Neural correlates of age-related changes in cortical neurophysiology

Fundamental differences in callosal structure, neurophysiologic function, and bimanual control in young and older adults

Bimanual actions involve coordinated motion but often rely on the movements performed with each hand to be different. Older adults exhibit differentially greater variability for bimanual actions in which each hand has an independent movement goal. Such actions rely on interhemispheric communication via the corpus callosum, including both facilitatory and inhibitory interactions. Here, we investigated whether age differences in callosal structure and interhemispheric function contribute to this selective movement difficulty. Participants performed 3 force production tasks: 1) unimanual, 2) bimanual simultaneous, and 3) bimanual independent. Older adults had significantly greater interhemispheric facilitation during voluntary muscle activation. We also report a fundamental shift with age in the relationship between callosal tract microstructural integrity and interhemispheric inhibition (IHI). Specifically, older adults with relatively greater callosal tract microstructural integrity have less IHI. Furthermore, greater IHI was related to poorer bimanual performance (assessed by dominant hand force variability) in older adults on all tasks, whereas this relationship was only observed in young adults for the bimanual independent condition. These findings indicate changes in interhemispheric communication with advancing age such that older adults may rely on bilateral cortical cooperation to a greater extent than young adults for manual actions.

Contralesional Motor Cortex Activation Depends on Ipsilesional Corticospinal Tract Integrity in Well-Recovered Subcortical Stroke Patients

Background. The relationship between structural and functional integrity of descending motor pathways can predict the potential for motor recovery after stroke. The authors examine the relationship between brain imaging biomarkers within contralesional and ipsilesional hemispheres and hand function in well-recovered patients after subcortical stroke at the level of the internal capsule. Objective. Measures of functional activation and integrity of the ipsilesional corticospinal tract might predict paretic hand function. Methods. A total of 14 patients in the chronic stable phase of motor recovery after subcortical stroke and 24 healthy age-matched individuals participated in the study. Functional MRI was used to examine BOLD contrast during passive wrist flexion–extension and paced or maximum-velocity active fist clenching. Functional integrity of the corticospinal pathway was assessed by transcranial magnetic stimulation to obtain motor-evoked potentials (MEPs) in the first dorsal interosseus muscle of the paretic and nonparetic hands. Fractional anisotropy and the proportion of traces between hemispheres in the posterior limb of both internal capsules were quantified using diffusion-weighted MRI. Results. Patients with smaller MEPs had a weaker paretic hand and more primary motor cortex activation in their affected hemisphere. Asymmetry between white matter tracts of either hemisphere was associated with reduced precision grip strength and increased BOLD activation within the contralesional dorsal premotor cortex for demanding hand tasks. Conclusion. There may be beneficial reorganization in contralesional secondary motor areas with increasing damage to the corticospinal tract after subcortical stroke. Associations between clinical, functional, and structural integrity measures in chronic stroke may lead to a better understanding of motor recovery processes.

Age-related changes in causal interactions between cortical motor regions during hand grip

Brain activity during motor performance becomes more widespread and less lateralized with advancing age in response to ongoing degenerative processes. In this study, we were interested in the mechanism by which this change in the pattern of activity supports motor performance with advancing age. We used both transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) to assess age related changes in motor system connectivity during isometric hand grip. Paired pulse TMS was used to measure the change in interhemispheric inhibition (IHI) from contralateral M1 (cM1) to ipsilateral M1 (iM1) during right hand grip. Dynamic Causal Modelling (DCM) of fMRI data was used to investigate the effect of age on causal interactions throughout the cortical motor network during right hand grip. Bayesian model selection was used to identify the causal model that best explained the data for all subjects. Firstly, we confirmed that the TMS and DCM measures both demonstrated a less inhibitory/more facilitatory influence of cM1 on iM1 during hand grip with advancing age. These values correlated with one another providing face validity for our DCM measures of connectivity. We found increasing reciprocal facilitatory influences with advancing age (i) between all ipsilateral cortical motor areas and (ii) between cortical motor areas of both hemispheres and iM1. There were no differences in the performance of our task with ageing suggesting that the ipsilateral cortical motor areas, in particular iM1, play a central role in maintaining performance levels with ageing through increasingly facilitatory cortico-cortical influences.

