such as headaches, concentration deficits, fatigue, depression and anxiety that have an enormous impact on patients' lives. There is currently no diagnostic biomarker for PCS. There have been attempts at identifying structural and functional brain changes in patients with PCS, using diffusion tensor imaging (DTI) and functional MRI (fMRI), respectively, and relate them to specific PCS symptoms. In this scoping review, we appraised, synthesised and summarised all empirical studies that (1) investigated structural or functional brain changes in PCS using DTI or fMRI, respectively, and (2) assessed behavioural alterations in patients with PCS. We performed a literature search in MEDLINE (Ovid), Embase (Ovid) and PsycINFO (Ovid) for primary research articles published up to February 2020. We identified 8306 articles and included 45 articles that investigated the relationship between DTI and fMRI parameters and behavioural changes in patients with PCS: 20 diffusion, 20 fMRI studies and 5 papers with both modalities. Most frequently studied structures were the corpus callosum, superior longitudinal fasciculus in diffusion and the dorsolateral prefrontal cortex and default mode network in the fMRI literature. Although some white matter and fMRI changes were correlated with cognitive or neuropsychiatric symptoms, there were no consistent, converging findings on the relationship between neuroimaging abnormalities and behavioural changes which could be largely due to the complex and heterogeneous presentation of PCS. Furthermore, the heterogeneity of symptoms in PCS may preclude discovery of one biomarker for all patients. Further research should take advantage of multimodal neuroimaging to better understand the brain-behaviour relationship, with a focus on individual differences rather than on group comparisons.

Noninvasive magnetic resonance imaging techniques in mild traumatic brain injury research and diagnosis

Mild traumatic brain injury (mTBI), frequently referred to as concussion, is one of the most common neurological disorders. The underlying neural mechanisms of functional disturbances in the brains of concussed individuals remain elusive. Novel forms of brain imaging have been developed to assess patients postconcussion, including functional magnetic resonance imaging (fMRI), susceptibility-weighted imaging (SWI), diffusion MRI (dMRI), and perfusion MRI [arterial spin labeling (ASL)], but results have been mixed with a more common utilization in the research environment and a slower integration into the clinical setting. In this review, the benefits and drawbacks of the methods are described: fMRI is an effective method in the diagnosis of concussion but it is expensive and time-consuming making it difficult for regular use in everyday practice; SWI allows detection of microhemorrhages in acute and chronic phases of concussion; dMRI is primarily used for the detection of white matter abnormalities, especially axonal injury, specific for mTBI; and ASL is an alternative to the BOLD method with its ability to track cerebral blood flow alterations. Thus, the absence of a universal diagnostic neuroimaging method suggests a need for the adoption of a multimodal approach to the neuroimaging of mTBI. Taken together, these methods, with their underlying functional and structural features, can contribute from different angles to a deeper understanding of mTBI mechanisms such that a comprehensive diagnosis of mTBI becomes feasible for the clinician.

Longitudinal fMRI task reveals neural plasticity in default mode network with disrupted executive-default coupling and selective attention after traumatic brain injury

Executive dysfunctions are common in individuals with Traumatic Brain Injury (TBI). However, change in functional neural coupling of default and executive networks in the post-acute phase (≥ 1 month after injury) patients over time has yet to be understood. During a 5-week observation period, we examined changes in the goal-oriented executive function networks in 20 TBI participants, using a face/scene matching 1-back fMRI task (Chen et al. 2011). We conducted multivariate pattern analysis to assess working memory and visual selective attention, followed by a repeat-measures ANOVA to examine longitudinal changes, with a cluster FDR at p = .001. Results showed that task accuracy significantly improved after follow-up. Significantly increased activity patterns over time were observed in the right dorsolateral prefrontal cortex and right insula. Decreased activity patterns were seen in the left posterior cingulate cortex (PCC), bilateral precuneus, right inferior occipital gyrus and right temporo-occipital junction. Improvement in task accuracy correlated with decreased activity patterns in the PCC (r = -0.478, p = 0.031) and temporo-occipital junction (r = -0.592, p = 0.006), which were interpreted as neural plastic changes. However, we did not observe the default mode network (DMN)-executive network decoupling during task performance that is found in other studies. These results suggest that fMRI of attentional task performance could serve as a potential biomarker for neural plasticity of selective attention in TBI patients in the post-acute phase.

