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1.  Neuromarkers of the common angiotensinogen polymorphism in healthy older adults: A comprehensive assessment of white matter integrity and cognition 
Behavioural brain research  2015;296:85-93.
The common angiotensinogen (AGT) M268T polymorphism (rs699; historically referred to as M235T) has been identified as a significant risk factor for cerebrovascular pathologies, yet it is unclear if healthy older adults carrying the threonine amino acid variant have a greater risk for white matter damage in specific fiber tracts. Further, the impact of the threonine variant on cognitive function remains unknown. The present study utilized multiple indices of diffusion tensor imaging (DTI) and neuropsychological assessment to examine the integrity of specific white matter tracts and cognition between individuals with homozygous genotypes of M268T (MetMet n = 27, ThrThr n = 27). Differences in subcortical hyperintensity (SH) volume were also examined between groups. Results indicated that the threonine variant was associated with significantly reduced integrity in the superior longitudinal fasciculus (SLF) and the cingulate gyrus segment of the cingulum bundle (cingulum CG) compared to those with the methionine variant, and poorer cognitive performance on tests of attention/processing speed and language. Despite these associations, integrity of these tracts did not significantly mediate relationships between cognition and genetic status, and SH did not differ significantly between groups. Collectively our results suggest that the threonine variant ofM268T is a significant risk factor for abnormalities in specific white matter tracts and cognitive domains in healthy older adults, independent of SH burden.
doi:10.1016/j.bbr.2015.08.028
PMCID: PMC4692153  PMID: 26318936
AGT; M235T; M268T; rs699; white matter; cognition
2.  Fiber bundle length and cognition: A length-based tractography MRI study 
Brain imaging and behavior  2015;9(4):765-775.
Executive function (EF) and cognitive processing speed (CPS) are two cognitive performance domains that decline with advanced age. Reduced EF and CPS are known to correlate with age-related frontal-lobe volume loss. However, it remains unclear whether white matter microstructure in these regions is associated with age-related decline in EF and/or CPS. We utilized quantitative tractography metrics derived from diffusion-tensor MRI to investigate the relationship between the mean fiber bundle lengths (FBLs) projecting to different lobes, and EF/CPS performance in 73 healthy aging adults. We measured aspects of EF and CPS with the Trail Making Test (TMT), Color-Word Interference Test, Letter-Number Sequencing (L-N Seq), and Symbol Coding. Results revealed that parietal and occipital FBLs explained a significant portion of variance in EF. Frontal, temporal and occipital FBLs explained a significant portion of variance in CPS. Shorter occipital FBLs were associated with poorer performance on the EF tests TMT-B and CWIT 3. Shorter frontal, parietal, and occipital FBLs were associated with poorer performance on L-N Seq and Symbol Coding. Shorter frontal and temporal FBLs were associated with lower performance on CPS tests TMT-A and CWIT 1. Shorter FBLs were also associated with increased age. Results suggest an age-related FBL shortening in specific brain regions related to poorer EF and CPS performance among older adults. Overall, results support both the frontal aging hypothesis and processing speed theory, suggesting that each mechanism is contributing to age-related cognitive decline.
doi:10.1007/s11682-014-9334-8
PMCID: PMC4424188  PMID: 25376332
Fiber Bundle Lengths; DTI; White Matter; Cognitive Processing Speed; Executive Function; Aging
3.  Prevalence of non-febrile seizures in children with idiopathic autism spectrum disorder and their unaffected siblings: a retrospective cohort study 
BMC Neurology  2016;16:245.
Background
Autism spectrum disorder (ASD) is a heterogeneous disorder characterized not only by deficits in communication and social interactions but also a high rate of co-occurring disorders, including metabolic abnormalities, gastrointestinal and sleep disorders, and seizures. Seizures, when present, interfere with cognitive development and are associated with a higher mortality rate in the ASD population.
Methods
To determine the relative prevalence of non-febrile seizures in children with idiopathic ASD from multiplex and simplex families compared with the unaffected siblings in a cohort of 610 children with idiopathic ASD and their 160 unaffected siblings, participating in the Autism Genetic Resource Exchange project, the secondary analysis was performed comparing the life-time prevalence of non-febrile seizures. Statistical models to account for non-independence of observations, inherent with the data from multiplex families, were used in assessing potential confounding effects of age, gender, and history of febrile seizures on odds of having non-febrile seizures.
