Modifying the discrete sequence production task for a multiday tDCS study in young and older adults

The discrete sequence production (DSP) task is an explicit motor learning sequence task where two 6-item sequences are presented one item at a time. Over many repetitions, participants eventually execute a 6-item sequence as 2 or more segments, an indication of distinct motor chunks. Previous work has demonstrated that older adults exhibit a reduction in chunk length and have an impaired explicit memory, relative to young adults. Right and left dorsolateral prefrontal cortex (DLPFC) have been demonstrated to be involved in early explicit sequence learning as well as early adaptation. Primary motor cortex (M1) has been shown to be involved in explicit sequence learning and retention. Further, premotor cortex has been shown to be involved in memory consolidation in sequence learning. Here, we use transcranial direct current stimulation (tDCS), a non-invasive form of brain stimulation, to facilitate early learning and chunking in both younger and older adults in a truncated version of the traditional DSP task. Participants attend three sessions over the course of a week, and are randomized into one of five tDCS conditions (right DLPFC, left DLPFC, M1, premotor, or sham). Over the three sessions, participants complete a battery of cognitive and motor tasks that correlate with motor learning ability and executive functioning in order to characterize the participant, use later as covariates in analysis, and understand how these cognitive and motor tasks might change from baseline as a function of the tDCS condition. Participants also practice the DSP task while receiving tDCS for up to 25 minutes during sessions 1 and 2. During session three, participants are tested on their ability to remember the sequence of the DSP task without stimulation. We hypothesize that tDCS to right DLPFC will facilitate early learning in both older and younger adults, with older adults receiving the most benefit from the tDCS stimulation. We also predict that tDCS over premotor cortex will help facilitate chunking in older adults, relative to older adults in the sham group. We expect that stimulation to M1, left DLPFC, and premotor in younger adults will change the rate of motor learning relative to young adults in the sham tDCS group. Our preliminary results suggest that younger adults without tDCS are still able to chunk with fewer trials in the DSP task over three sessions.