Altered Brain Activation in Youth following Concussion: Using a Dual-task Paradigm

A concussion is known as a functional injury affecting brain communication, integration, and processing. There is a need to objectively measure how concussions disrupt brain activation while completing ecologically relevant tasks.The objective of this study was to compare brain activation patterns between concussion and comparison groups (non-concussed youth) during a cognitive-motor single and dual-task paradigm utilizing functional near-infrared spectroscopy (fNIRS) in regions of the frontal-parietal attention network and compared to task performance.Youth with concussion generally exhibited hyperactivation and recruitment of additional brain regions in the dorsal lateral prefrontal (DLPFC), superior (SPC) and inferior parietal cortices (IPC), which are associated with processing, information integration, and response selection. Additionally, hyper- or hypo-activation patterns were associated with slower processing speed on the cognitive task. Our findings corroborate the growing literature suggesting that neural recovery may be delayed compared to the restoration of behavioral performance post-concussion.Concussion, near-infrared spectroscopy, dual-task paradigm, cognitive, motor, brain activation.

White Matter Integrity and Its Relationship to Cognitive-Motor Integration in Females with and without Post-Concussion Syndrome

Fifteen percent of individuals who sustain a concussion go on to develop post-concussion syndrome (PCS). These persistent symptoms are believed to be attributed to damage to white matter tracts and impaired neurotransmission. Specifically, declines in white matter integrity after concussion have been found along the long-coursing axons underlying the frontoparietal network. This network is essential for the performance of visuomotor transformation tasks requiring cognitive-motor integration (CMI). We have previously observed deficits in performance on CMI-based tasks in those who have a history of concussion, but were asymptomatic. The aim of this study was to investigate performance on a CMI task, as well as white matter integrity differences along frontoparietal-cerebellar white matter tracts, in those with PCS compared to healthy controls. We hypothesized an association between the behavioral and brain structural measures. Twenty-six female participants (13 with PCS for ≥6 months and 13 healthy controls) completed four computer-based visuomotor CMI tasks. In addition, diffusion tensor images (DTIs) were acquired. No statistically significant differences were found in CMI performance between groups (p > 0.05). Further, there were no statistically significant differences between groups on any DTI metrics (p > 0.05). However, examination of the data collapsed across participants revealed significant associations between performance on a CMI task and white matter integrity. Further investigation into additional causes of symptoms in those with PCS (including psychological and cervicogenic factors) will strengthen our understanding of this diverse group. Nonetheless, this study demonstrates that white matter integrity is related to levels of performance in tasks that require rule-based movement control.

Concomitant modulation of BOLD responses in white matter pathways and cortex

In response to a flickering visual stimulus, the BOLD response in primary visual cortex varies with the flickering frequency and is maximal when it is close to 8Hz. In previous studies we demonstrated that BOLD signals in specific white matter (WM) pathways covary with the alternations between stimulus conditions in a block design in similar manner to gray matter (GM) regions. Here we investigated whether WM tracts show varying responses to changes in flicker frequency and are modulated in the same manner as cortical areas. We used a Fourier analysis of BOLD signals to measure the signal amplitude and phase at the fundamental frequency of a block-design task in which flickering visual stimuli alternated with blank presentations, avoiding the assumption of any specific hemodynamic response function. The BOLD responses in WM pathways and the primary visual cortex were evaluated for flicker frequencies varying between 2 and 14Hz. The variations with frequency of BOLD signals in specific WM tracts followed closely those in primary visual cortex, suggesting that variations in cortical activation are directly coupled to corresponding BOLD signals in connected WM tracts. Statistically significant differences in the timings of BOLD responses were also measured between visual cortex and specific WM bundles. These results confirm that when cortical BOLD responses are modulated by selecting different task parameters, relevant WM tracts exhibit corresponding BOLD signals that are also affected.

Neuroimaging in the Diagnosis of Chronic Traumatic Encephalopathy: A Systematic Review

OBJECTIVE

Chronic traumatic encephalopathy (CTE) is a neurodegenerative tauopathy associated with repeated subconcussive and concussive head injury. Clinical features include cognitive, behavioral, mood, and motor impairments. Definitive diagnosis is only possible at postmortem. Here, the utility of neuroimaging in the diagnosis of CTE is evaluated by systematically reviewing recent evidence for changes in neuroimaging biomarkers in suspected cases of CTE compared with controls.

