The influence of neuropathology on the FMRI signal: a measurement of brain or vein?

ENIGMA's simple seven: Recommendations to enhance the reproducibility of resting-state fMRI in traumatic brain injury

Resting state functional magnetic resonance imaging (rsfMRI) provides researchers and clinicians with a powerful tool to examine functional connectivity across large-scale brain networks, with ever-increasing applications to the study of neurological disorders, such as traumatic brain injury (TBI). While rsfMRI holds unparalleled promise in systems neurosciences, its acquisition and analytical methodology across research groups is variable, resulting in a literature that is challenging to integrate and interpret. The focus of this narrative review is to address the primary methodological issues including investigator decision points in the application of rsfMRI to study the consequences of TBI. As part of the ENIGMA Brain Injury working group, we have collaborated to identify a minimum set of recommendations that are designed to produce results that are reliable, harmonizable, and reproducible for the TBI imaging research community. Part one of this review provides the results of a literature search of current rsfMRI studies of TBI, highlighting key design considerations and data processing pipelines. Part two outlines seven data acquisition, processing, and analysis recommendations with the goal of maximizing study reliability and between-site comparability, while preserving investigator autonomy. Part three summarizes new directions and opportunities for future rsfMRI studies in TBI patients. The goal is to galvanize the TBI community to gain consensus for a set of rigorous and reproducible methods, and to increase analytical transparency and data sharing to address the reproducibility crisis in the field.

Power shift and connectivity changes in healthy aging during resting-state EEG

The neural activity of human brain changes in healthy individuals during aging. The most frequent variation in patterns of neural activity are a shift from posterior to anterior areas and a reduced asymmetry between hemispheres. These patterns are typically observed during task execution and by using functional magnetic resonance imaging data. In the present study we investigated whether analogous effects can also be detected during rest and by means of source-space time series reconstructed from electroencephalographic recordings. By analyzing oscillatory power distribution across the brain we indeed found a shift from posterior to anterior areas in older adults. We additionally examined this shift by evaluating connectivity and its changes with age. The findings indicated that inter-area connections among frontal, parietal and temporal areas were strengthened in older individuals. A more complex pattern was shown in intra-area connections, where age-related activity was enhanced in parietal and temporal areas, and reduced in frontal areas. Finally, the resulting network exhibits a loss of modularity with age. Overall, the results extend to resting-state condition the evidence of an age-related shift of brain activity from posterior to anterior areas, thus suggesting that this shift is a general feature of the aging brain rather than being task-specific. In addition, the connectivity results provide new information on the reorganization of resting-state brain activity in aging.

ENIGMA brain injury: Framework, challenges, and opportunities

Traumatic brain injury (TBI) is a major cause of disability worldwide, but the heterogeneous nature of TBI with respect to injury severity and health comorbidities make patient outcome difficult to predict. Injury severity accounts for only some of this variance, and a wide range of preinjury, injury-related, and postinjury factors may influence outcome, such as sex, socioeconomic status, injury mechanism, and social support. Neuroimaging research in this area has generally been limited by insufficient sample sizes. Additionally, development of reliable biomarkers of mild TBI or repeated subconcussive impacts has been slow, likely due, in part, to subtle effects of injury and the aforementioned variability. The ENIGMA Consortium has established a framework for global collaboration that has resulted in the largest-ever neuroimaging studies of multiple psychiatric and neurological disorders. Here we describe the organization, recent progress, and future goals of the Brain Injury working group.

Functional Brain Hyperactivations Are Linked to an Electrophysiological Measure of Slow Interhemispheric Transfer Time after Pediatric Moderate/Severe Traumatic Brain Injury

Increased task-related blood oxygen level dependent (BOLD) activation is commonly observed in functional magnetic resonance imaging (fMRI) studies of moderate/severe traumatic brain injury (msTBI), but the functional relevance of these hyperactivations and how they are linked to more direct measures of neuronal function remain largely unknown. Here, we investigated how working memory load (WML)-dependent BOLD activation was related to an electrophysiological measure of interhemispheric transfer time (IHTT) in a sample of 18 msTBI patients and 26 demographically matched controls from the UCLA RAPBI (Recovery after Pediatric Brain Injury) study. In the context of highly similar fMRI task performance, a subgroup of TBI patients with slow IHTT had greater BOLD activation with higher WML than both healthy control children and a subgroup of msTBI patients with normal IHTT. Slower IHTT treated as a continuous variable was also associated with BOLD hyperactivation in the full TBI sample and in controls. Higher WML-dependent BOLD activation was related to better performance on a clinical cognitive performance index, an association that was more pronounced within the patient group with slow IHTT. Our previous work has shown that a subgroup of children with slow IHTT after pediatric msTBI has increased risk for poor white matter organization, long-term neurodegeneration, and poor cognitive outcome. BOLD hyperactivations after msTBI may reflect neuronal compensatory processes supporting higher-order capacity demanding cognitive functions in the context of inefficient neuronal transfer of information. The link between BOLD hyperactivations and slow IHTT adds to the multi-modal validation of this electrophysiological measure as a promising biomarker.