Results
The life-time prevalence of non-febrile seizures was 8.2% among children with ASD and 2.5% among their unaffected siblings. In a logistic regression analysis that adjusted for familial clustering, children with ASD had 5.27 (95%CI: 1.51–18.35) times higher odds of having non-febrile seizures compared to their unaffected siblings. In this comparison, age, presence of gastrointestinal dysfunction, and history of febrile seizures were significantly associated with the prevalence of non-febrile seizures.
Conclusion
Children with idiopathic ASD are significantly more likely to have non-febrile seizures than their unaffected siblings, suggesting that non-febrile seizures may be ASD-specific. Further studies are needed to determine modifiable risk factors for non-febrile seizures in ASD.
doi:10.1186/s12883-016-0764-3
PMCID: PMC5126876  PMID: 27894273
Autism; Idiopathic; Seizures; Epilepsy; Genetic; Prevalence; Familial; Siblings; AGRE
4.  Genetic markers of cholesterol transport and gray matter diffusion: A preliminary study of the CETP I405V polymorphism 
Variations of the cholesteryl ester transfer protein polymorphism (CETP I405V/rs5882) have been associated with an increased risk for neurodegeneration, particularly when examined in conjunction with the epsilon 4 isoform of apolipoprotein E (ApoE4). Despite these identified relationships, the impact of I405V on gray matter microstructure remains unknown. The present study examined the impact of the CETP I405V polymorphism on gray matter integrity among 52 healthy adults between ages 51–85. Gray matter was measured bilaterally using diffusion tensor imaging (DTI) metrics of fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD). Participants were grouped according to a dominant statistical model (II genotype vs. IV/VV genotypes) and secondary analyses were completed to examine the interactive effects of CETP and ApoE4 on DTI metrics. Compared to individuals with the IV/VV genotypes, II homozygotes demonstrated significantly higher MD in bilateral temporal, parietal, and occipital gray matter. Secondary analyses revealed higher FA and AD in the left temporal lobe of IV/VV genotypes with an ApoE4 allele. Our results provide preliminary evidence that CETP II homozygosity is a predisposing risk factor for gray matter abnormalities in posterior brain regions in healthy older adults, independent of an ApoE4 allele.
doi:10.1007/s00702-015-1434-0
PMCID: PMC4618053  PMID: 26253899
CETP; APOE; Gray Matter; DTI
5.  REDUCING CSF PARTIAL VOLUME EFFECTS TO ENHANCE DIFFUSION TENSOR IMAGING METRICS OF BRAIN MICROSTRUCTURE 
Technology and innovation  2016;18(1):5-20.
Technological advances over recent decades now allow for in vivo observation of human brain tissue through the use of neuroimaging methods. While this field originated with techniques capable of capturing macrostructural details of brain anatomy, modern methods such as diffusion tensor imaging (DTI) that are now regularly implemented in research protocols have the ability to characterize brain microstructure. DTI has been used to reveal subtle micro-anatomical abnormalities in the prodromal phase ofº various diseases and also to delineate “normal” age-related changes in brain tissue across the lifespan. Nevertheless, imaging artifact in DTI remains a significant limitation for identifying true neural signatures of disease and brain-behavior relationships. Cerebrospinal fluid (CSF) contamination of brain voxels is a main source of error on DTI scans that causes partial volume effects and reduces the accuracy of tissue characterization. Several methods have been proposed to correct for CSF artifact though many of these methods introduce new limitations that may preclude certain applications. The purpose of this review is to discuss the complexity of signal acquisition as it relates to CSF artifact on DTI scans and review methods of CSF suppression in DTI. We will then discuss a technique that has been recently shown to effectively suppress the CSF signal in DTI data, resulting in fewer errors and improved measurement of brain tissue. This approach and related techniques have the potential to significantly improve our understanding of “normal” brain aging and neuropsychiatric and neurodegenerative diseases. Considerations for next-level applications are discussed.
doi:10.21300/18.1.2016.5
PMCID: PMC5054978  PMID: 27721931
MRI; DTI; Partial volume effects; CSF suppression
6.  Brain structure and cognitive correlates of body mass index in healthy older adults 
Behavioural brain research  2014;278:342-347.