DATA SOURCES

Providing an update on a previous systematic review of articles published until December 2014, we searched for articles published between December 2014 and July 2016. We searched PubMed for studies assessing neuroimaging changes in symptomatic suspected cases of CTE with a history of repeated subconcussive or concussive head injury or participation in contact sports involving direct impact to the head. Exclusion criteria were case studies, review articles, and articles focusing on repetitive head trauma from military service, head banging, epilepsy, physical abuse, or animal models.

MAIN RESULTS

Seven articles met the review criteria, almost all of which studied professional athletes. The range of modalities were categorized into structural magnetic resonance imaging (MRI), diffusion MRI, and radionuclide studies. Biomarkers which differed significantly between suspected CTE and controls were Evans index (P = 0.05), cavum septum pellucidum (CSP) rate (P < 0.0006), length (P < 0.03) and ratio of CSP length to septum length (P < 0.03), regional differences in axial diffusivity (P < 0.05) and free/intracellular water fractions (P < 0.005), single-photon emission computed tomography perfusion abnormalities (P < 0.01), positron emission tomography (PET) signals from tau-binding, glucose-binding, and GABA receptor-binding radionuclides (P < 0.0001, P < 0.005, and P < 0.005, respectively). Important limitations include low specificity in identification of suspected cases of CTE across studies, the need for postmortem validation, and a lack of generalizability to nonprofessional athletes.

CONCLUSIONS

The most promising biomarker is tau-binding radionuclide PET signal because it is most specific to the underlying neuropathology and differentiated CTE from both controls and patients with Alzheimer disease (P < 0.0001). Multimodal imaging will improve specificity further. Future research should minimize variability in identification of suspected cases of CTE using published clinical criteria.

Task-related functional magnetic resonance imaging activations in patients with acute and subacute mild traumatic brain injury: A coordinate-based meta-analysis

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ALE meta-analysis revealed functional activation differences in mTBI.

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Reduced activation identified within the right middle frontal gyrus.

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Suggests alteration of prefrontal region, associated with executive functioning.

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Need for addressing subject- and task-specific variation in future studies.

Task-based functional magnetic resonance imaging (fMRI) has been used to examine neuroanatomical and functional changes following mild traumatic brain injury (mTBI). Prior studies have lacked consistency in identifying common regions of altered neural activity during cognitive tasks. This may be partly due to differences in task paradigm, patient heterogeneity, and methods of fMRI analysis. We conducted a meta-analysis using an activation likelihood estimation (ALE) method to identify regions of differential brain activation in patients with mTBI compared to healthy controls. We included experiments that performed scans from acute to subacute time points post-injury. The seven included studies recruited a total sample of 174 patients with mTBIs and 139 control participants. The results of our coordinate based meta-analysis revealed a single cluster of reduced activation within the right middle frontal gyrus (MFG) that differentiated mTBI from healthy controls. We conclude that the cognitive impairments in memory and attention typically reported in mTBI patients may be associated with a deficit in the right MFG, which impacts the recruitment of neural networks important for attentional control.

Activations in gray and white matter are modulated by uni-manual responses during within and inter-hemispheric transfer: effects of response hand and right-handedness

Because the visual cortices are contra-laterally organized, inter-hemispheric transfer tasks have been used to behaviorally probe how information briefly presented to one hemisphere of the visual cortex is integrated with responses resulting from the ipsi- or contra-lateral motor cortex. By forcing rapid information exchange across diverse regions, these tasks robustly activate not only gray matter regions, but also white matter tracts. It is likely that the response hand itself (dominant or non-dominant) modulates gray and white matter activations during within and inter-hemispheric transfer. Yet the role of uni-manual responses and/or right hand dominance in modulating brain activations during such basic tasks is unclear. Here we investigated how uni-manual responses with either hand modulated activations during a basic visuo-motor task (the established Poffenberger paradigm) alternating between inter- and within-hemispheric transfer conditions. In a large sample of strongly right-handed adults (n = 49), we used a factorial combination of transfer condition [Inter vs. Within] and response hand [Dominant(Right) vs. Non-Dominant (Left)] to discover fMRI-based activations in gray matter, and in narrowly defined white matter tracts. These tracts were identified using a priori probabilistic white matter atlases. Uni-manual responses with the right hand strongly modulated activations in gray matter, and notably in white matter. Furthermore, when responding with the left hand, activations during inter-hemispheric transfer were strongly predicted by the degree of right-hand dominance, with increased right-handedness predicting decreased fMRI activation. Finally, increasing age within the middle-aged sample was associated with a decrease in activations. These results provide novel evidence of complex relationships between uni-manual responses in right-handed subjects, and activations during within- and inter-hemispheric transfer suggest that the organization of the motor system exerts sophisticated functional effects. Moreover, our evidence of activation in white matter tracts is consistent with prior studies, confirming fMRI-detectable white matter activations which are systematically modulated by experimental condition.