Regenerative Capacity of Endogenous Factor: Growth Differentiation Factor 11; a New Approach of the Management of Age-Related Cardiovascular Events

Aging is a complicated pathophysiological process accompanied by a wide array of biological adaptations. The physiological deterioration correlates with the reduced regenerative capacity of tissues. The rejuvenation of tissue regeneration in aging organisms has also been observed after heterochronic parabiosis. With this model, it has been shown that exposure to young blood can rejuvenate the regenerative capacity of peripheral tissues and brain in aged animals. An endogenous compound called growth differentiation factor 11 (GDF11) is a circulating negative regulator of cardiac hypertrophy, suggesting that raising GDF11 levels could potentially treat or prevent cardiac diseases. The protein GDF11 is found in humans as well as animals. The existence of endogenous regulators of regenerative capacity, such as GDF11, in peripheral tissues and brain has now been demonstrated. It will be important to investigate the mechanisms with therapeutic promise that induce the regenerative effects of GDF11 for a variety of age-related diseases.

Surface-Based fMRI-Driven Diffusion Tractography in the Presence of Significant Brain Pathology: A Study Linking Structure and Function in Cerebral Palsy

Diffusion MRI (dMRI) tractography analyses are difficult to perform in the presence of brain pathology. Automated methods that rely on cortical parcellation for structural connectivity studies often fail, while manually defining regions is extremely time consuming and can introduce human error. Both methods also make assumptions about structure-function relationships that may not hold after cortical reorganisation. Seeding tractography with functional-MRI (fMRI) activation is an emerging method that reduces these confounds, but inherent smoothing of fMRI signal may result in the inclusion of irrelevant pathways. This paper describes a novel fMRI-seeded dMRI-analysis pipeline based on surface-meshes that reduces these issues and utilises machine-learning to generate task specific white matter pathways, minimising the requirement for manually-drawn ROIs. We directly compared this new strategy to a standard voxelwise fMRI-dMRI approach, by investigating correlations between clinical scores and dMRI metrics of thalamocortical and corticomotor tracts in 31 children with unilateral cerebral palsy. The surface-based approach successfully processed more participants (87%) than the voxel-based approach (65%), and provided significantly more-coherent tractography. Significant correlations between dMRI metrics and five clinical scores of function were found for the more superior regions of these tracts. These significant correlations were stronger and more frequently found with the surface-based method (15/20 investigated were significant; R² = 0.43–0.73) than the voxelwise analysis (2 sig. correlations; 0.38 & 0.49). More restricted fMRI signal, better-constrained tractography, and the novel track-classification method all appeared to contribute toward these differences.

Modeling distinct imaging hemodynamics early after TBI: the relationship between signal amplitude and connectivity

Over the past decade, fMRI studies of cognitive change following traumatic brain injury (TBI) have investigated blood oxygen level dependent (BOLD) activity during working memory (WM) performance in individuals in early and chronic phases of recovery. Recently, BOLD fMRI work has largely shifted to focus on WM and resting functional connectivity following TBI. However, fundamental questions in WM remain. Specifically, the effects of injury on the basic relationships between local and interregional functional neuroimaging signals during WM processing early following moderate to severe TBI have not been examined. This study employs a mixed effects model to examine prefrontal cortex and parietal lobe signal change during a WM task, the n-back, and whether there is covariance between regions of high amplitude signal change, (synchrony of elicited activity (SEA) very early following TBI. We also examined whether signal change and SEA differentially predict performance during WM. Overall, percent signal change in the right prefrontal cortex (rPFC) was and important predictor of both reaction time (RT) and SEA in early TBI and matched controls. Right prefrontal cortex (rPFC) percent signal change positively predicted SEA within and between persons regardless of injury status, suggesting that the link between these neurodynamic processes in WM-activated regions remains unaffected even very early after TBI. Additionally, rPFC activity was positively related to RT within and between persons in both groups. Right parietal (rPAR) activity was negatively related to RT within subjects in both groups. Thus, the local signal intensity of the rPFC in TBI appears to be a critical property of network functioning and performance in WM processing and may be a precursor to recruitment observed in chronic samples. The present results suggest that as much research moves toward large scale functional connectivity modeling, it will be essential to develop integrated models of how local and distant neurodynamics promote WM performance after TBI.

