Abstract
The contribution of poor finger force control to age-related decline in manual dexterity is above and beyond ubiquitous behavioral slowing. Altered control of the finger forces can impart unwanted torque on the object affecting its orientation, thus impairing manual performance. Anodal transcranial direct current stimulation (tDCS) over primary motor cortex (M1) has been shown to improve the performance speed on manual tasks in older adults. However, the effects of anodal tDCS over M1 on the finger force control during object manipulation in older adults remain to be fully explored. Here we determined the effects of anodal tDCS over M1 on the control of grip force in older adults while they manipulated an object with an uncertain mechanical property. Eight healthy older adults were instructed to grip and lift an object whose contact surfaces were unexpectedly made more or less slippery across trials using acetate and sandpaper surfaces, respectively. Subjects performed this task before and after receiving anodal or sham tDCS over M1 on two separate sessions using a cross-over design. We found that older adults used significantly lower grip force following anodal tDCS compared to sham tDCS. Friction measured at the finger-object interface remained invariant after anodal and sham tDCS. These findings suggest that anodal tDCS over M1 improved the control of grip force during object manipulation in healthy older adults. Although the cortical networks for representing objects and manipulative actions are complex, the reduction in grip force following anodal tDCS over M1 might be due to a cortical excitation yielding improved processing of object-specific sensory information and its integration with the motor commands for production of manipulative forces. Our findings indicate that tDCS has a potential to improve the control of finger force during dexterous manipulation in older adults.
Highlights
Aging often impairs the ability to perform dexterous manipulation
We found that the reduction in grip force measured at lift-onset from pre- to post-intervention was marginally different between anodal and sham transcranial direct current stimulation (tDCS) (-13.9% versus -1.02%; posthoc paired t-test: t7 = -1.95; p = 0.09; Fig 2A)
The novel finding from this study is that a single session of anodal transcranial direct current stimulation in healthy older adults significantly improved the control of grip force during object manipulation in an unpredictable environment
Summary
Aging often impairs the ability to perform dexterous manipulation. Poor finger force control seems likely to contribute to this impairment, and may partially explain ubiquitous age-related behavioral slowing [1,2]. Adults over 60 years demonstrate altered control of the magnitude and direction of finger forces applied to the object during functionally-relevant tasks [2,3,4,5]. Older adults usually apply greater grip force on the object than younger adults during object manipulation [3,6]. Exertion of unnecessarily greater grip force can be detrimental to task performance because small finger misalignments can lead to unwanted torque on the object, affecting its orientation [3]. Improving older adults’ ability to control grip forces may improve manual performance on skillful tasks
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