Functional Magnetic Resonance Imaging and Oculomotor Dysfunction in Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is a significant cause of disability, especially when symptoms become chronic. This chronicity is often linked to oculomotor dysfunction (OMD). To our knowledge, this is the first prospective study to localize aberrations in brain function between mTBI cohorts, by comparing patients with mTBI with OMD with an mTBI control group without OMD, using task and resting-state functional magnetic resonance imaging (fMRI). Ten subjects with mTBI who had OMD (OMD group) were compared with nine subjects with mTBI who had no findings of OMD (control group). These groups were determined by a developmental optometrist using objective testing for OMD. The (convergence) task fMRI data demonstrated significantly decreased brain activity, measured as decreases in the blood oxygen level dependent (BOLD) signal, in the OMD group compared with the control group in three brain regions: the left posterior lingual gyrus, the bilateral anterior lingual gyrus and cuneus, and the parahippocampal gyrus. When doing a seed-based resting state fMRI analysis in the lingual/parahippocampal region, a large cluster covering the left middle frontal gyrus and the dorsolateral pre-frontal cortex (Brodmann areas 9 and 10), with decreased functional correlation in the OMD group, was identified. Together these observations provide evidence for neural networks of interactions involving the control of eye movement for visual processing, reading comprehension, spatial localization and navigation, and spatial working memory that appear to be decreased in mTBI patients with OMD compared with mTBI patients without OMD. The clinical symptomatology associated with post-traumatic OMD correlates well with these MRI findings.

Association of Visual Tracking Metrics With Post-concussion Symptomatology

Attention impairment may provide a cohesive neurobiological explanation for clusters of clinical symptoms that occur after a concussion; therefore, objective quantification of attention is needed. Visually tracking a moving target is an attention-dependent sensorimotor function, and eye movement can be recorded easily and objectively to quantify performance. Our previous work suggested the utility of gaze-target synchronization metrics of a predictive visual tracking task in concussion screening and recovery monitoring. Another objectively quantifiable performance measure frequently suggested for concussion screening is simple visuo-manual reaction time (simple reaction time, SRT). Here, we used visual tracking and SRT tasks to assess changes between pre- and within-2-week post-concussion performances and explore their relationships to post-concussion symptomatology. Athletes participating in organized competitive sports were recruited. Visual tracking and SRT records were collected from the recruited athlete pool as baseline measures over a 4-year period. When athletes experienced a concussion, they were re-assessed within 2 weeks of their injury. We present the data from a total of 29 concussed athletes. Post-concussion symptom burden was assessed with the Rivermead Post-Concussion Symptoms Questionnaire and subscales of the Brain Injury Screening Questionnaire. Post-concussion changes in visual tracking and SRT performance were examined using a paired t-test. Correlations of changes in visual tracking and SRT performance to symptom burden were examined using Pearson's coefficients. Post-concussion changes in visual tracking performance were not consistent among the athletes. However, changes in several visual tracking metrics had moderate to strong correlations to symptom scales (r up to 0.68). On the other hand, while post-concussion SRT performance was reduced (p < 0.01), the changes in the performance metrics were not meaningfully correlated to symptomatology (r ≤ 0.33). Results suggest that visual tracking performance metrics reflect clinical symptoms when assessed within 2 weeks of concussion. Evaluation of concussion requires assessments in multiple domains because the clinical profiles are heterogeneous. While most individuals show recovery within a week of injury, others experience prolonged recovery periods. Visual tracking performance metrics may serve as a biomarker of debilitating symptoms of concussion implicating attention as a root cause of such pathologies.

Assessment of Oculomotor Function in Patients With Postconcussion Syndrome: A Systematic Review

BACKGROUND

Concussion and its associated sequel, postconcussion syndrome (PCS), have a debilitating impact on the lives of concussed patients. However, a diagnostic biomarker for this condition is lacking. Recently, there has been a surge of interest in using oculomotor function testing as an objective assessment of patients with PCS.