Obesity, commonly measured with body mass index (BMI), is associated with numerous deleterious health conditions including alterations in brain integrity related to advanced age. Prior research has suggested that white matter integrity observed using diffusion tensor imaging (DTI) is altered in relation to high BMI, but the integrity of specific white matter tracts remains poorly understood. Additionally, no studies have examined white matter tract integrity in conjunction with neuropsychological evaluation associated with BMI among older adults. The present study examined white matter tract integrity using DTI and cognitive performance associated with BMI in 62 healthy older adults (20 males, 42 females) aged 51 to 81. Results revealed that elevated BMI was associated with lower fractional anisotropy (FA) in the uncinate fasciculus, though there was no evidence of an age by BMI interaction relating to FA in this tract. No relationships were observed between BMI and other white matter tracts or cognition after controlling for demographic variables. Findings suggest that elevated BMI is associated with lower structural integrity in a brain region connecting frontal and temporal lobes and this alteration precedes cognitive dysfunction. Future studies should examine biological mechanisms that mediate the relationships between BMI and white matter tract integrity, as well as the evolution of these abnormalities utilizing longitudinal designs.
doi:10.1016/j.bbr.2014.10.010
PMCID: PMC4382378  PMID: 25448431
BMI; Tractography; DTI; Cognition; White Matter; Aging
7.  Posterior brain white matter abnormalities in older adults with probable mild cognitive impairment 
Objective
Much of the mild cognitive impairment (MCI) neuroimaging literature has exclusively focused on regions associated with Alzheimer’s disease. Little research has examined white matter abnormalities of other brain regions, including those associated with visual processing, despite evidence that other brain abnormalities appear in these regions in early disease stages.
Method
Diffusion tensor imaging (DTI) was utilized to examine participants (n = 44) that completed baseline imaging as part of a longitudinal healthy aging study. Participants were divided into two groups based on scores from the Montreal Cognitive Assessment (MoCA), a brief screening tool for MCI. Participants who scored < 26 were defined as “probable MCI” while those who scored ≥ 26 were labled cognitively healthy. Two DTI indices were analyzed including fractional anisotropy (FA) and mean diffusivity (MD). DTI values for white matter in the lingual gyrus, cuneus, pericalcarine, fusiform gyrus and all four lobes were compared using MANOVA. Regression analyses examined the relationship between DTI indices and total MoCA score.
Results
Results revealed significantly lower FA in the probable MCI group in the cuneus, fusiform, pericalcarine and occipital lobe, and significantly higher MD in the temporal lobe. Fusiform FA and temporal lobe MD were significantly related to total MoCA score after accounting for age and education.
Conclusions
Results indicate that there are posterior white matter microstructural changes in individuals with probable MCI. These differences demonstrate that white matter abnormalities are evident among individuals with probable MCI in regions beyond those commonly associated with Alzheimer’s disease and anterior brain aging patterns.
doi:10.1080/13803395.2014.985636
PMCID: PMC4355053  PMID: 25523313
8.  Impact of the AGTR1 A1166C polymorphism on subcortical hyperintensities and cognition in healthy older adults 
Age  2014;36(4):9664.
Vascular aging consists of complex and multifaceted processes that may be influenced by genetic polymorphisms of the renin-angiotensin system. A polymorphism in the angiotensin II type 1 receptor gene (AGTR1/rs5186) has been associated with an increased risk for arterial stiffness, hypertension, and ischemic stroke. Despite these identified relationships, the impact of AGTR1 A1166C on white matter integrity and cognition is less clear in a healthy aging population. The present study utilized indices of neuroimaging and neuropsychological assessment to examine the impact of the A1166C polymorphism on subcortical hyperintensities (SH) and cognition in 49 healthy adults between ages 51–85. Using a dominant statistical model (CC + CA (risk) vs. AA), results revealed significantly larger SH volume for individuals with the C1166 variant (p < 0.05, partial eta2 = 0.117) compared with those with the AA genotype. Post hoc analyses indicated that increased SH volume in C allele carriers could not be explained by vascular factors such as pulse pressure or body mass index. In addition, cognitive performance did not differ significantly between groups and was not significantly associated with SH in this cohort. Results suggest that presence of the C1166 variant may serve as a biomarker of risk for suboptimal brain integrity in otherwise healthy older adults prior to changes in cognition.
doi:10.1007/s11357-014-9664-x
PMCID: PMC4150909  PMID: 24981111
AGTR1; A1166C; Cerebrovascular aging; Subcortical hyperintensities; Cognition
9.  White matter changes with age utilizing quantitative diffusion MRI 
Neurology  2014;83(3):247-252.
