False memories can be modulated by transcranial direct current stimulation (TDCS)

Studying false memories gives insights into basic memory (control) mechanisms and executive functioning. One paradigm that allows for the systematic investigation of false memories - directed forgetting (DF) - has been investigated in a multitude of design variants and populations. In its item-method variant, participants are presented an array of stimuli, each of which is followed by a remember (R) or a forget (F) instruction. On a subsequent surprise memory test, recognition rate for R stimuli is generally higher than for F stimuli. Previous studies of the authors suggest this effect to be borne not only by veridical but also by false memories. The present study aimed at modulating false alarms by transcranial direct current stimulation (TDCS). TDCS is a non-invasive, well-tolerated technique that is used to investigate healthy and impaired neuronal functions. Cortical activity is thought to be enhanced by anodal TDCS and to be reduced by cathodal TDCS. Eighty-three healthy subjects participated in a double-blind sham-controlled crossover study. TDCS (20min, 1mA) to the left dorsolateral prefrontal cortex (dlPFC) or sham stimulation were applied during the encoding phase of a DF task. Whereas hits yielded the classic DF effect in all three groups, false alarm patterns differed massively. Under anodal stimulation, false alarm rates were significantly higher than under sham stimulation. Under cathodal stimulation, in turn, they were significantly lower. No between-group response bias difference was observed. To our knowledge, this is the first study that shows a dose effect of brain stimulation on a complex cognitive task.