Effect of modulated anodal stimulation over the dorsolateral prefrontal cortex on working memory: a preliminary study

Objectives: Working memory has two components: temporary storage and manipulation of the information necessary for cognitive behavior through the central executive system. This study aimed to investigate the effects of modulated anodal transcranial direct current stimulation (mtDCS) of the dorsolateral prefrontal cortex on working memory. Materials and Methods: Twelve volunteers without neurological or psychiatric disorders and without drug use participated. Working memory performance was assessed with a visual 3-back task using consonant letters during stimulation. A 20/minute training session was conducted to facilitate learning, and participants who achieved an accuracy rate of 50% or higher proceeded to the main experiment. Four stimulation conditions were applied, each lasting 10 minutes: mtDCS-11 Hz and mtDCS-22 Hz (1.70 mA offset, 0.35 mA peak/to/peak), direct current (DC) (2 mA), and sham. Reaction times and total correct responses were recorded. Results: Statistical analysis of the left dorsolateral prefrontal cortex (DLPFC) stimulation revealed a significant difference in the mean numbers of correct responses among the mtDCS and transcranial direct current stimulation (tDCS) conditions, but not compared with sham. The mean number of correct responses under mtDCS-11Hz and mtDCS-22Hz was significantly lower than under DC stimulation; however the performance decrement under 11 Hz mtDCS was the most pronounced among the active conditions. Conclusion: mtDCS-11Hz and mtDCS-22Hz negated the subtle facilitation of tDCS might have provided.