OBJECTIVES

To systematically synthesize, appraise, and summarize all published empirical studies that have assessed alteration of oculomotor functions in patients with PCS.

METHODS

Medline, Embase, PsychINFO, and CINAHL databases searched up to July 2016 for studies that used oculomotor function assessment in patients with postconcussion symptoms.

RESULTS

The search identified 1637 citations, and finally 8 case-control studies were included. Of these, 5 studies used a similar task with a target moving in a circular trajectory. Three other studies measured conventional oculomotor tasks such as saccade, vergence, and smooth pursuit eye movements.

CONCLUSIONS

Currently, there is limited support for the recommendation of oculomotor function assessments for diagnosis and identification of patients with PCS following head trauma. Therefore, more rigorous studies assessing oculomotor function changes in patients with PCS are warranted.

Effects of Tissue-Specific Functional Magnetic Resonance Imaging Signal Regression on Resting-State Functional Connectivity

Neuroimaging studies typically consider white matter as unchanging in different neural and metabolic states. However, a recent study demonstrated that white matter signal regression (WMSR) produced a similar loss of neurometabolic information to global (whole-brain) signal regression (GSR) in resting-state functional magnetic resonance imaging (R-fMRI) data. This was unexpected as the loss of information would normally be attributed to neural activity within gray matter correlating with the global R-fMRI signal. Indeed, WMSR has been suggested as an alternative to avoid such pitfalls in GSR. To address these concerns about tissue-specific regression in R-fMRI data analysis, we performed GSR, WMSR, and gray matter signal regression (GMSR) on R-fMRI data from the 1000 Functional Connectomes Project. We describe several regional and motion-related differences between different types of regressions. However, the overall effects of concern, particularly network-specific alteration of correlation coefficients, are present for all regressions. This suggests that tissue-specific regression is not an adequate strategy to counter pitfalls of GSR. Conversely, if GSR is desired, but the studied disease state excludes either gray matter or white matter from analysis (e.g., due to tissue atrophy), our results indicate that WMSR or GMSR may reproduce the gross effects of GSR.

Traumatic Brain Injury and Alzheimer's Disease: The Cerebrovascular Link

Traumatic brain injury (TBI) and Alzheimer's disease (AD) are devastating neurological disorders, whose complex relationship is not completely understood. Cerebrovascular pathology, a key element in both conditions, could represent a mechanistic link between Aβ/tau deposition after TBI and the development of post concussive syndrome, dementia and chronic traumatic encephalopathy (CTE). In addition to debilitating acute effects, TBI-induced neurovascular injuries accelerate amyloid β (Aβ) production and perivascular accumulation, arterial stiffness, tau hyperphosphorylation and tau/Aβ-induced blood brain barrier damage, giving rise to a deleterious feed-forward loop. We postulate that TBI can initiate cerebrovascular pathology, which is causally involved in the development of multiple forms of neurodegeneration including AD-like dementias. In this review, we will explore how novel biomarkers, animal and human studies with a focus on cerebrovascular dysfunction are contributing to the understanding of the consequences of TBI on the development of AD-like pathology.

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Cerebrovascular dysfunction (CVD) is emerging as a key element in the development of neurodegeneration after TBI.

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We propose that TBI initiates CVD, accelerating Aβ/tau deposition and leading to neurodegeneration and dementias.

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Clarifying this connection will support the development of novel biomarkers and therapeutic approaches for both TBI and AD.

Feasibility of using normobaric hypoxic stress in mTBI research

Studies of mild traumatic brain injury (mTBI) recovery generally assess patients in unstressed conditions that permit compensation for impairments through increased effort expenditure. This possibility may explain why a subgroup of individuals report persistent mTBI symptoms yet perform normally on objective assessment. Accordingly, the development and utilization of stress paradigms may be effective for enhancing the sensitivity of mTBI assessment. Previous studies, discussed here, indirectly but plausibly support the use of normobaric hypoxia as a stressor in uncovering latent mTBI symptoms due to the overlapping symptomatology induced by both normobaric hypoxia and mTBI. Limited studies by our group and others further support this plausibility through proof-of-concept demonstrations that hypoxia reversibly induces disproportionately severe impairments of oculomotor, pupillometric, cognitive and autonomic function in mTBI individuals.

