The role of anodal transcranial direct current stimulation (tDCS) in the consolidation of visual perceptual learning is mediated by the wake/sleep cycle

Abstract

Visual skills can be improved substantially after extensive training, a phenomenon called visual perceptual learning (VPL). It has been documented that both sleep and noninvasive brain stimulation (NIBS) (e.g., transcranial direct current stimulation (tDCS)) can enhance learning and memory, especially during consolidation, a vital stage of transforming fragile memory engram into long-term storage. However, how sleep and NIBS interact during VPL consolidation remains unclear. In this study, we investigated this issue using a 2 (25 minutes of 2 mA anodal tDCS vs. sham stimulation) × 2 (morning-evening condition vs. evening-morning condition) between-subject design. Four groups of subjects were randomly assigned to one of the four experimental conditions and were trained with an orientation discrimination task. TDCS was applied over the visual cortex immediately after the end of the training session. In the morning-evening condition, subjects completed a training session in the morning and a test session in the evening of the same day. They kept awake during the daytime. We found that, for the tDCS group, compared with the morning training session, the orientation discrimination threshold in the evening test session decreased significantly. No such an effect was found in the sham stimulation group. In the evening-morning condition, subjects completed a training session in the evening and a test session in the next morning. They had a night sleep at home. After sleep, there was a significant decrease in discrimination threshold for the sham stimulation group, whereas the discrimination threshold increased significantly for the tDCS group. Our results demonstrated that VPL consolidation was sleep-dependent and the effect of anodal tDCS on VPL consolidation was mediated by the wake/sleep cycle, implying that the homeostatic plasticity may play a critical role in VPL consolidation during sleep.