Interactions between the aging brain and motor task complexity across the lifespan: balancing brain activity resource demand and supply.

The Compensation Related Utilization of Neural Circuits Hypothesis (CRUNCH) proposes a framework for understanding task-related brain activity changes as a function of healthy aging and task complexity. Specifically, it affords the following predictions: (i) all adult age groups display more brain activation with increases in task complexity, (ii) older adults show more brain activation compared with younger adults at low task complexity levels, and (iii) disproportionately increase brain activation with increased task complexity, but (iv) show smaller (or no) increases in brain activation at the highest complexity levels. To test these hypotheses, performance on a bimanual tracking task at 4 complexity levels and associated brain activation were assessed in 3 age groups (20-40, 40-60, and 60-80 years, n = 99). All age groups showed decreased tracking accuracy and increased brain activation with increased task complexity, with larger performance decrements and activation increases in the older age groups. Older adults exhibited increased brain activation at a lower complexity level, but not the predicted failure to further increase brain activity at the highest complexity level. We conclude that older adults show more brain activation than younger adults and preserve the capacity to deploy increased neural resources as a function of task demand.

Task-related measures of short-interval intracortical inhibition and GABA levels in healthy young and older adults: A multimodal TMS-MRS study.

Establishing the associations between magnetic resonance spectroscopy (MRS)-assessed gamma-aminobutyric acid (GABA) levels and transcranial magnetic stimulation (TMS)-derived 'task-related' modulations in GABAA receptor-mediated inhibition and how these associations change with advancing age is a topic of interest in the field of human neuroscience. In this study, we identified the relationship between GABA levels and task-related modulations in GABAA receptor-mediated inhibition in the dominant (left) and non-dominant (right) sensorimotor (SM) cortices. GABA levels were measured using edited MRS and task-related GABAA receptor-mediated inhibition was measured using a short-interval intracortical inhibition (SICI) TMS protocol during the preparation and premotor period of a choice reaction time (CRT) task in 25 young (aged 18-33 years) and 25 older (aged 60-74 years) adults. Our results demonstrated that GABA levels in both SM voxels were lower in older adults as compared to younger adults; and higher SM GABA levels in the dominant as compared to the non-dominant SM voxel pointed to a lateralization effect, irrespective of age group. Furthermore, older adults showed decreased GABAA receptor-mediated inhibition in the preparation phase of the CRT task within the dominant primary motor cortex (M1), as compared to young adults. Finally, results from an exploratory correlation analysis pointed towards positive relationships between MRS-assessed GABA levels and TMS-derived task-related SICI measures. However, after correction for multiple comparisons none of the correlations remained significant.

Age-Related Declines in Motor Performance are Associated With Decreased Segregation of Large-Scale Resting State Brain Networks.

Aging is typically associated with substantial declines in motor functioning as well as robust changes in the functional organization of brain networks. Previous research has investigated the link between these 2 age-varying factors but examinations were predominantly limited to the functional organization within motor-related brain networks. Little is known about the relationship between age-related behavioral impairments and changes in functional organization at the whole brain (i.e., multiple network) level. This knowledge gap is surprising given that the decreased segregation of brain networks (i.e., increased internetwork connectivity) can be considered a hallmark of the aging process. Accordingly, we investigated the association between declines in motor performance across the adult lifespan (20-75 years) and age-related modulations of functional connectivity within and between resting state networks. Results indicated that stronger internetwork resting state connectivity observed as a function of age was significantly related to worse motor performance. Moreover, performance had a significantly stronger association with the strength of internetwork as compared with intranetwork connectivity, including connectivity within motor networks. These findings suggest that age-related declines in motor performance may be attributed to a breakdown in the functional organization of large-scale brain networks rather than simply age-related connectivity changes within motor-related networks.

The effect of anodal transcranial direct current stimulation on multi-limb coordination performance.

Motor coordination is the combination of body movements performed in a well-planned and controlled manner based upon motor commands from the brain. Several interventions have been in practice to improve motor control. Transcranial direct current stimulation (tDCS) is getting a lot of attention these days for its effect in improving motor functions. Studies focusing on the ability of tDCS to improve motor control, inhibition and coordination are sparse. Therefore, the influence of tDCS stimulation at the right dorsolateral prefrontal cortex (DLPFC) on motor control and coordination was investigated, in a sham-controlled double-blinded pseudo-randomized design, with a multi-limb coordination task in healthy young subjects. Number of errors and reaction time were used as outcome parameters. Our findings showed that, anodal tDCS reduced the number of errors only in the heterolateral coordination condition, however there was no change in reaction time. No changes were found for the homolateral and three-limb coordination conditions.

Bimanual motor deficits in older adults predicted by diffusion tensor imaging metrics of corpus callosum subregions.

Age-related changes in the microstructural organization of the corpus callosum (CC) may explain declines in bimanual motor performance associated with normal aging. We used diffusion tensor imaging in young (n = 33) and older (n = 33) adults to investigate the microstructural organization of seven specific CC subregions (prefrontal, premotor, primary motor, primary sensory, parietal, temporal and occipital). A set of bimanual tasks was used to assess various aspects of bimanual motor functioning: the Purdue Pegboard test, simultaneous and alternating finger tapping, a choice reaction time test and a complex visuomotor tracking task. The older adults showed age-related deficits on all measures of bimanual motor performance. Correlation analyses within the older group showed that white matter fractional anisotropy of the CC occipital region was associated with bimanual fine manipulation skills (Purdue Pegboard test), whereas better performance on the other bimanual tasks was related to higher fractional anisotropy in the more anterior premotor, primary motor and primary sensory CC subregions. Such associations were less prominent in the younger group. Our findings suggest that structural alterations of subregional callosal fibers may account for bimanual motor declines in normal aging.

Long-term TENS treatment decreases cortical motor representation in multiple sclerosis.

This study investigated the effects of a long-term transcutaneous electrical nerve stimulation (TENS) treatment on cortical motor representation in patients with multiple sclerosis (MS). In this double-blind crossover design, patients received either TENS or sham stimulation for 3 weeks (1h per day) on the median nerve region of the most impaired hand, followed by the other stimulation condition after a washout period of 6 months. Cortical motor representation was mapped using transcranial magnetic stimulation (TMS) at the baseline and after the 3-week stimulation protocol. Our results revealed that 3 weeks of daily stimulation with TENS significantly decreased the cortical motor representation of the stimulated muscle in MS patients. Although the mechanisms underlying this decrease remain unclear, our findings indicate that TENS has the ability to induce long-term reorganization in the motor cortex of MS patients.