Functional connectivity and activity of white matter in somatosensory pathways under tactile stimulations

Functional MRI has proven to be effective in detecting neural activity in brain cortices on the basis of blood oxygenation level dependent (BOLD) contrast, but has relatively poor sensitivity for detecting neural activity in white matter. To demonstrate that BOLD signals in white matter are detectable and contain information on neural activity, we stimulated the somatosensory system and examined distributions of BOLD signals in related white matter pathways. The temporal correlation profiles and frequency contents of BOLD signals were compared between stimulation and resting conditions, and between relevant white matter fibers and background regions, as well as between left and right side stimulations. Quantitative analyses show that, overall, MR signals from white matter fiber bundles in the somatosensory system exhibited significantly greater temporal correlations with the primary sensory cortex and greater signal power during tactile stimulations than in a resting state, and were stronger than corresponding measurements for background white matter both during stimulations and in a resting state. The temporal correlation and signal power under stimulation were found to be twice those observed from the same bundle in a resting state, and bore clear relations with the side of stimuli. These indicate that BOLD signals in white matter fibers encode neural activity related to their functional roles connecting cortical volumes, which are detectable with appropriate methods.

Pathological vascular and inflammatory biomarkers of acute- and chronic-phase traumatic brain injury

Given the demand for developing objective methods for characterizing traumatic brain injury (TBI), research dedicated to evaluating putative biomarkers has burgeoned over the past decade. Since it is critical to elucidate the underlying pathological processes that underlie the higher diverse outcomes that follow neurotrauma, considerable efforts have been aimed at identifying biomarkers of both the acute- and chronic-phase TBI. Such information is not only critical for helping to elucidate the pathological changes that lead to poor long-term outcomes following TBI but it may also assist in the identification of possible prevention and interventions for individuals who sustain head trauma. In the current review, we discuss the potential role of vascular dysfunction and chronic inflammation in both acute- and chronic-phase TBI, and we also highlight existing studies that have investigated inflammation biomarkers associated with poorer injury outcome.

Reliability of the depth-dependent high-resolution BOLD hemodynamic response in human visual cortex and vicinity

Functional magnetic resonance imaging (fMRI) often relies on a hemodynamic response function (HRF), the stereotypical blood oxygen level dependent (BOLD) response elicited by a brief (<4s) stimulus. Early measurements of the HRF used coarse spatial resolutions (≥3mm voxels) that would generally include contributions from white matter, gray matter, and the extra-pial compartment (the space between the pial surface and skull including pial blood vessels) within each voxel. To resolve these contributions, high-resolution fMRI (0.9-mm voxels) was performed at 3T in early visual cortex and its apposed white-matter and extra-pial compartments. The results characterized the depth dependence of the HRF and its reliability during nine fMRI sessions. Significant HRFs were observed in white-matter and extra-pial compartments as well as in gray matter. White-matter HRFs were faster and weaker than in the gray matter, while extra-pial HRFs were comparatively slower and stronger. Depth trends of the HRF peak amplitude were stable throughout a broad depth range that included all three compartments for each session. Across sessions, however, the depth trend of HRF peak amplitudes was stable only in the white matter and deep-intermediate gray matter, while there were strong session-to-session variations in the superficial gray matter and the extra-pial compartment. Thus, high-resolution fMRI can resolve significant and dynamically distinct HRFs in gray matter, white matter, and extra-pial compartments.

The relationship between BOLD fMRI response and the underlying white matter as measured by fractional anisotropy (FA): A systematic review

Despite the relationship between brain structure and function being of fundamental interest in cognitive neuroscience, the relationship between the brain's white matter, measured using fractional anisotropy (FA), and the functional magnetic resonance imaging (fMRI) blood oxygen level dependent (BOLD) response is poorly understood. A systematic review of literature investigating the association between FA and fMRI BOLD response was conducted following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. The PubMed and Web of Knowledge databases were searched up until 22.04.2016 using a predetermined set of search criteria. The search identified 363 papers, 28 of which met the specified inclusion criteria. Positive relationships were mainly observed in studies investigating the primary sensory and motor systems and in resting state data. Both positive and negative relationships were seen in studies using cognitive tasks. This systematic review suggests that there is a relationship between FA and the fMRI BOLD response and that the relationship is task and region dependent. Behavioural and/or clinical variables were shown to be essential in interpreting the relationships between imaging measures. The results highlight the heterogeneity in the methods used across papers in terms of fMRI task, population investigated and data analysis techniques. Further investigation and replication of current findings are required before definitive conclusions can be drawn.

