Nap after anodal transcranial direct current stimulation (tDCS) disrupts consolidation of visual perceptual learning- an EEG study

In visual perceptual learning (VPL), consolidation is a crucial process that stabilizes and even further improves performance on trained tasks without additional practice. Our previous study found that sleep after transcranial direct current stimulation (tDCS) disrupted VPL consolidation, yet the underlying neural mechanisms of such phenomenon are still unclear. The current study incorporated an adaptation sleep and two intervention days containing formal sleep. On each intervention day, participants were trained on an orientation discrimination task (ODT) in the morning following resting-state EEG recording. In each ODT trial, two Gaussian-enveloped sinusoidal gratings (Gabor patch) with slightly different orientations were presented in random order in the lower left or right visual field (VF). The order of VFs was counterbalanced across subjects, and the orientations of Gabors in the two VFs were orthogonal. After training, 2.0 mA anodal tDCS was delivered over the occipital regions contralateral to the VF in training for 25 minutes for active and 30 seconds for sham conditions, respectively. Immediately after brain stimulation, resting-state EEG was recorded for a second time. In the afternoon, participants napped in the sleep room with polysomnography (PSG) recording for 60-90 minutes. A post-test was completed after the nap. Results showed that, following a nap, participants’ orientation discrimination thresholds declined in the sham condition but not in the active tDCS condition. Moreover, in N2 sleep, delta power in the tDCS-stimulated occipital area was lower in the active condition compared to that in the sham condition, and the sleep spindle power in the frontal area contralateral to the stimulated hemisphere was also lower in the active condition. Our findings indicate that the consolidation of VPL can be disrupted by the interplay of post-training anodal tDCS and subsequent nap, suggesting that anodal tDCS-induced hyper-excited brain activity might disturb the system consolidation process during sleep.