Registered report: Does transcranial direct current stimulation of the primary motor cortex improve implicit motor sequence learning in Parkinson's disease?

Abstract

Implicit motor sequence learning (IMSL) is a cognitive function that is known to be directly associated with impaired motor function in Parkinson's disease (PD).Research on healthy young participants shows the potential for transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technique, over the primary motor cortex (M1) to enhance IMSL. tDCS has direct effects on the underlying cortex, but also induces distant (basal ganglia) network effects-hence its potential value in PD, a prime model of basal ganglia dysfunction. To date, only null effects have been reported in persons with PD. However, these studies did not determine the reacquisition effects, although previous studies in healthy young adults suggest that tDCS specifically exerts its beneficial effects on IMSL on reacquisition rather than acquisition. In the current study, we will therefore establish possible reacquisition effects, which are of a particular interest, as long-term effects are vital for the successful functional rehabilitation of persons with PD. Using a sham-controlled, counterbalanced design, we will investigate the potential of tDCS delivered over M1 to enhance IMSL, as measured by the serial reaction time task, in persons with PD and a neurologically healthy age-and sex-matched control (HC) group. Multilevel Mixed Models will be implemented to analyze the sequence-specific aspect of IMSL (primary outcome) and general learning (secondary outcome). We will determine not only the immediate effects that may occur concurrently with the application of tDCS but also the short-term (5 min post-tDCS) and long-term (1 week post-tDCS) reacquisition effects.

Differential effects of conventional and high‐definition transcranial direct‐current stimulation of the motor cortex on implicit motor sequence learning

“…An electrode angle orientation of 45° (i.e., with 45° deviation from the midsagittal plane) was used as this positioning induces superior neuroplastic effects of M1 due to a better alignment with the motor cortex (Foerster et al, 2019). The reference electrode was positioned on the supraorbital area, ipsilateral to the dominant hand (Firouzi, Baetens, et al, 2021).…”

“…(right) The response keys C, V, B, and N were the only visible keys, and all other keys were covered. Taken from Firouzi, Baetens, et al (2021).…”

“…The SRT task, identical to Deroost et al (2018), Firouzi, Baetens, et al (2021), andFirouzi, Van Herk, et al (2021), was performed on laptops using E-Prime ® software (Psychology Software Tools, Inc., Pittsburgh, PA, USA). See Figure 2 for a schematic representation of the task.…”

“…Direct current was delivered through square rubber electrodes placed in rectangular saline-soaked sponges (size 35 cm 2 ). To stimulate M1, electrodes were placed over C3 or C4 according to the international 10-20 electroencephalogram (EEG) system, matching with M1 contralateral to the performing, dominant hand (Firouzi, Baetens, et al, 2021;Klem et al, 1999). An electrode angle orientation of 45 F I G U R E 1 Simulation of electric field strengths (V/m) using a 1 Â 1 conventional C3/Fp2 (top) and 4 * 1 HD C3/Cz/F3/T7/P3 (bottom) montage on a standard MNI 152 head.…”

“…This effect represents the overall improvement in motor performance and reflects more general aspects of motor skill acquisition. However, the relative increase in RTs when the sequence is interrupted (i.e., in a random block) relative to preceding and subsequent sequential blocks denotes sequence‐specific learning (primary outcome) (Firouzi, Baetens, et al, 2021). Investigating sequence‐specific learning effects (SSLE) allows us to assess the impact of tDCS on the acquisition and consolidation of the imposed motor sequence.…”