Assessment of cortical reorganisation for hand function after stroke

Abstract

Stroke often leads to impairment of hand function. Over the following months a variable amount of recovery can be seen. The evidence from animal and human studies suggests that reorganization rather than repair is the key. Surviving neural networks are important for recovery of function and non-invasive techniques such as functional magnetic resonance imaging allow us to study them in humans. For example, initial attempts to move a paretic limb following stroke are associated with widespread activity within the distributed motor system in both cerebral hemispheres, more so in patients with greater impairment. Disruption of activity in premotor areas using transcranial magnetic stimulation prior to movement can impair motor performance in stroke patients but not in controls suggesting that these new patterns of brain activity can support recovered function. In other words, this reorganisation is functionally relevant. More recently, research has been directed at understanding how surviving brain regions influence one another during movement. This opens the way for functional brain imaging to become a clinically useful tool in rehabilitation. Understanding the dynamic process of systems level reorganization will allow greater understanding of the mechanisms of recovery and potentially improve our ability to deliver effective restorative therapy.

Age-related changes in the topological architecture of the brain during hand grip

Neuroanatomical changes in the aging brain are widely distributed rather than focal. We investigated age-related changes in large-scale functional brain networks by applying graph theory to functional magnetic resonance imaging data acquired during a simple grip task with either dominant or nondominant hand. We measured the effect of age on efficiency of information transfer within a series of hierarchical functional networks composed of the whole brain or component parts of the whole brain. Global efficiency was maintained with aging during dominant hand use, primarily due to increased efficiency in parietal-occipital-cerebellar-related networks. During nondominant hand use, global efficiency, as well as efficiency within ipsilateral hemisphere and between hemispheres declined with age. This was attributable largely to frontal-temporal-limbic-cerebellar-related networks. Increased efficiency with age was seen in networks involving parietal-occipital regions, but unlike for dominant hand use, this topological reconfiguration could not maintain the level of global efficiency. Here, graph theoretical approaches have demonstrated both compensatory and noncompensatory changes in topological configuration of large-scale networks during aging depending on the task.

Evidence for motor cortex dedifferentiation in older adults

Older adults (OA) show more diffuse brain activity than young adults (YA) during the performance of cognitive, motor, and perceptual tasks. It is unclear whether this overactivation reflects compensation or dedifferentiation. Typically, these investigations have not evaluated the organization of the resting brain, which can help to determine whether more diffuse representations reflect physiological or task-dependent effects. In the present study we used transcranial magnetic stimulation (TMS) to determine whether there are differences in motor cortex organization of both brain hemispheres in young and older adults. We measured resting motor threshold, motor evoked potential (MEP) latency and amplitude, and extent of first dorsal interosseous representations, in addition to a computerized measure of reaction time. There was no significant age difference in motor threshold, but we did find that OA had larger contralateral MEP amplitudes and a longer contralateral MEP latency. Furthermore, the spatial extent of motor representations in OA was larger. We found that larger dominant hemisphere motor representations in OA were associated with higher reaction times, suggesting dedifferentiation rather than compensation effects.

Age differences in spatial working memory contributions to visuomotor adaptation and transfer

Throughout our life span we encounter challenges that require us to adapt to the demands of our changing environment; this entails learning new skills. Two primary components of motor skill learning are motor acquisition, the initial process of learning the skill, and motor transfer, when learning a new skill is benefitted by the overlap with a previously learned one. Older adults typically exhibit declines in motor acquisition compared to young adults, but remarkably, do not demonstrate deficits in motor transfer [10]. Our recent work demonstrates that a failure to engage spatial working memory (SWM) is associated with skill learning deficits in older adults [16]. Here, we investigate the role that SWM plays in both motor learning and transfer in young and older adults. Both age groups exhibited performance savings, or positive transfer, at transfer of learning for some performance variables. Measures of spatial working memory performance and reaction time correlated with both motor learning and transfer for young adults. Young adults recruited overlapping brain regions in prefrontal, premotor, parietal and occipital cortex for performance of a SWM and a visuomotor adaptation task, most notably during motor learning, replicating our prior findings [12]. Neural overlap between the SWM task and visuomotor adaptation for the older adults was limited to parietal cortex, with minimal changes from motor learning to transfer. Combined, these results suggest that age differences in engagement of cognitive strategies have a differential impact on motor learning and transfer.