Interpreting Intervention Induced Neuroplasticity with fMRI: The Case for Multimodal Imaging Strategies

Direct measurement of recovery from brain injury is an important goal in neurorehabilitation, and requires reliable, objective, and interpretable measures of changes in brain function, referred to generally as “neuroplasticity.” One popular imaging modality for measuring neuroplasticity is task-based functional magnetic resonance imaging (t-fMRI). In the field of neurorehabilitation, however, assessing neuroplasticity using t-fMRI presents a significant challenge. This commentary reviews t-fMRI changes commonly reported in patients with cerebral palsy or acquired brain injuries, with a focus on studies of motor rehabilitation, and discusses complexities surrounding their interpretations. Specifically, we discuss the difficulties in interpreting t-fMRI changes in terms of their underlying causes, that is, differentiating whether they reflect genuine reorganisation, neurological restoration, compensation, use of preexisting redundancies, changes in strategy, or maladaptive processes. Furthermore, we discuss the impact of heterogeneous disease states and essential t-fMRI processing steps on the interpretability of activation patterns. To better understand therapy-induced neuroplastic changes, we suggest that researchers utilising t-fMRI consider concurrently acquiring information from an additional modality, to quantify, for example, haemodynamic differences or microstructural changes. We outline a variety of such supplementary measures for investigating brain reorganisation and discuss situations in which they may prove beneficial to the interpretation of t-fMRI data.

Attempted and successful compensation in preclinical and early manifest neurodegeneration - a review of task FMRI studies

Several models of neural compensation in healthy aging have been suggested to explain brain activity that aids to sustain cognitive function. Applying recently suggested criteria of "attempted" and "successful" compensation, we reviewed existing literature on compensatory mechanisms in preclinical Huntington's disease (HD) and amnestic mild cognitive impairment (aMCI). Both disorders constitute early stages of neurodegeneration ideal for examining compensatory mechanisms and developing targeted interventions. We strived to clarify whether compensation criteria derived from healthy aging populations can be applied to early neurodegeneration. To concentrate on the close coupling of cognitive performance and brain activity, we exclusively addressed task fMRI studies. First, we found evidence for parallels in compensatory mechanisms between healthy aging and neurodegenerative disease. Several studies fulfilled criteria of attempted compensation, while reports of successful compensation were largely absent, which made it difficult to conclude on. Second, comparing working memory studies in preclinical HD and aMCI, we identified similar compensatory patterns across neurodegenerative disorders in lateral and medial prefrontal cortex. Such patterns included an inverted U-shaped relationship of neurodegeneration and compensatory activity spanning from preclinical to manifest disease. Due to the lack of studies systematically targeting all criteria of compensation, we propose an exemplary study design, including the manipulation of compensating brain areas by brain stimulation. Furthermore, we delineate the benefits of targeted interventions by non-invasive brain stimulation, as well as of unspecific interventions such as physical activity or cognitive training. Unambiguously detecting compensation in early neurodegenerative disease will help tailor interventions aiming at sustained overall functioning and delayed clinical disease onset.

The rich get richer: brain injury elicits hyperconnectivity in core subnetworks

There remains much unknown about how large-scale neural networks accommodate neurological disruption, such as moderate and severe traumatic brain injury (TBI). A primary goal in this study was to examine the alterations in network topology occurring during the first year of recovery following TBI. To do so we examined 21 individuals with moderate and severe TBI at 3 and 6 months after resolution of posttraumatic amnesia and 15 age- and education-matched healthy adults using functional MRI and graph theoretical analyses. There were two central hypotheses in this study: 1) physical disruption results in increased functional connectivity, or hyperconnectivity, and 2) hyperconnectivity occurs in regions typically observed to be the most highly connected cortical hubs, or the "rich club". The current findings generally support the hyperconnectivity hypothesis showing that during the first year of recovery after TBI, neural networks show increased connectivity, and this change is disproportionately represented in brain regions belonging to the brain's core subnetworks. The selective increases in connectivity observed here are consistent with the preferential attachment model underlying scale-free network development. This study is the largest of its kind and provides the unique opportunity to examine how neural systems adapt to significant neurological disruption during the first year after injury.

