Repeated tDCS at Clinically Relevant Field Intensity Can Boost Concurrent Motor Learning in Rats

Abstract

Clinical trials with transcranial direct current stimulation (tDCS) use weak electric fields that have yet to demonstrate measurable behavioral effects in animal models. We hypothesized that weak stimulation will produce sizable effects, provided it is applied concurrently with behavioral training and repeated over multiple sessions. We tested this in a rodent model of dexterous motor skill learning using a pellet-reaching task in ad libitum behaving rats. The task was automated to minimize experimenter bias. We measured field magnitudes intracranially to calibrate the stimulation current. Male rats were trained for 20 min with concurrent epicranial tDCS over 10 daily sessions. We developed a new electrode montage that enabled stable stimulation over the 10 sessions with a field intensity of 2 V/m at the motor cortex. Behavior was recorded with high-speed video to quantify reaching dynamics. We also measured motor-evoked potentials (MEPs) bilaterally with epidural microstimulation. The number of successful reaches improved across days of training, and the rate of learning was higher in the anodal group as compared with sham-control animals ( F (1)  = 7.12; p  = 0.008; N  = 24). MEPs were not systematically affected by tDCS. Post hoc analysis suggests that tDCS modulated motor learning only for right-pawed animals, improving success of reaching but limiting stereotypy in these animals. Repeated and concurrent anodal tDCS can boost motor skill learning at clinically relevant field intensities. In this animal model, the effect interacted with paw preference and was not associated with corticospinal excitability.