Objective:
To investigate the relationship between older age and mean cerebral white matter fiber bundle lengths (FBLs) in specific white matter tracts in the brain using quantified diffusion MRI.
Methods:
Sixty-three healthy adults older than 50 years underwent diffusion tensor imaging. Tractography tracings of cerebral white matter fiber bundles were derived from the diffusion tensor imaging data.
Results:
Results revealed significantly shorter FBLs in the anterior thalamic radiation for every 1-year increase over the age of 50 years.
Conclusions:
We investigated the effects of age on FBL in specific white matter tracts in the brains of healthy older individuals utilizing quantified diffusion MRI. The results revealed a significant inverse relationship between age and FBL. Longitudinal studies of FBL across a lifespan are needed to examine the specific changes to the integrity of white matter.
doi:10.1212/WNL.0000000000000597
PMCID: PMC4117364  PMID: 24928121
10.  Neuronal fiber bundle lengths in healthy adult carriers of the ApoE4 allele: A quantitative tractography DTI study 
Brain imaging and behavior  2013;7(3):10.1007/s11682-013-9225-4.
The epsilon 4 (e4) isoform of apolipoprotein E (ApoE) is a known genetic risk factor for suboptimal brain health. Morphometry studies of brains with Alzheimer’s disease have reported significant alterations in temporal lobe brain structure of e4 carriers, yet it remains unclear if the presence of an e4 allele is associated with alterations in the microstructure of white matter fiber bundles in healthy populations. The present study used quantitative tractography based on diffusion tensor imaging (qtDTI) to examine the influence of the e4 allele on temporal lobe fiber bundle lengths (FBLs) in 64 healthy older adults with at least one e4 allele (carriers, N=23) versus no e4 allele (non-carriers, N=41). Subtests from the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) were also analyzed to examine memory performance between groups. Analyses revealed shorter FBLs in the left uncinate fasciculus (UF) (p=.038) of e4 carriers compared to non-carriers. By contrast, neither FBLs specific to the temporal lobe nor memory performances differed significantly between groups. Increased age correlated significantly with shorter FBL in the temporal lobe and UF, and with decreased performance on tests of memory. This is the first study to utilize qtDTI to examine relationships between FBL and ApoE genotype. Results suggest that FBL in the UF is influenced by the presence of an ApoE e4 allele (ApoE4) in healthy older adults. Temporal lobe FBLs, however, are more vulnerable to aging than the presence of an e4 allele.
doi:10.1007/s11682-013-9225-4
PMCID: PMC3726531  PMID: 23475756
ApoE; ApoE4; Tractography; DTI; White matter; Fiber bundle lengths; Aging
11.  Impact of body mass index on neuronal fiber bundle lengths among healthy older adults 
Brain imaging and behavior  2013;7(3):10.1007/s11682-013-9230-7.
Increased body mass index (BMI) has been linked to various detrimental health outcomes, including cognitive dysfunction. Recent work investigating associations between obesity and the brain has revealed decreased white matter microstructural integrity in individuals with elevated BMI, independent of age or comorbid health conditions. However, the relationship between high BMI and white matter fiber bundle length (FBL), which represents a novel metric of microstructural brain integrity, remains unknown. The present study utilized quantitative tractography based on diffusion tensor imaging (DTI) to investigate the relationship between BMI and FBL in 72 otherwise healthy older adults (24 males, 48 females). All participants were between 51 and 85 years of age (M = 63.26, SD = 8.76). Results revealed that elevated BMI was associated with shorter FBL in the temporal lobe, independent of age (p < .01). In addition, increased age was associated with shorter frontal, temporal, and whole brain FBL (all p values < .01). These findings indicate that, while increased age is an important factor associated with reduced FBL, high BMI is uniquely associated with reduced FBL in the temporal lobe. These data offer evidence for additive adverse effects of high BMI on the brain, especially in areas already vulnerable to aging processes and age-related neurodegenerative diseases. Further research is necessary to determine the physiological mechanisms associated with the shortening of FBL in individuals with high BMI.
