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.

Keywords: brain stimulation; motor learning; reaching behavior.

Figure 1.

The training chamber and video recording. A, An overhead view of the chamber apparatus for automated reach training. Each trial required the rat to traverse to the rear end of the chamber to trigger a sensor (infrared beam) that initiates automated loading of a food pellet on a pedestal into target position outside an aperture The pedestal is offset to the side of the opening to be sure it can only be reached with the preferred paw. B, Arrangement of three mirrors to capture the movement of the rats' paws in 3D. High-speed video recording is triggered by another sensor outside the opening.

Figure 2.

A, Schematic of the montage (Adapted from Tang, 2020). B, A cathode grid electrode implanted in the chest. C, Left, A conventional epicranial electrode holder with a radius of 3 mm; right, Ag/AgCl stimulation electrode. D, Left, Custom-made epicranial electrode holder made from dental cement; right, a stimulation electrode 3 × 3 mm² platinum plate. E, Recording shanks with two contacts per shank forming a rectangle of 1 × 1 mm to measure the E-field in 2D. F, Measurement of the electric field amplitude (combining vertical and horizontal components) at 10, 100, and 1,000 Hz at different current intensities. These were measured with different electrode holders in different animals (one animal per stimulation configuration). G, An anatomically detailed current-flow model of the current montage as described in Farahani et al. (2024). H, A model of electric field magnitudes when stimulating with a current of 150 µA applied continuously over the 20 min of stimulation. The increased intensity in the deeper tissue is due to increased resistivity assigned to this white matter structure.

Figure 3.

Effect of tDCS on motor skill learning. A, Experiment timeline from acclimatization, surgery, training, and MEP recording. B, The frontal image of the rat reaching for a food pellet (white). Mirrors on top, left, and right facilitate 3D recording of the paw motion with a single video camera. C, The number of successful reaching attempts across days of training. Lines indicate mean (and SEM) across animals in the anodal (red, N = 12) and control groups (blue, N = 12). D, The success rate across days of training.

Figure 4.

MEP. A, MEPs are measured with electrodes placed on the trained (green) and untrained wrist extensor muscles (black). The “trained” paw is the one with which the animal learned to grab the food pellet (yellow) through a narrow slit. The tDCS electrode is placed over the contralateral motor cortex of the trained paw during 10 d of training. In terminal MEP experiments, bipolar electrodes are placed in direct epidural contact over both left and right motor cortices (ipsi- and contralateral sides relative to recording electrodes). (Extended Data Fig. 4-1 shows examples of MEP traces). B, MEP amplitude measured in trained and untrained paw with ipsi- and contralateral epidural pulsed stimulation. Motor recruitment curves can be obtained by adjusting the pulse intensity as a percentage of motor thresholds. Lines indicate mean (and SEM) across animals in the anodal (red, N = 10) and control groups (blue, N = 9). C, MEP amplitude of trained and untrained paw in response to stimulation of the contralateral hemisphere at 140% of threshold. Each line is one animal. D, MEP amplitude at 140% threshold in the trained paw with contralateral epidural pulsed stimulation versus behavioral performance at the end of learning (number of successful reaching attempts on the 10th day). Each point is one animal.

Figure 5.

Effect of motor training and tDCS on stereotypy. A, A snapshot of a paw with the colored dots indicating the position labels. B, Sample trajectories for each digit in a single grab. C, Reaching trajectories of the middle digit (#3) across several trials in the first session for a right-pawed rat. D, Reaching trajectories of the middle finger across several trials in the last session for the same rat. E, Stereotypy across training sessions for right-pawed and left-pawed animals (solid and dashed lines, respectively; error bars indicate SEM). (Extended Data Fig. 5-2 shows reaching trajectories for 4 digits as a sample). F, The number of successes separating now by paw preference and stimulation conditions (red anodal tDCS and blue control). G, The same as F but for stereotypy. H, Schematic of the results of statistical analysis for right-pawed animals. Green is the planned primary outcome measure, and blue is the secondary measure (Extended Data Fig. 5-1 shows schematic of the results of statistical analysis including paw preference as a factor).