Site Dependency of Anodal Transcranial Direct-Current Stimulation on Reaction Time and Transfer of Learning during a Sequential Visual Isometric Pinch Task

Abstract

Considering the advantages of brain stimulation techniques in detecting the role of different areas of the brain in human sensorimotor behaviors, we used anodal transcranial direct-current stimulation (a-tDCS) over three different brain sites of the frontoparietal cortex (FPC) in healthy participants to elucidate the role of these three brain areas of the FPC on reaction time (RT) during a sequential visual isometric pinch task (SVIPT). We also aimed to assess if the stimulation of these cortical sites affects the transfer of learning during SVIPT. A total of 48 right-handed healthy participants were randomly assigned to one of the four a-tDCS groups: (1) left primary motor cortex (M1), (2) left dorsolateral prefrontal cortex (DLPFC), (3) left posterior parietal cortex (PPC), and (4) sham. A-tDCS (0.3 mA, 20 min) was applied concurrently with the SVIPT, in which the participants precisely controlled their forces to reach seven different target forces from 10 to 40% of the maximum voluntary contraction (MVC) presented on a computer screen with the right dominant hand. Four test blocks were randomly performed at the baseline and 15 min after the intervention, including sequence and random blocks with either hand. Our results showed significant elongations in the ratio of RTs between the M1 and sham groups in the sequence blocks of both the right-trained and left-untrained hands. No significant differences were found between the DLPFC and sham groups and the PPC and sham groups in RT measurements within the SVIPT. Our findings suggest that RT improvement within implicit learning of an SVIPT is not mediated by single-session a-tDCS over M1, DLPFC, or PPC. Further research is needed to understand the optimal characteristics of tDCS and stimulation sites to modulate reaction time in a precision control task such as an SVIPT.

Keywords: a-tDCS; anodal transcranial direct-current stimulation; dorsolateral prefrontal cortex; posterior parietal cortex; primary motor cortex; reaction time; transfer learning.

Figure 1

CONSORT flow diagram.

Figure 2

Experimental set up. The participants were instructed to squeeze a force transducer as precisely as possible to reach each target force that appeared on the computer screen. Each sequence block consisted of eight trials, which included seven different target forces from 10 to 40% of their MVC. In a sequence block, the target forces appeared in a sequence order (10, 35, 20, 40, 25, 15, and 30% of the MVC), while the target forces were randomly presented in a random block. They were asked to complete each block as quickly and accurately as possible. The RT was measured as a temporal variable for each target force. SVIPT: sequential visual isometric pinch task; A-tDCS: anodal transcranial direct-current stimulation; M1: primary motor cortex; DLPFC: dorsolateral prefrontal cortex; PPC: posterior parietal cortex; S: sequence block; R: random block; RT: reaction time; SR: sequence right; SL: sequence left; RR: random right; and RL: random left.

Figure 3

The mean rank of the RT ratio in the sequential right hand block assessment test among four tDCS stimulation sites (M1, DLPFC, PPC, and sham) (* p < 0.05, ** p < 0.01).

Figure 4

The mean rank ratio RT in the sequence left hand block assessment test among four tDCS stimulation sites (M1, DLPFC, PPC, and sham) (* p < 0.05, ** p < 0.01).

Figure 5

The mean rank ratio of the RT in the random right-hand block assessment test among four tDCS stimulation sites (M1, DLPFC, PPC, and sham) (* p < 0.05, ** p < 0.01).

Figure 6

Mean rank ratio RT in the random left-hand block assessment test among four tDCS stimulation sites (M1, DLPFC, PPC, and sham).