doi:10.1007/s11682-013-9230-7
PMCID: PMC3830712  PMID: 23564371
Tractography; BMI; DTI; White Matter; Fiber Bundle Length; Aging
12.  Neuronal fiber pathway abnormalities in autism: An initial MRI diffusion tensor tracking study of hippocampo-fusiform and amygdalo-fusiform pathways 
MRI diffusion-tensor tracking (DTT) was performed in 17 high-functioning adolescents/adults with autism and 17 pairwise-matched controls. White matter pathways involved in face processing were examined due to the relevance of face perception to the social symptoms of autism, and due to known behavioral and functional imaging findings in autism. The hippocampo-fusiform (HF) and amygdalo-fusiform (AF) pathways had normal size and shape but abnormal microstructure in the autism group. The right HF had reduced across-fiber diffusivity (D-min) compared with controls, opposite to the whole-brain effect of increased D-min. In contrast, left HF, right AF, and left AF had increased D-min and increased along-fiber diffusivity (D-max), more consistent with the whole-brain effect. There was a general loss of lateralization compared with controls. The right HF D-min was markedly low in the autism subgroup with lower Benton face recognition scores, compared with the lower-Benton control subgroup, and compared with the higher-Benton autism subgroup. Similar behavioral relationships were found for performance IQ. Such results suggest an early functionally-significant pathological process in right HF consistent with small-diameter axons (with correspondingly slower neural transmission) and/or higher packing density. In left AF and HF, changes were interpreted as secondary, possibly reflecting axonal loss and/or decreased myelination.
doi:10.1017/S1355617708081381
PMCID: PMC3298449  PMID: 18954474
Autism; Diffusion tensor MRI; White matter fiber tracking; Fusiform face area; Amygdala; Hippocampus; Face recognition; DTT; White matter pathways
13.  Anterior Thalamic Radiation Integrity in Schizophrenia: A Diffusion-Tensor Imaging Study 
Psychiatry research  2010;183(2):10.1016/j.pscychresns.2010.04.013.
The anterior limb of the internal capsule (ALIC) is a white matter structure, the medial portion of which includes the anterior thalamic radiation (ATR) carrying nerve fibers between thalamus and prefrontal cortex. ATR abnormalities have a possible link with cognitive abnormalities and negative symptoms in schizophrenia. We aimed to study the fiber integrity of the ATR more selectively by isolating the medial portion of the ALIC using region-of-interest based methodology. Diffusion-tensor imaging was used to measure the anisotropy of total ALIC (tALIC) and medial ALIC (mALIC) in 39 schizophrenia and 33 control participants, matched for age/gender/handedness. Relationships between anisotropy, psychopathology, and cognitive performance were analyzed. Compared to controls, schizophrenia participants had 4.55% lower anisotropy in right tALIC, and 5.38% lower anisotropy in right mALIC. There were no significant group anisotropy differences on the left. Significant correlations were observed between right ALIC integrity and relevant domains of cognitive function (e.g., executive function, working memory). Our study suggests an asymmetric microstructural change in ALIC in schizophrenia involving the right side, which is only minimally stronger in mALIC, and which correlates with cognitive impairment. Microstructural changes in the ALIC may be linked to cognitive dysfunction in schizophrenia.
doi:10.1016/j.pscychresns.2010.04.013
PMCID: PMC3887223  PMID: 20619618
Schizophrenia; Anterior Thalamic Radiation; Internal Capsule; DTI; Diffusion Tensor Imaging; ROI; Thalamus; anisotropy; working memory; executive function
14.  Neuroimaging Signatures and Cognitive Correlates of the Montreal Cognitive Assessment Screen in a Nonclinical Elderly Sample 
The Montreal Cognitive Assessment (MoCA) screen was developed as a brief instrument to identify mild cognitive impairment and dementia among older individuals. To date, limited information is available regarding the neuroimaging signatures associated with performance on the scale, or the relationship between the MoCA and more comprehensive cognitive screening measures. The present study examined performances on the MoCA among 111 non-clinical older adults (ages 51–85) enrolled in a prospective study of cognitive aging. Participants were administered the MoCA, Mini-Mental State Exam (MMSE), and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). A subset of participants (N = 69) underwent structural 3 T magnetic resonance imaging (MRI) to define the volumes of total frontal gray matter, total hippocampus, T2-weighted subcortical hyperintensities (SH), and total brain volume. The results revealed significant correlations between the total score on the MoCA and total score on the RBANS and MMSE, though the strength of the correlations was more robust between the MoCA and the RBANS. Modest correlations between individual subscales of the MoCA and neuroimaging variables were evident, but no patterns of shared variance emerged between the MoCA total score and neuroimaging indices. In contrast, total brain volume correlated significantly with total score on the RBANS. These results suggest that additional studies are needed to define the significance of MoCA scores relative to brain integrity among an older population.
doi:10.1093/arclin/acr017
PMCID: PMC3142949  PMID: 21642663
Mild cognitive impairment; Neuroimaging (structural)
15.  Quantitative Analysis of the Shape of the Corpus Callosum in Autistic Individuals 
Autism  2011;15(2):223-238.