Altered Cognitive Control Activations after Moderate-to-Severe Traumatic Brain Injury and Their Relationship to Injury Severity and Everyday-Life Function

This study investigated how the neuronal underpinnings of both adaptive and stable cognitive control processes are affected by traumatic brain injury (TBI). Functional magnetic resonance imaging (fMRI) was undertaken in 62 survivors of moderate-to-severe TBI (>1 year after injury) and 68 healthy controls during performance of a continuous performance test adapted for use in a mixed block- and event-related design. Survivors of TBI demonstrated increased reliance on adaptive task control processes within an a priori core region for cognitive control in the medial frontal cortex. TBI survivors also had increased activations related to time-on-task effects during stable task-set maintenance in right inferior parietal and prefrontal cortices. Increased brain activations in TBI survivors had a dose-dependent linear positive relationship to injury severity and were negatively correlated with self-reported cognitive control problems in everyday-life situations. Results were adjusted for age, education, and fMRI task performance. In conclusion, evidence was provided that the neural underpinnings of adaptive and stable control processes are differently affected by TBI. Moreover, it was demonstrated that increased brain activations typically observed in survivors of TBI might represent injury-specific compensatory adaptations also utilized in everyday-life situations.

The cognitive neuroscience of ageing

The availability of neuroimaging technology has spurred a marked increase in the human cognitive neuroscience literature, including the study of cognitive ageing. Although there is a growing consensus that the ageing brain retains considerable plasticity of function, currently measured primarily by means of functional MRI, it is less clear how age differences in brain activity relate to cognitive performance. The field is also hampered by the complexity of the ageing process itself and the large number of factors that are influenced by age. In this Review, current trends and unresolved issues in the cognitive neuroscience of ageing are discussed.

A perfusion fMRI study of the neural correlates of sustained-attention and working-memory deficits in chronic traumatic brain injury

BACKGROUND

Given that traumatic brain injury (TBI) results in chronic alteration of baseline cerebral perfusion, a perfusion functional MRI (fMRI) method that dissociates resting- and task-related cerebral blood flow (CBF) changes can be useful in noninvasively investigating the neural correlates of cognitive dysfunction and recovery in TBI.

OBJECTIVE

The authors used continuous arterial spin-labeled (ASL) perfusion fMRI to characterize CBF at rest and during sustained-attention and working-memory tasks.

METHODS

A total of 18 to 21 individuals with moderate to severe TBI and 14 to 18 demographically matched healthy controls completed 3 continuous 6-minute perfusion fMRI scans (resting, visual sustained attention, and 2-back working memory).

RESULTS

For both tasks, TBI participants showed worse behavioral performance than controls. Voxelwise neuroimaging analysis of the 2-back task found that group differences in task-induced CBF changes were localized to bilateral superior occipital cortices and the left superior temporal cortex. Whereas controls deactivated these areas during task performance, TBI participants tended to activate these same areas. These regions were among those found to be disproportionately hypoperfused at rest after TBI. For both tasks, the control and TBI groups showed different patterns of correlation between performance and task-related CBF changes.

CONCLUSIONS

ASL perfusion fMRI demonstrated differences between individuals with TBI and healthy controls in resting perfusion and in task-evoked CBF changes as well as different patterns of performance-activation correlation. These results are consistent with the notion that sensory/attentional modulation deficits contribute to higher cognitive dysfunction in TBI.

The relationship of resting cerebral blood flow and brain activation during a social cognition task in adolescents with chronic moderate to severe traumatic brain injury: a preliminary investigation

