Impact of tDCS on performance and learning of target detection: interaction with stimulus characteristics and experimental design.

We have previously found that transcranial direct current stimulation (tDCS) over right inferior frontal cortex (RIFC) enhances performance during learning of a difficult visual target detection task (Clark et al., 2012). In order to examine the cognitive mechanisms of tDCS that lead to enhanced performance, here we analyzed its differential effects on responses to stimuli that varied by repetition and target presence, differences related to expectancy by comparing performance in single- and double-blind task designs, and individual differences in skin stimulation and mood. Participants were trained for 1h to detect target objects hidden in a complex virtual environment, while anodal tDCS was applied over RIFC at 0.1 mA or 2.0 mA for the first 30 min. Participants were tested immediately before and after training and again 1h later. Higher tDCS current was associated with increased performance for all test stimuli, but was greatest for repeated test stimuli with the presence of hidden-targets. This finding was replicated in a second set of subjects using a double-blind task design. Accuracy for target detection discrimination sensitivity (d'; Z(hits)-Z(false alarms)) was greater for 2.0 mA current (1.77) compared with 0.1 mA (0.95), with no differences in response bias (ß). Taken together, these findings indicate that the enhancement of performance with tDCS is sensitive to stimulus repetition and target presence, but not to changes in expectancy, mood, or type of blinded task design. The implications of these findings for understanding the cognitive mechanisms of tDCS are discussed.

Transcranial direct current stimulation's effect on novice versus experienced learning.

Transcranial direct current stimulation (TDCS) is a non-invasive form of brain stimulation applied via a weak electrical current passed between electrodes on the scalp. In recent studies, TDCS has been shown to improve learning when applied to the prefrontal cortex (e.g., Kincses et al. in Neuropsychologia 42:113-117, 2003; Clark et al. Neuroimage in 2010). The present study examined the effects of TDCS delivered at the beginning of training (novice) or after an hour of training (experienced) on participants' ability to detect cues indicative of covert threats. Participants completed two 1-h training sessions. During the first 30 min of each training session, either 0.1 mA or 2.0 mA of anodal TDCS was delivered to the participant. The anode was positioned near F8, and the cathode was placed on the upper left arm. Testing trials immediately followed training. Accuracy in classification of images containing and not-containing threat stimuli during the testing sessions indicated: (1) that mastery of threat detection significantly increased with training, (2) that anodal TDCS at 2 mA significantly enhanced learning, and (3) TDCS was significantly more effective in enhancing test performance when applied in novice learners than in experienced learners. The enhanced performance following training with TDCS persisted into the second session when TDCS was delivered early in training.