Multiple studies suggest that the corpus callosum in patients with autism is reduced in size. This study attempts to elucidate the nature of this morphometric abnormality by analyzing the shape of this structure in 17 high-functioning patients with autism and an equal number of comparison participants matched for age, sex, IQ, and handedness. The corpus callosum was segmented from T1 weighted images acquired with a Siemens 1.5 T scanner. Transformed coordinates of the curvilinear axis were aggregated into a parametric map and compared across series to derive regions of statistical significance. Our results indicate that in subjects with autism reduction in size of the corpus callosum occurs over all of its subdivisions (genu, body, splenium) with a small area of overgrowth at its caudal pole. Since the commisural fibers that traverse the different anatomical compartments of the corpus callosum originate in disparate brain regions our results suggest the presence of widely distributed cortical abnormalities in people with autism.
doi:10.1177/1362361310386506
PMCID: PMC3349188  PMID: 21363871
Autistic Disorder; Cerebral Cortex/growth & development; Corpus Callosum; Magnetic Resonance Imaging
16.  MRI diffusion tensor tracking of a new amygdalo-fusiform and hippocampo-fusiform pathway system in humans 
Purpose
To use MRI diffusion-tensor tracking (DTT) to test for the presence of unknown neuronal fiber pathways interconnecting mid-fusiform cortex and anteromedial temporal lobe in humans. Such pathways are hypothesized to exist because these regions co-activate in functional MRI (fMRI) studies of emotion-valued faces and words, suggesting a functional link that could be mediated by neuronal connections.
Materials and Methods
15 normal human subjects were studied using unbiased DTT approaches designed for probing unknown pathways, including whole-brain seeding and large pathway-selection volumes. Several quality-control steps verified the results.
Results
Parallel amygdalo-fusiform and hippocampo-fusiform pathways were found in all subjects. The pathways begin/end at mid-fusiform gyrus above the lateral occipitotemporal sulcus bilaterally. The superior pathway ends/begins at the superolateral amygdala. The inferior pathway crosses medially and ends/begins at the hippocampal head. The pathways are left-lateralized, with consistently larger cross-sectional area, higher anisotropy, and lower minimum eigenvalue (D-min) on the left, where D-min assesses intrinsic cross-fiber diffusivity independent of curvature.
Conclusion
A previously-undescribed pathway system interconnecting mid-fusiform region with amygdala/hippocampus has been revealed. This pathway system may be important for recognition, memory consolidation, and emotional modulation of face, object, and lexical information, which may be disrupted in conditions such as Alzheimer's disease.
doi:10.1002/jmri.21692
PMCID: PMC2720528  PMID: 19418556
MRI; diffusion-tensor fiber tracking (DTT); amygdala/hippocampus; temporal lobes; face/object recognition; emotion
17.  BOLD Correlates of Trial-by-Trial Reaction Time Variability in Gray and White Matter: A Multi-Study fMRI Analysis 
PLoS ONE  2009;4(1):e4257.
Background
Reaction time (RT) is one of the most widely used measures of performance in experimental psychology, yet relatively few fMRI studies have included trial-by-trial differences in RT as a predictor variable in their analyses. Using a multi-study approach, we investigated whether there are brain regions that show a general relationship between trial-by-trial RT variability and activation across a range of cognitive tasks.
Methodology/Principal Findings
The relation between trial-by-trial differences in RT and brain activation was modeled in five different fMRI datasets spanning a range of experimental tasks and stimulus modalities. Three main findings were identified. First, in a widely distributed set of gray and white matter regions, activation was delayed on trials with long RTs relative to short RTs, suggesting delayed initiation of underlying physiological processes. Second, in lateral and medial frontal regions, activation showed a “time-on-task” effect, increasing linearly as a function of RT. Finally, RT variability reliably modulated the BOLD signal not only in gray matter but also in diffuse regions of white matter.
Conclusions/Significance
The results highlight the importance of modeling trial-by-trial RT in fMRI analyses and raise the possibility that RT variability may provide a powerful probe for investigating the previously elusive white matter BOLD signal.
doi:10.1371/journal.pone.0004257
PMCID: PMC2622763  PMID: 19165335

Results 1-17 (17)