Mapping white-matter functional organization at rest and during naturalistic visual perception

Despite the wide applications of functional magnetic resonance imaging (fMRI) to mapping brain activation and connectivity in cortical gray matter, it has rarely been utilized to study white-matter functions. In this study, we investigated the spatiotemporal characteristics of fMRI data within the white matter acquired from humans both in the resting state and while watching a naturalistic movie. By using independent component analysis and hierarchical clustering, resting-state fMRI data in the white matter were de-noised and decomposed into spatially independent components, which were further assembled into hierarchically organized axonal fiber bundles. Interestingly, such components were partly reorganized during natural vision. Relative to resting state, the visual task specifically induced a stronger degree of temporal coherence within the optic radiations, as well as significant correlations between the optic radiations and multiple cortical visual networks. Therefore, fMRI contains rich functional information about the activity and connectivity within white matter at rest and during tasks, challenging the conventional practice of taking white-matter signals as noise or artifacts.

Effects of anesthesia on resting state BOLD signals in white matter of non-human primates

Resting state functional magnetic resonance imaging (rsfMRI) has been widely used to measure functional connectivity between cortical regions of the brain. However, there have been minimal reports of bold oxygenation level dependent (BOLD) signals in white matter, and even fewer attempts to detect resting state connectivity. Recently, there has been growing evidence that suggests that reliable detection of white matter BOLD signals may be possible. We have previously shown that nearest neighbor inter-voxel correlations of resting state BOLD signal fluctuations in white matter are anisotropic and can be represented by a functional correlation tensor, but the biophysical origins of these signal variations are not clear. We aimed to assess whether MRI signal fluctuations in white matter vary for different baseline levels of neural activity. We performed imaging studies on live squirrel monkeys under different levels of isoflurane anesthesia at 9.4T. We found 1) the fractional power (0.01-0.08Hz) in white matter was between 60 to 75% of the level in gray matter; 2) the power in both gray and white matter low frequencies decreased monotonically in similar manner with increasing levels of anesthesia; 3) the distribution of fractional anisotropy values of the functional tensors in white matter were significantly higher than those in gray matter; and 4) the functional tensor eigenvalues decreased with increasing level of anesthesia. Our results suggest that as anesthesia level changes baseline neural activity, white matter signal fluctuations behave similarly to those in gray matter, and functional tensors in white matter are affected in parallel.

Assessing behavioral syndromes in patients with brain tumors using the frontal systems behavior scale (FrSBe)

BACKGROUND

Personality changes following brain tumors may be due to disruption of frontal-subcortical networks. The relation between personality changes and tumor parameters such as volumes of the surgical cavity, residual tumor, or nonspecific white matter abnormalities is unknown. In this study we examined the relation between these tumor parameters and abnormal behaviors typically associated with frontal lobe dysfunction.

METHODS

Thirty-one patients with intracranial tumors who completed the Frontal Systems Behavior Scale (FrSBe) during clinical neuropsychological assessment and had a solitary, well-delimited brain lesion on MRI within 3 months of that assessment were included. Tumor parameters were manually segmented using OsiriX. Nonparametric statistics were used to determine the relationship between tumor parameters and frontal behavioral dysfunction as measured by FrSBe scores.

RESULTS

Patients reported significantly more behavior problems after tumor diagnosis. Tumor cavity volume was correlated with self-reported Executive Dysfunction (rho = 0.450, P = .047), and there was a trend in the relationship with self-reported Apathy (rho = 0.438, P = .053). Nonspecific white matter abnormality volume was also correlated with self-reported Apathy (rho = 0.810, P = .01). There were no correlations between FrSBe scores and residual tumor volume or summed volumes of tumor-related parameters.

CONCLUSION

Our results suggest that tumor parameters have differential effects on behaviors associated with frontal-subcortical networks and corroborate the high frequency of behavioral dysfunction in brain tumor patients. Examination of these relationships in a prospective trial is warranted to establish incidence, prevalence, risk factors, and consequences of behavioral disturbances in brain tumor patients.

