Effects of posterior parietal cortex anodal transcranial direct current stimulation on ankle tracking visuomotor control in healthy young adults

Ankle motor control is crucial for balance maintenance and fall prevention. Neurocomputational models of motor control suggest that the posterior parietal cortex (PPC) plays a critical role in estimating body and environmental states, a process fundamental to motor control. Anodal transcranial direct current stimulation (atDCS) has been shown to modulate cortical excitability and alter behaviors accordingly. This study explored the impact of atDCS over the PPC on ankle tracking visuomotor control using a motor adaptation research paradigm in healthy young adults. Thirty-eight participants were randomly assigned to either an atDCS or sham control group. All participants completed an ankle tracking experiment divided into three phases: pre-adaptation, adaptation, and re-adaptation, with each phase comprising eight blocks of five trials. During the experiment, each participant wore a sensor on the non-dominant foot and performed continuous dorsiflexion and plantarflexion movements to track a target cursor on a screen. Visual feedback of the foot position was provided, with a 1:1 feedback ratio in the pre- and re-adaptation phases and a 2.5:1 ratio in the adaptation phase to promote visual-motor remapping. The atDCS group received 20 min of 2 mA atDCS over the PPC during the adaptation phase. Tracking performance on each trial was measured as the root mean squared error (RMSE) between the target and actual movement trajectories. Both groups showed similar RMSEs in the pre-adaptation phase (p > 0.05). However, in the adaptation phase, the atDCS group demonstrated a significant reduction from block 1 to block 2 (p = 0.001, Cohen's d = 0.86) and maintained this improved performance in the following blocks, while the sham group showed no significant changes throughout this phase (p > 0.05). In the re-adaptation phase, both groups quickly returned to their pre-adaptation performance levels. These findings indicate that neither the atDCS nor the sham group adapted to the high visual feedback ratio. However, the early reduction in RMSE observed in the atDCS group suggests that atDCS over the PPC may transiently enhance ankle tracking visuomotor control under the heightened visual feedback ratio condition, resulting in short-term improvements. Future research is warranted to explore whether multiple atDCS sessions over the PPC could provide long-term benefits for lower extremity visuomotor control.