Enhancing motor learning in healthy older adults through non-invasive brain stimulation

Transcranial direct current stimulation (tDCS) is a technique that can increase
excitability of the cortex. It has been used in many studies, including young
and older adults (YA and OA), to enhance cognitive performance. In the light of
the current aging population, enhancing cognition in healthy OA is a promising
direction of research. Unfortunately, results have not been consistent and
differential findings between YA and OA are understood poorly. Accordingly,
there has been a call to advance the field by taking a more systematic
approach. Therefore, we aim to conduct a study that replicates an important
existing result, namely, that tDCS can enhance motor (sequence) learning in OA.
To better understand previous findings of differential results between age
groups, we will extend the existing study with an extra learning condition that
will allow us to tear apart contributions of tDCS to task-general and
sequence-specific learning in OA and YA. Because higher age is often related to
reduced dexterity, we expect a larger effect of tDCS on task-general learning
in OA compared to YA.

We focus on a previous study showing enhanced motor sequence learning
performance in OA when coupled with active tDCS stimulation. In their study,
Zimerman et al. (2013) found a facilitating effect of M1 anodal tDCS on
sequence learning in OA, these effects were still present during retention
tests. Surprisingly, they found no effect of atDCS on learning in YA and no
learning at all in the OA sham control condition. The Zimerman et al. (2013)
study warrants replication in order to: 1) confirm the important main result of
facilitation of sequence learning in OA; 2) confirm the unexpected findings
regarding the YA and sham conditions; and 3) to potentially extend these
results by testing additional hypotheses. Specifically, an opportunity to
extend on the previous findings is to include random sequences in the practice
and retention sessions. This allows us to distinguish between tDCS
contributions to task-general versus sequence-specific learning.

The study follows a two (Age Group: YA versus OA) * two (tDCS Condition: atDCS
versus sham, within subjects) design. tDCS Conditions will be separated by at
least 10 days and blinded to participants and experiment leader.

We will treat participants with a Neuroelectrics Starstim tCS, a class IIa
device according to the classification in the Council Directive 93/42/CEE for
medical devices. TDCS will be delivered through rubber electrodes housed in 25
cm2 saline-soaked sponges. In both stimulation conditions, the anode electrode
will be placed above the hand area of the left primary motor cortex (C3 will be
localized using the 10-20 EEG system); the cathode electrode will be placed on
the right supraorbital region. Impedance is checked before and during
stimulation to ensure good contact quality of the sponges with the scalp. The
stimulation will be started at the onset of the practice phase (practice block
1) of the sequence-learning task. For both stimulation conditions, the current
will be ramped up to 1 mA (0.04 mA / cm2) over 8 seconds. In the sham control
stimulation condition, the current will be delivered for 30 seconds before
being ramped down. In the stimulation condition, the current will be delivered
for 20 minutes before being ramped down.

The burden and risks associated with the behavioural aspects (visiting the lab;
administering questionnaires and computer task) of this study are low. The
applied tDCS intervention is well tolerated and has generally been associated
only with minor adverse effects in healthy humans, such as headache, moderate
fatigue, nausea, and an itching sensation as well as skin irritation under the
electrodes. Regarding tDCS, standardized protocols and considerable literature
including clinical studies exists, and a recent review of 172 articles
concludes that *tDCS is a safe technique when used in 1*2 sessions for healthy
volunteers* (Brunoni et al., 2011). The participants will not have any direct
personal benefit from participation; the main benefit is an increased
fundamental understanding of the possibilities to enhance motor learning in
different age groups. In the light of the current aging population, enhancing
cognition in healthy OA is a highly relevant and promising direction of
research. An example of a potential specific application of insights in the
effects of tDCS on motor learning is for example improved re-learning of motor
skills after stroke.