Effects of transcranial direct current stimulation on the control of finger force during dexterous manipulation in healthy older adults

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.

Fig 1. The test object and the experimental procedure.

(A) Grip-lift object: Strain gauges embedded in the object measured grip force and load force at contact surfaces of both digits. The contact surfaces were covered either with acetate or sandpaper surface. (B) Experimental procedure: The intervention was delivered for 20 minutes. Subjects practiced a motor task during the intervention period.

Fig 2. Effects of tDCS on the grip force measured at lift-onset.

(A) Changes in the grip force (in %) following anodal and sham tDCS measured with respect to the baseline (pre) measurements for both acetate and sandpaper surfaces. Asterisk indicates p<0.025. # indicates p<0.1. (B) Subject-wise grip force (N) data averaged across acetate trials obtained before (pre) and after (post) anodal tDCS.

Fig 3. Inverse coefficient of friction for acetate and sandpaper surfaces measured before (pre) and after (post) anodal and sham tDCS.

Asterisk indicates p<0.001.