Anodal transcranial direct current stimulation of the prefrontal cortex enhances complex verbal associative thought

Abstract

The remote associates test (RAT) is a complex verbal task with associations to both creative thought and general intelligence. RAT problems require not only lateral associations and the internal production of many words but a convergent focus on a single answer. Complex problem-solving of this sort may thus require both substantial verbal processing and strong executive function capacities. Previous studies have provided evidence that verbal task performance can be enhanced by noninvasive transcranial direct current stimulation (tDCS). tDCS modulates excitability of neural tissue depending on the polarity of the current. The after-effects of this modulation may have effects on task performance if the task examined draws on the modulated region. Studies of verbal cognition have focused largely on the left dorsolateral prefrontal cortex (F3 of the 10-20 EEG system) as a region of interest. We planned to assess whether modulating excitability at F3 could affect complex verbal abilities. In Experiment 1 (anodal, cathodal, or sham stimulation over F3 with the reference electrode over the contralateral supraorbital region), we found a significant overall effect of stimulation condition on RAT performance. Post hoc tests showed an increase in performance after anodal stimulation (1 mA) compared to sham (p = .025) and to cathodal stimulation (p = .038). In Experiment 2 (either anodal stimulation at F3 or separately at its homologue F4), we replicated the anodal effect of the first study, but also showed that anodal stimulation of F4 had no effect on RAT performance. These data provide evidence that anodal stimulation of the left dorsolateral prefrontal cortex can improve performance on a complex verbal problem-solving task believed to require significant executive function capacity.

Figure 1

Schematic of one iteration of the experimental design of Experiment 1 showing the timing of verbal fluency (VF) and remote associates test (RAT) in relation to the stimulation sessions. Three equally difficult versions of the RAT were used. Versions of VF (e.g., A–S or C–L) and the RAT (e.g., Problem Set 1 or Problem Set 2) were counterbalanced across stimulation conditions. The order of stimulation sessions was counterbalanced across subjects. In Experiment 2, both VF and RAT were administered as posttests, and order of completion was counterbalanced.

Figure 2

Mean number and standard deviation of remote associates problems solved correctly in each of the stimulation conditions.

Figure 3

Mean number and standard deviation of RAT problems solved by order of testing session, regardless of stimulation condition.

Figure 4

Pretest and posttest verbal fluency scores and standard deviations in each of the stimulation conditions.

Figure 5

Mean number and standard deviation of RAT problems solved after anodal or sham stimulation of F3 and F4.