UC Irvine

Transcranial Direct Current Stimulation (tDCS) is a non-invasive means of electrical brain stimulation that can influence the neural activity of the underlying cortex, and is becoming a popular means of cognitive enhancement. However, some meta-analyses arrive at inconclusive results, and the efficacy of tDCS is controversial, especially during the earlier years of this dissertation work. One reason that could account for the low reliability between some studies is the possibility of delayed effects that may not occur until some hours or days after a session. Evidence for this phenomenon has been accruing with longitudinal studies and is thought to relate to downstream effects of the stimulation on promoting LTP and LTP-like plasticity within task-relevant neurons. The primary aim of this thesis is to explore the conditions under which such long-term changes can occur, and the extent to which they can influence human learning. To do so, three empirical studies were conducted. First, we established that tDCS could improve cognitive function during a week-long period of working memory (WM) training, with follow-up effects lasting up to a year. Our second study investigated the neural underpinnings of these longitudinal effects by using EEG to show greater evoked responses to a visual flicker roughly 24 hours after tDCS. Finally, we repeated our WM training design, but manipulated the stimulation timing window (before, during, or after task performance), in order to optimize our protocol to manifest greater long-term effects. However, we unexpectedly found that tDCS actually impaired performance relative to sham, particularly when applied before or after training. Post-hoc analyses looking at the combined data from our first and third experiments revealed an interesting baseline-dependency that may reconcile our discrepant results. We found that tDCS was only effective for individuals who started off with either low or high WM ability, but ineffective or possibly even detrimental for individuals starting off with more average ability levels. Overall, this thesis argues for a promising outlook for the use of tDCS for increasing long-term learning, but cautions that the strength and direction of effects can vary wildly depending on a variety of individual difference and other factors.