Disability, atrophy and cortical reorganization following spinal cord injury

The impact of traumatic spinal cord injury on structural integrity, cortical reorganization and ensuing disability is variable and may depend on a dynamic interaction between the severity of local damage and the capacity of the brain for plastic reorganization. We investigated trauma-induced anatomical changes in the spinal cord and brain, and explored their relationship to functional changes in sensorimotor cortex. Structural changes were assessed using cross-sectional cord area, voxel-based morphometry and voxel-based cortical thickness of T₁-weighted images in 10 subjects with cervical spinal cord injury and 16 controls. Cortical activation in response to right-sided (i) handgrip; and (ii) median and tibial nerve stimulation were assessed using functional magnetic resonance imaging. Regression analyses explored associations between cord area, grey and white matter volume, cortical activations and thickness, and disability. Subjects with spinal cord injury had impaired upper and lower limb function bilaterally, a 30% reduced cord area, smaller white matter volume in the pyramids and left cerebellar peduncle, and smaller grey matter volume and cortical thinning in the leg area of the primary motor and sensory cortex compared with controls. Functional magnetic resonance imaging revealed increased activation in the left primary motor cortex leg area during handgrip and the left primary sensory cortex face area during median nerve stimulation in subjects with spinal cord injury compared with controls, but no increased activation following tibial nerve stimulation. A smaller cervical cord area was associated with impaired upper limb function and increased activations with handgrip and median nerve stimulation, but reduced activations with tibial nerve stimulation. Increased sensory deficits were associated with increased activations in the left primary sensory cortex face area due to median nerve stimulation. In conclusion, spinal cord injury leads to cord atrophy, cortical atrophy of primary motor and sensory cortex, and cortical reorganization of the sensorimotor system. The degree of cortical reorganization is predicted by spinal atrophy and is associated with significant disability.

Deficit of sensorimotor integration in normal aging

Sensorimotor performance declines with normal aging. The present study explored age-related changes in sensorimotor integration by conditioning a supra-threshold transcranial magnetic stimulation pulse with a peripheral nerve shock at different interstimulus intervals. Cortical motor threshold of the abductor pollicis brevis muscle, intracortical inhibition and facilitation were measured. We also assessed the influence of median nerve stimulation on motor cortex excitability at intervals which evoked short- and long-latency afferent inhibition (SAI and LAI, respectively) and afferent-induced facilitation (AIF). We observed a marked decrease of the long latency influence of proprioceptive inputs on M1 excitability in the elderly, with the loss of AIF and LAI. The SAI, motor thresholds and intracortical inhibition and facilitation were not age-related. Decreased sensorimotor performance with aging appears to be associated with a decrease in the influence of proprioceptive inputs on motor cortex excitability at longer intervals (probably via higher order cortical areas).

Central adaptations to repetitive grasping in healthy aging

Augmented cortical activity during repetitive grasping mitigates repetition-related decrease in cortical efficiency in young adults. It is unclear if similar processes occur with healthy aging. We recorded movement-related cortical potentials (MRCP) during 150 repetitive handgrip contractions at 70% of maximal voluntary contraction (MVC) in healthy young (n = 10) and old (n = 10) adults. Repetitions were grouped into two Blocks (Block 1 and 2: repetitions 1-60 and 91-150, respectively) and analyzed separately to assess the effects of aging and block. EMG of the flexor digitorum superficialis and handgrip force were also recorded. No changes in EMG or MVC were observed across blocks for either group. Significant interactions (P < 0.05) were observed for MRCPs recorded from mesial (FCz, Cz, CPz) and motor (C1, C3, Cz) electrode sites, with younger adults demonstrating significant increases in MRCP amplitude. Focal MRCP activity in response to repetitive grasping resulted in minimal changes (i.e. Block 1 versus Block 2) in older adults. Central adaptive processes change across the lifespan, showing increasingly less focal activation in older adults during repetitive grasping. Our findings are consistent with previous paradigms demonstrating more diffuse cortical activation during motor tasks in older adults.