Sports-related brain injuries: connecting pathology to diagnosis

Brain injuries are becoming increasingly common in athletes and represent an important diagnostic challenge. Early detection and management of brain injuries in sports are of utmost importance in preventing chronic neurological and psychiatric decline. These types of injuries incurred during sports are referred to as mild traumatic brain injuries, which represent a heterogeneous spectrum of disease. The most dramatic manifestation of chronic mild traumatic brain injuries is termed chronic traumatic encephalopathy, which is associated with profound neuropsychiatric deficits. Because chronic traumatic encephalopathy can only be diagnosed by postmortem examination, new diagnostic methodologies are needed for early detection and amelioration of disease burden. This review examines the pathology driving changes in athletes participating in high-impact sports and how this understanding can lead to innovations in neuroimaging and biomarker discovery.

Chronic Post-Concussion Neurocognitive Deficits. II. Relationship with Persistent Symptoms

Individuals who sustain a concussion may continue to experience problems long after their injury. However, it has been postulated in the literature that the relationship between a concussive injury and persistent complaints attributed to it is mediated largely by the development of symptoms associated with posttraumatic stress disorder (PTSD) and depression. We sought to characterize cognitive deficits of adult patients who had persistent symptoms after a concussion and determine whether the original injury retains associations with these deficits after accounting for the developed symptoms that overlap with PTSD and depression. We compared the results of neurocognitive testing from 33 patients of both genders aged 18-55 at 3 months to 5 years post-injury with those from 140 control subjects. Statistical comparisons revealed that patients generally produced accurate responses on reaction time-based tests, but with reduced efficiency. On visual tracking, patients increased gaze position error variability following an attention demanding task, an effect that may reflect greater fatigability. When neurocognitive performance was examined in the context of demographic- and symptom-related variables, the original injury retained associations with reduced performance at a statistically significant level. For some patients, reduced cognitive efficiency and fatigability may represent key elements of interference when interacting with the environment, leading to varied paths of recovery after a concussion. Poor recovery may be better understood when these deficits are taken into consideration.

Chronic Post-Concussion Neurocognitive Deficits. I. Relationship with White Matter Integrity

We previously identified visual tracking deficits and associated degradation of integrity in specific white matter tracts as characteristics of concussion. We re-explored these characteristics in adult patients with persistent post-concussive symptoms using independent new data acquired during 2009–2012. Thirty-two patients and 126 normal controls underwent cognitive assessments and MR-DTI. After data collection, a subset of control subjects was selected to be individually paired with patients based on gender and age. We identified patients’ cognitive deficits through pairwise comparisons between patients and matched control subjects. Within the remaining 94 normal subjects, we identified white matter tracts whose integrity correlated with metrics that indicated performance degradation in patients. We then tested for reduced integrity in these white matter tracts in patients relative to matched controls. Most patients showed no abnormality in MR images unlike the previous study. Patients’ visual tracking was generally normal. Patients’ response times in an attention task were slowed, but could not be explained as reduced integrity of white matter tracts relating to normal response timing. In the present patient cohort, we did not observe behavioral or anatomical deficits that we previously identified as characteristic of concussion. The recent cohort likely represented those with milder injury compared to the earlier cohort. The discrepancy may be explained by a change in the patient recruitment pool circa 2007 associated with an increase in public awareness of concussion.

Exploring the physiological correlates of chronic mild traumatic brain injury symptoms

We report on the results of a multimodal imaging study involving behavioral assessments, evoked and resting-state BOLD fMRI, and DTI in chronic mTBI subjects. We found that larger task-evoked BOLD activity in the MT+/LO region in extra-striate visual cortex correlated with mTBI and PTSD symptoms, especially light sensitivity. Moreover, higher FA values near the left optic radiation (OR) were associated with both light sensitivity and higher BOLD activity in the MT+/LO region. The MT+/LO region was localized as a region of abnormal functional connectivity with central white matter regions previously found to have abnormal physiological signals during visual eye movement tracking (Astafiev et al., 2015). We conclude that mTBI symptoms and light sensitivity may be related to excessive responsiveness of visual cortex to sensory stimuli. This abnormal sensitivity may be related to chronic remodeling of white matter visual pathways acutely injured.

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We report a multimodal imaging study of symptoms in chronic mTBI.

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Higher BOLD activity in the MT +/LO region correlated with mTBI symptoms.

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Higher FA near the left optic radiation was associated with light sensitivity.