Transcranial direct current stimulation to the left dorsolateral prefrontal cortex enhances early dexterity skills with the left non-dominant hand : a randomized controlled trial

Background The left dorsolateral prefrontal cortex (DLPFC) is involved in early-phase manual dexterity skill acquisition when cognitive control processes, such as integration and complexity demands, are required. However, the effectiveness of left DLPFC transcranial direct current stimulation (tDCS) on early-phase motor learning and whether its effectiveness depends on the cognitive demand of the target task are unclear. This study aimed to investigate whether tDCS over the left DLPFC improves non-dominant hand dexterity performance and determine if its efficacy depends on the cognitive demand of the target task.Methods In this randomized, double-blind, sham-controlled trial, 70 healthy, right-handed, young adult participants were recruited. They were randomly allocated to the active tDCS (2 mA for 20 min) or sham groups and repeatedly performed the Purdue Pegboard Test (PPT) left-handed peg task and left-handed assembly task three times: pre-tDCS, during tDCS, and post tDCS.Results The final sample comprised 66 healthy young adults (mean age, 22.73 +/- 1.57 years). There were significant interactions between group and time in both PPT tasks, indicating significantly higher performance of those in the active tDCS group than those in the sham group post tDCS (p < 0.001). Moreover, a greater benefit was observed in the left-handed assembly task performance than in the peg task performance (p < 0.001). No significant correlation between baseline performance and benefits from tDCS was observed in either task.Conclusions These results demonstrated that prefrontal tDCS significantly improved early-phase manual dexterity skill acquisition, and its benefits were greater for the task with high cognitive demands. These findings contribute to a deeper understanding of the underlying neurophysiological mechanisms of the left DLPFC in the modulation of early-phase dexterity skill acquisition

Effect of Audiovisual Cross-Modal Conflict during Working Memory Tasks : A Near-Infrared Spectroscopy Study

Cognitive conflict effects are well characterized within unimodality. However, little is known about cross-modal conflicts and their neural bases. This study characterizes the two types of visual and auditory cross-modal conflicts through working memory tasks and brain activities. The participants consisted of 31 healthy, right-handed, young male adults. The Paced Auditory Serial Addition Test (PASAT) and the Paced Visual Serial Addition Test (PVSAT) were performed under distractor and no distractor conditions. Distractor conditions comprised two conditions in which either the PASAT or PVSAT was the target task, and the other was used as a distractor stimulus. Additionally, oxygenated hemoglobin (Oxy-Hb) concentration changes in the frontoparietal regions were measured during tasks. The results showed significantly lower PASAT performance under distractor conditions than under no distractor conditions, but not in the PVSAT. Oxy-Hb changes in the bilateral ventrolateral prefrontal cortex (VLPFC) and inferior parietal cortex (IPC) significantly increased in the PASAT with distractor compared with no distractor conditions, but not in the PVSAT. Furthermore, there were significant positive correlations between Delta task performance accuracy and Delta Oxy-Hb in the bilateral IPC only in the PASAT. Visual cross-modal conflict significantly impairs auditory task performance, and bilateral VLPFC and IPC are key regions in inhibiting visual crossmodal distractors

The impact of visual cross-modal conflict with semantic and nonsemantic distractors on working memory task : A functional near-infrared spectroscopy study

Cross-modal conflicts arise when information from multisensory modalities is incongruent. Most previous studies investigating audiovisual cross-modal conflicts have focused on visual targets with auditory distractors, and only a few studies have focused on auditory targets with visual distractors. Moreover, no study has investigated the differences in the impact of visual cross-modal conflict with semantic and nonsemantic competition and its neural basis. This cross-sectional study aimed to characterize the impact of 2 types of visual cross-modal conflicts with semantic and nonsemantic distractors through a working memory task and associated brain activities. The participants were 33 healthy, right-handed, young male adults. The paced auditory serial addition test was performed under 3 conditions: no-distractor and 2 types of visual distractor conditions (nonsemantic and semantic distractor conditions). Symbols and numbers were used as nonsemantic and semantic distractors, respectively. The oxygenated hemoglobin (Oxy-Hb) concentration in the frontoparietal regions, bilateral ventrolateral prefrontal cortex (VLPFC), dorsolateral prefrontal cortex, and inferior parietal cortex (IPC) were measured during the task under each condition. The results showed significantly lower paced auditory serial addition test performances in both distractor conditions than in the no-distractor condition, but no significant difference between the 2 distractor conditions. For brain activity, a significantly increased Oxy-Hb concentration in the right VLPFC was only observed in the nonsemantic distractor condition (corrected P = .015; Cohen d = .46). The changes in Oxy-Hb in the bilateral IPC were positively correlated with changes in task performance for both types of visual cross-modal distractor conditions. Visual cross-modal conflict significantly impairs auditory working memory task performance, regardless of the presence of semantic or nonsemantic distractors. The right VLPFC may be a crucial region to inhibit visual nonsemantic information in cross-modal conflict situations, and bilateral IPC may be closely linked with the inhibition of visual cross-modal distractor, regardless of the presence of semantic or nonsemantic distractors