Convergence of human brain mapping tools: neuronavigated TMS parameters and fMRI activity in the hand motor area

Functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) are well-established tools for investigating the human motor system in-vivo. We here studied the relationship between movement-related fMRI signal changes in the primary motor cortex (M1) and electrophysiological properties of the hand motor area assessed with neuronavigated TMS in 17 healthy subjects. The voxel showing the highest task-related BOLD response in the left hand motor area during right hand movements was identified for each individual subject. This fMRI peak voxel in M1 served as spatial target for coil positioning during neuronavigated TMS. We performed correlation analyses between TMS parameters, BOLD signal estimates and effective connectivity parameters of M1 assessed with dynamic causal modeling (DCM). The results showed a negative correlation between the movement-related BOLD signal in left M1 and resting as well as active motor threshold (MT) obtained for left M1. The DCM analysis revealed that higher excitability of left M1 was associated with a stronger coupling between left supplementary motor area (SMA) and M1. Furthermore, BOLD activity in left M1 correlated with ipsilateral silent period (ISP), i.e. the stronger the task-related BOLD response in left M1, the higher interhemispheric inhibition effects targeting right M1. DCM analyses revealed a positive correlation between the coupling of left SMA with left M1 and the duration of ISP. The data show that TMS parameters assessed for the hand area of M1 do not only reflect the intrinsic properties at the stimulation site but also interactions with remote areas in the human motor system.

Physical activity and neural correlates of aging: a combined TMS/fMRI study

Aerobic exercise has been suggested to ameliorate aging-related decline in humans. Recently, evidence has indicated chronological aging is associated with decreases in measures of interhemispheric inhibition during unimanual movements, but that such decreases may be mitigated by long-term physical fitness. The present study investigated measures of ipsilateral (right) primary motor cortex activity during right-hand movements using functional magnetic resonance imaging and transcranial magnetic stimulation (TMS). Healthy, right-handed participant groups were comprised of 12 sedentary older adults, 12 physically active older adults, and 12 young adults. Active older adults and younger adults evidenced longer ipsilateral silent periods (iSP) and less positive BOLD of ipsilateral motor cortex (iM1) as compared to sedentary older adults. Across groups, duration of iSP from TMS was inversely correlated with BOLD activity in iM1 during unimanual movement. These findings suggest that increased physical activity may have a role in decreasing aging-related losses of interhemispheric inhibition.

Age differences in interhemispheric interactions: callosal structure, physiological function, and behavior

There is a fundamental gap in understanding how brain structural and functional network connectivity are interrelated, how they change with age, and how such changes contribute to older adults’ sensorimotor deficits. Recent neuroimaging approaches including resting state functional connectivity MRI (fcMRI) and diffusion tensor imaging (DTI) have been used to assess brain functional (fcMRI) and structural (DTI) network connectivity, allowing for more integrative assessments of distributed neural systems than in the past. Declines in corpus callosum size and microstructure with advancing age have been well documented, but their contributions to age deficits in unimanual and bimanual function are not well defined. Our recent work implicates age-related declines in callosal size and integrity as a key contributor to unimanual and bimanual control deficits. Moreover, our data provide evidence for a fundamental shift in the balance of excitatory and inhibitory interhemispheric processes that occurs with age, resulting in age differences in the relationship between functional and structural network connectivity. Training studies suggest that the balance of interhemispheric interactions can be shifted with experience, making this a viable target for future interventions.