Alterations in cerebrovascular function are evident acutely in moderate to severe traumatic brain injury (TBI), although less is known about their chronic effects. Adolescent and adult patients with moderate to severe TBI have been reported to demonstrate diffuse activation throughout the brain during functional magnetic resonance imaging (fMRI). Because fMRI is a measure related to blood flow, it is possible that any deficits in blood flow may alter activation. An arterial spin labeling (ASL) perfusion sequence was performed on seven adolescents with chronic moderate to severe TBI and seven typically developing (TD) adolescents during the same session in which they had performed a social cognition task during fMRI. In the TD group, prefrontal CBF was positively related to prefrontal activation and negatively related to non-prefrontal, posterior, brain activation. This relationship was not seen in the TBI group, who demonstrated a greater positive relationship between prefrontal CBF and non-prefrontal activation than the TD group. An analysis of CBF data independent of fMRI showed reduced CBF in the right non-prefrontal region (p<.055) in the TBI group. To understand any role reduced CBF may play in diffuse extra-activation, we then related the right non-prefrontal CBF to activation. CBF in the right non-prefrontal region in the TD group was positively associated with prefrontal activation, suggesting an interactive role of non-prefrontal and prefrontal blood flow throughout the right hemisphere in healthy brains. However, the TBI group demonstrated a positive association with activation constrained to the right non-prefrontal region. These data suggest a relationship between impaired non-prefrontal CBF and the presence of non-prefrontal extra-activation, where the region with more limited blood flow is associated with activation limited to that region. In a secondary analysis, pathology associated with hyperintensities on T2-weighted FLAIR imaging over the whole brain was related to whole brain activation, revealing a negative relationship between lesion volume and frontal activation, and a positive relationship between lesion volume and posterior activation. These preliminary data, albeit collected with small sample sizes, suggest that reduced non-prefrontal CBF, and possibly pathological tissue associated with T2-hyperintensities, may provide contributions to the diffuse, primarily posterior extra-activation observed in adolescents following moderate to severe TBI.

Differentiation between vergence and saccadic functional activity within the human frontal eye fields and midbrain revealed through fMRI

PURPOSE

Eye movement research has traditionally studied solely saccade and/or vergence eye movements by isolating these systems within a laboratory setting. While the neural correlates of saccadic eye movements are established, few studies have quantified the functional activity of vergence eye movements using fMRI. This study mapped the neural substrates of vergence eye movements and compared them to saccades to elucidate the spatial commonality and differentiation between these systems.

METHODOLOGY

The stimulus was presented in a block design where the 'off' stimulus was a sustained fixation and the 'on' stimulus was random vergence or saccadic eye movements. Data were collected with a 3T scanner. A general linear model (GLM) was used in conjunction with cluster size to determine significantly active regions. A paired t-test of the GLM beta weight coefficients was computed between the saccade and vergence functional activities to test the hypothesis that vergence and saccadic stimulation would have spatial differentiation in addition to shared neural substrates.

RESULTS

Segregated functional activation was observed within the frontal eye fields where a portion of the functional activity from the vergence task was located anterior to the saccadic functional activity (z>2.3; p<0.03). An area within the midbrain was significantly correlated with the experimental design for the vergence but not the saccade data set. Similar functional activation was observed within the following regions of interest: the supplementary eye field, dorsolateral prefrontal cortex, ventral lateral prefrontal cortex, lateral intraparietal area, cuneus, precuneus, anterior and posterior cingulates, and cerebellar vermis. The functional activity from these regions was not different between the vergence and saccade data sets assessed by analyzing the beta weights of the paired t-test (p>0.2).

CONCLUSION

Functional MRI can elucidate the differences between the vergence and saccade neural substrates within the frontal eye fields and midbrain.

The less BOLD, the wiser: support for the latent resource hypothesis after traumatic brain injury

Previous studies of the BOLD response in the injured brain have revealed neural recruitment relative to controls during working memory tasks in several brain regions, most consistently the right prefrontal cortex and anterior cingulate cortices. We previously proposed that the recruitment observed in this literature represents auxiliary support resources, and that recruitment of PFC is not abnormal or injury specific and should reduce as novelty and challenge decrease. The current study directly tests this hypothesis in the context of practice of a working memory task. It was hypothesized that individuals with brain injury would demonstrate recruitment of previously indicated regions, behavioral improvement following task practice, and a reduction in the BOLD signal in recruited regions after practice. Individuals with traumatic brain injury and healthy controls performed the n-back during fMRI acquisition, practiced each task out of the scanner, and returned to the scanner for additional fMRI n-back acquisition. Statistical parametric maps demonstrated a number of regions of recruitment in the 1-back in individuals with brain injury and a number of corresponding regions of reduced activation in individuals with brain injury following practice in both the 1-back and 2-back. Regions of interest demonstrated reduced activation following practice, including the anterior cingulate and right prefrontal cortices. Individuals with brain injury demonstrated modest behavioral improvements following practice. These findings suggest that neural recruitment in brain injury does not represent reorganization but a natural extension of latent mechanisms that engage transiently and are contingent upon cerebral challenge.

The nature of processing speed deficits in traumatic brain injury: is less brain more?

