LAUSR

tDCS is a non-invasive brain stimulation approach in which low level currents are applied across the scalp to influence underlying brain function [1e3]. The Serial Reaction Time Task (SRTT) is commonly used to investigate neural mechanisms underlying motor-learning [4]. In the SRTT, subjects make a series of button presses based upon visual location cues. When the sequence is random, mean RT remains relatively stable over time. However, when the sequence is repetitive and fixed, individuals show a progressive motor learning reflected in a reduction in reaction time (RT) across trials even if they are not told of the sequence in advance. The rate of motor learning may be modulated by tDCS applied to the visuomotor learning circuit consisting of dorsalstream visual cortex and motor/premotor regions of frontal cortex [5,6]. We recently [7] demonstrated that RT distributions during the fixed version of the SRTT are bimodal, with intermixed fast, “proactive” and slow, “reactive” trials, and that tDCS functions primarily by altering the ratio between trials reflecting enhanced motor learning. Further, we have demonstrated that the different trial types are associated with differential connectivity patterns within the visuomotor network. Finally, we demonstrated that the shift in connectivity pattern explained the shift in RT distribution. Specifically, cathodal stimulation of dorsal-stream visual cortex, with cathode at POz and anode at Cz according to the 10e20 EEG system, brought about a change in connectivity between motor and visual cortices accompanied by improved task performance. Traditional tDCS uses relatively large (3 3 cm) pads placed over specific scalp regions, which leads to relatively coarse targeting of the electrical field within underlying brain regions. More recently, high-definition (HD-tDCS) approaches have been developed to better focus the energy to key underlying brain regions [8]. Here we investigate the relative effectiveness of HD-vs. conventional tDCS [9] in improving motor learning when applied to the visual node of the visuomotor network, alongwith the relative effects on underlying brain connectivity patterns. We predicted increased efficacy of HD-vs. conventional tDCS, reflecting its greater focality within target regions. This study involved 10 healthy participants (3 females, 7 males), mean age 41.7 ± 9.6. All subjects provided written informed consent, and the procedures were approved by the Nathan Kline Institute Review Board. All participants reported normal vision. All were right-handed. Cathodal (2 mA) or sham tDCS over visual cortex was administered using the Soterix Medical HD-tDCS 4 1 stimulator while subjects performed repeat trial blocks (“runs”) of the SRTT using a previously described paradigm. Simultaneous EEG was recorded