Excitability of the motor cortex ipsilateral to the moving body side depends on spatio-temporal task complexity and hemispheric specialization

Unilateral movements are mainly controlled by the contralateral hemisphere, even though the primary motor cortex ipsilateral (M1_ipsi) to the moving body side can undergo task-related changes of activity as well. Here we used transcranial magnetic stimulation (TMS) to investigate whether representations of the wrist flexor (FCR) and extensor (ECR) in M1_ipsi would be modulated when unilateral rhythmical wrist movements were executed in isolation or in the context of a simple or difficult hand-foot coordination pattern, and whether this modulation would differ for the left versus right hemisphere. We found that M1_ipsi facilitation of the resting ECR and FCR mirrored the activation of the moving wrist such that facilitation was higher when the homologous muscle was activated during the cyclical movement. We showed that this ipsilateral facilitation increased significantly when the wrist movements were performed in the context of demanding hand-foot coordination tasks whereas foot movements alone influenced the hand representation of M1_ipsi only slightly. Our data revealed a clear hemispheric asymmetry such that MEP responses were significantly larger when elicited in the left M1_ipsi than in the right. In experiment 2, we tested whether the modulations of M1_ipsi facilitation, caused by performing different coordination tasks with the left versus right body sides, could be explained by changes in short intracortical inhibition (SICI). We found that SICI was increasingly reduced for a complex coordination pattern as compared to rest, but only in the right M1_ipsi. We argue that our results might reflect the stronger involvement of the left versus right hemisphere in performing demanding motor tasks.

Association between muscle activation and metabolic cost of walking in young and old adults

BACKGROUND

The net metabolic cost of walking (C(w)) as well as the level of neural activation of agonist and antagonist leg muscles are higher in healthy old compared with young adults. This study examined the association between C(w) and agonist muscle activity and antagonist coactivity in young and old adults.

METHODS

Young and old adults walked at 0.98 m/s on a treadmill set at 6% decline, level, and 6% incline, while C(w) and neural activation of leg muscles were measured.

RESULTS

C(w) was 7.0% (incline), 19.2% (level), and 47.3% (decline) higher in old adults (overall 18.3%). Old (67.1%) versus young (40.1%) adults activated their leg muscles 67.3% more during the gait tasks and had 152.8% higher antagonist muscle coactivation (old: 67.1%, young: 19.9%). Agonist muscle activation was unrelated to C(w) on incline, but it explained up to 42% (level), 48% (decline), and 70% (three tasks combined) of variance in C(w). Antagonist coactivation accounted for up to 41% (incline), 45% (level), 59% (decline), 39% (three tasks combined) of variance in C(w).

CONCLUSIONS

Age-related adaptations in the recruitment pattern of leg muscles during gait significantly contribute to the high C(w) in old adults. Clinical interventions optimizing the neural control of leg muscles during gait could reduce C(w) consequently the relative effort needed for exercise and activities of daily living in old adults.

Non-invasive brain stimulation: enhancing motor and cognitive functions in healthy old subjects

Healthy aging is accompanied by changes in cognitive and motor functions that result in impairment of activities of daily living. This process involves a number of modifications in the brain and is associated with metabolic, structural, and physiological changes; some of these serving as adaptive responses to the functional declines. Up to date there are no universally accepted strategies to ameliorate declining functions in this population. An essential basis to develop such strategies is a better understanding of neuroplastic changes during healthy aging. In this context, non-invasive brain stimulation techniques, such as transcranial direct current or transcranial magnetic stimulation, provide an attractive option to modulate cortical neuronal assemblies, even with subsequent changes in neuroplasticity. Thus, in the present review we discuss the use of these techniques as a tool to study underlying cortical mechanisms during healthy aging and as an interventional strategy to enhance declining functions and learning abilities in aged subjects.