The cognitive constructs working memory (WM) and processing speed are fundamental components to general intellectual functioning in humans and highly susceptible to disruption following neurological insult. Much of the work to date examining speeded working memory deficits in clinical samples using functional imaging has demonstrated recruitment of network areas including prefrontal cortex (PFC) and anterior cingulate cortex (ACC). What remains unclear is the nature of this neural recruitment. The goal of this study was to isolate the neural networks distinct from those evident in healthy adults and to determine if reaction time (RT) reliably predicts observable between-group differences. The current data indicate that much of the neural recruitment in TBI during a speeded visual scanning task is positively correlated with RT. These data indicate that recruitment in PFC during tasks of rapid information processing are at least partially attributable to normal recruitment of PFC support resources during slowed task processing.

Auditory orienting and inhibition of return in mild traumatic brain injury: a FMRI study

The semiacute phase of mild traumatic brain injury (mTBI) is associated with deficits in the cognitive domains of attention, memory, and executive function, which previous work suggests may be related to a specific deficit in disengaging attentional focus. However, to date, there have only been a few studies that have employed dynamic imaging techniques to investigate the potential neurological basis of these cognitive deficits during the semiacute stage of injury. Therefore, event-related functional magnetic resonance imaging was used to investigate the neurological correlates of attentional dysfunction in a clinically homogeneous sample of 16 patients with mTBI during the semiacute phase of injury (<3 weeks). Behaviorally, patients with mTBI exhibited deficits in disengaging and reorienting auditory attention following invalid cues as well as a failure to inhibit attentional allocation to a cued spatial location compared to a group of matched controls. Accordingly, patients with mTBI also exhibited hypoactivation within thalamus, striatum, midbrain nuclei, and cerebellum across all trials as well as hypoactivation in the right posterior parietal cortex, presupplementary motor area, bilateral frontal eye fields, and right ventrolateral prefrontal cortex during attentional disengagement. Finally, the hemodynamic response within several regions of the attentional network predicted response times better for controls than for patients with mTBI. These objective neurological findings represent a potential biomarker for the behavioral deficits in spatial attention that characterize the initial recovery phase of mTBI.

Resting network plasticity following brain injury

The purpose of this study was to examine neural network properties at separate time-points during recovery from traumatic brain injury (TBI) using graph theory. Whole-brain analyses of the topological properties of the fMRI signal were conducted in 6 participants at 3 months and 6 months following severe TBI. Results revealed alterations of network properties including a change in the degree distribution, reduced overall strength in connectivity, and increased "small-worldness" from 3 months to 6 months post injury. The findings here indicate that, during recovery from injury, the strength but not the number of network connections diminishes, so that over the course of recovery, the network begins to approximate what is observed in healthy adults. These are the first data examining functional connectivity in a disrupted neural system during recovery.

Cognitive neuroscience of aging

The number of reports on the cognitive neuroscience of aging has increased in recent years, and most of these studies have found many similarities in the patterns of activity in young and old adults, indicating that basic neural mechanisms are maintained into older age. Despite these overall similarities, older adults often have less activity in some regions, such as medial temporal areas during memory processing and visual regions across a variety of cognitive domains. It seems clear that age reductions in cognitive function can be tied, at least in part, to these reductions in brain activity. On the other hand, older adults typically also overrecruit some brain areas, mainly the ventral or dorsal prefrontal cortex during memory tasks, as well as both the frontal and parietal regions during tasks engaging cognitive control processes, such as attention. Sometimes this overrecruitment appears to be in response to altered function in other brain regions and is often seen in those older adults who perform better on the task at hand. These findings have provided rather convincing support for the idea that overrecruitment can be compensatory in the elderly. Nevertheless, not all age increases can be interpreted as compensatory, and some are more indicative of neural inefficiency. The challenge facing future research will be to understand the task conditions that promote compensation in older adults, the role of the various brain areas in aiding cognitive function, and how these compensatory mechanisms can be elicited to enhance quality of life in the elderly.

Functional MRI and other non-invasive imaging technologies: providing visual biomarkers for spinal cord structure and function after injury

Substantial progress has been made towards understanding the molecular basis for limited endogenous central nervous system (CNS) axonal growth after injuries such as spinal cord trauma. Realization of the potential benefit of therapeutic interventions requires methods to assess axonal growth and functional reorganization over time after neurological damage. Here, we discuss the technical challenges of analyzing and interpreting the effects of various interventions on CNS repair, specifically in the context of spinal cord injury. Evolving technologies such as functional magnetic resonance imaging and other non-invasive imaging techniques will be reviewed. These technologies should revolutionize our ability to track changes in both CNS structure and function.