Absence of cross-limb transfer of performance gains following ballistic motor practice in older adults

The phenomenon of cross-limb transfer, in which unilateral strength training can result in bilateral strength gains, has recently been tested for ballistic movements. Performance gains associated with repetitive motor practice, and the associated transfer, occur within a few minutes. In this study, young and older adults were trained to perform ballistic abductions of their dominant (right) index finger as quickly as possible. Performance was assessed bilaterally before, during, and after this training. Both groups exhibited large performance gains in the right hand as a result of training (P < 0.001; young 84% improvement, older 70% improvement), which were not significantly different between groups (P = 0.40). Transcranial magnetic stimulation revealed that the performance improvements were accompanied by increases in excitability, together with decreases in intracortical inhibition, of the projections to both the trained muscle and the homologous muscle in the contralateral limb (P < 0.05). The young group also exhibited performance improvements as a result of cross-limb transfer in the left (untrained) hand (P < 0.005), equivalent to 75% of the performance increase in the trained hand. In contrast, there were no significant performance gains in the left hand for the older group (P = 0.23). This was surprising given that the older group exhibited a significantly greater degree of mirror activity than the young group (P < 0.01) in the left first dorsal interosseus muscle (FDI) during right hand movements. Our findings suggest that older adults exhibit a reduced capacity for cross-limb transfer, which may have implications for motor rehabilitation programs after stroke.

Differential callosal contributions to bimanual control in young and older adults

Our recent work has shown that older adults are disproportionately impaired at bimanual tasks when the two hands are moving out of phase with each other [Bangert, A. S., Reuter-Lorenz, P. A., Walsh, C. M., Schachter, A. B., & Seidler, R. D. Bimanual coordination and aging: Neurobehavioral implications. Neuropsychologia, 48, 1165-1170, 2010]. Interhemispheric interactions play a key role during such bimanual movements to prevent interference from the opposite hemisphere. Declines in corpus callosum (CC) size and microstructure with advancing age have been well documented, but their contributions to age deficits in bimanual function have not been identified. In the current study, we used structural magnetic resonance and diffusion tensor imaging to investigate age-related changes in the relationships between callosal macrostructure, microstructure, and motor performance on tapping tasks requiring differing degrees of interhemispheric interaction. We found that older adults demonstrated disproportionately poorer performance on out-of-phase bimanual control, replicating our previous results. In addition, older adults had smaller anterior CC size and poorer white matter integrity in the callosal midbody than their younger counterparts. Surprisingly, larger CC size and better integrity of callosal microstructure in regions connecting sensorimotor cortices were associated with poorer motor performance on tasks requiring high levels of interhemispheric interaction in young adults. Conversely, in older adults, better performance on these tasks was associated with larger size and better CC microstructure integrity within the same callosal regions. These findings implicate age-related declines in callosal size and integrity as a key contributor to bimanual control deficits. Further, the differential age-related involvement of transcallosal pathways reported here raises new questions about the role of the CC in bimanual control.

The role of contralesional dorsal premotor cortex after stroke as studied with concurrent TMS-fMRI

Contralesional dorsal premotor cortex (cPMd) may support residual motor function following stroke. We performed two complementary experiments to explore how cPMd might perform this role in a group of chronic human stroke patients. First, we used paired-coil transcranial magnetic stimulation (TMS) to establish the physiological influence of cPMd on ipsilesional primary motor cortex (iM1) at rest. We found that this influence became less inhibitory/more facilitatory in patients with greater clinical impairment. Second, we applied TMS over cPMd during functional magnetic resonance imaging (fMRI) in these patients to examine the causal influence of cPMd TMS on the whole network of surviving cortical motor areas in either hemisphere and whether these influences changed during affected hand movement. We confirmed that hand grip-related activation in cPMd was greater in more impaired patients. Furthermore, the peak ipsilesional sensorimotor cortex activity shifted posteriorly in more impaired patients. Critical new findings were that concurrent TMS-fMRI results correlated with the level of both clinical impairment and neurophysiological impairment (i.e., less inhibitory/more facilitatory cPMd-iM1 measure at rest as assessed with paired-coil TMS). Specifically, greater clinical and neurophysiological impairment was associated with a stronger facilitatory influence of cPMd TMS on blood oxygenation level-dependent signal in posterior parts of ipsilesional sensorimotor cortex during hand grip, corresponding to the posteriorly shifted sensorimotor activity seen in more impaired patients. cPMd TMS was not found to influence activity in other brain regions in either hemisphere. This state-dependent influence on ipsilesional sensorimotor regions may provide a mechanism by which cPMd supports recovered function after stroke.

Age effects on the asymmetry of the motor system: evidence from cortical oscillatory activity

Functional hemispheric asymmetry can be lost with aging. In this electroencephalographic study, we assessed hemispheric asymmetries in regulating motor responses by analyzing oscillatory brain activity during a go/nogo task in younger and older right-handed participants. Three conditions were embedded in the task: go, high-conflict and low-conflict nogo. The hand used to respond to go stimuli was varied block-wise. Independently of the go/nogo conditions and responding hand, young participants showed asymmetric desynchronizations in the mu (10 Hz) and beta (18-22 Hz) frequency bands that was stronger in the scalp sensorimotor region contralateral to the hand used for the go responses, while older adults showed a more symmetric pattern of desynchronization. These findings indicate that a loss of hemispheric asymmetry is a hallmark of the aging motor system, consistent with a decline of inter-hemispheric motor inhibition in normal aging.

Functional implications of age differences in motor system connectivity

Older adults show less lateralized task-related brain activity than young adults. One potential mechanism of this increased activation is that age-related degeneration of the corpus callosum (CC) may alter the balance of inhibition between the two hemispheres. To determine whether age differences in interhemispheric connectivity affect functional brain activity in older adults, we used magnetic resonance imaging (MRI) to assess resting functional connectivity and functional activation during a simple motor task. We found that older adults had smaller CC area compared to young adults. Older adults exhibited greater recruitment of ipsilateral primary motor cortex (M1), which was associated with longer reaction times. Additionally, recruitment of ipsilateral M1 in older adults was correlated with reduced resting interhemispheric connectivity and a larger CC. We suggest that reduced interhemispheric connectivity reflects a loss of the ability to inhibit the non-dominant hemisphere during motor task performance for older adults, which has a negative impact on performance.

Functional magnetic resonance imaging reveals age-related alterations to motor networks in weighted elbow flexion-extension movement

OBJECTIVE

we investigated whether motor system activity in normal aging shows age-related alteration during a non-isomeric elbow flexion-extension movement task using weights.

METHODS

Twenty-six healthy, right-handed elderly and 20 healthy, right-handed young subjects without any psychiatric, neurological, or medical disease participated in this study. All subjects underwent two consecutive scanning sessions: one without weights, and one with weights. During the weights session, each subject held a small non-metallic bar (weighing approximately 1 kg) with their dominant hand and performed elbow flexion and extension movements. Functional magnetic resonance imaging BOLD contrast was obtained for each subject using a 3.0 T MRI scanner. Image processing and statistical analyses were carried out using SPM2. In fMRI data group analysis, contrast images from the analysis of individual subjects were analysed by one-sample t-tests, thereby generating a random-effects model, allowing inference to the general population. The SPM{t}s were thresholded at P<0.01, false discovery rate (FDR) corrected for multiple comparisons across the whole brain. Finally, the resulting activation maps were created and displayed by projection onto the anatomically standardized mean T1 image of all subjects to identify the anatomical correlates of the activity.

RESULTS

It was revealed that the main change in the aging brain was significant activation of the ipsilateral basal ganglia-thalamo-cortical motor loop in older subjects, suggesting the recruitment of additional brain areas during the execution of a weighted elbow motor task as a compensation process for age-related neurobiological change.

CONCLUSION

The current study is the first to demonstrate significant differences in brain activation between old and young subjects during weighted elbow flexion-extension movement when both the old and young groups maintain the same performance level. In particular, overactivation of the basal ganglia in the aging brain appears to play a crucial role in counteracting age-related decline of force generation and to support the same level of performance as that of younger counterparts.

Tactile-dependant corticomotor facilitation is influenced by discrimination performance in seniors

Background

Active contraction leads to facilitation of motor responses evoked by transcranial magnetic stimulation (TMS). In small hand muscles, motor facilitation is known to be also influenced by the nature of the task. Recently, we showed that corticomotor facilitation was selectively enhanced when young participants actively discriminated tactile symbols with the tip of their index or little finger. This tactile-dependant motor facilitation reflected, for the large part, attentional influences associated with performing tactile discrimination, since execution of a concomitant distraction task abolished facilitation. In the present report, we extend these observations to examine the influence of age on the ability to produce extra motor facilitation when the hand is used for sensory exploration.

Methods

Corticomotor excitability was tested in 16 healthy seniors (58-83 years) while they actively moved their right index finger over a surface under two task conditions. In the tactile discrimination (TD) condition, participants attended to the spatial location of two tactile symbols on the explored surface, while in the non discrimination (ND) condition, participants simply moved their finger over a blank surface. Changes in amplitude, in latency and in the silent period (SP) duration were measured from recordings of motor evoked potentials (MEP) in the right first dorsal interosseous muscle in response to TMS of the left motor cortex.

Results

Healthy seniors exhibited widely varying levels of performance with the TD task, older age being associated with lower accuracy and vice-versa. Large inter-individual variations were also observed in terms of tactile-specific corticomotor facilitation. Regrouping seniors into higher (n = 6) and lower performance groups (n = 10) revealed a significant task by performance interaction. This latter interaction reflected differences between higher and lower performance groups; tactile-related facilitation being observed mainly in the former group. Latency measurements and SP durations were not affected by task conditions.

Conclusions

The present findings provide further insights into the factors influencing task-dependant changes in corticomotor excitability in the context of aging. Our results, in particular, highlight the importance of adjusting task demands and controlling for attention when attempting to elicit task-specific motor facilitation in older persons engaged in fine manual actions. Such information could be critical in the future for planning interventions to re-educate or maintain hand function in the presence of neurological impairments.

Facilitating skilled right hand motor function in older subjects by anodal polarization over the left primary motor cortex

Healthy ageing is accompanied by limitations in performance of activities of daily living and personal independence. Recent reports demonstrated improvements in motor function induced by noninvasive anodal direct current stimulation (tDCS) of the primary motor cortex (M1) in young healthy adults. Here we tested the hypothesis that a single session of anodal tDCS over left M1 could facilitate performance of right upper extremity tasks required for activities of daily living (Jebsen-Taylor hand function test, JTT) in older subjects relative to Sham in a double-blind cross-over study design. We found (a) significant improvement in JTT function with tDCS relative to Sham that outlasted the stimulation period by at least 30 min, (b) that the older the subjects the more prominent this improvement appeared and (c) that consistent with previous results in younger subjects, these effects were not accompanied by any overt undesired side effect. We conclude that anodal tDCS applied over M1 can facilitate performance of skilled hand functions required for activities of daily living in older subjects.

Unimanual muscle activation increases interhemispheric inhibition from the active to the resting hemisphere

Interhemispheric inhibition (IHI) is an important mechanism to maximize the independent functioning of each hemisphere and is most likely mediated by transcallosal fibres. IHI can be investigated by paired pulse transcranial magnetic stimulation (TMS) whereby, in half of the trials, a test stimulus (TS) over one hemisphere is preceded by a conditioning stimulus (CS) over the other hemisphere. Whereas various studies have investigated IHI in rest, less is known about interhemispheric interactions during voluntary muscle activation. Here, we investigated the influence of tonic muscle activity (5% of the maximal voluntary contraction) in either the right wrist flexor or extensor versus rest on IHI from the active (left) to the resting (right) hemisphere. Our main finding was that tonic activation of the right wrist flexor, led to an increase in IHI from the active (dominant left) to the resting (non-dominant right) hemisphere as compared to rest. A control experiment employed the same design but CS intensity was lowered to match MEP amplitudes of the conditioning hand between active and rest conditions. This resulted in a relative decrease of IHI. It is hypothesized that functional regulation of IHI might prevent the occurrence of mirror activity in the primary motor cortex (M1) of the resting hemisphere and, thus, might play an important role in the execution of unimanual actions.