Transcranial direct current stimulation (tDCS) and the 9-dot problem

Abstract

Our principal objective was to replicate the findings of Chi and Snyder (2012) who reported that subjects (45% of sample) receiving active transcranial direct current stimulation (tDCS) involving cathodal stimulation of the left anterior temporal lobe (ATL) and anodal stimulation of the right ATL were able to successfully complete a task considered exceedingly challenging to solve (i.e., the 9-dot problem) relative to participants receiving sham stimulation (solved problem = 0 %). There are, to our knowledge, no published replication studies of Chi and Snyder's original work targeting the ATL. A secondary objective was to determine whether participants who successfully complete the 9-dot problem following active tDCS demonstrate superior visuospatial skills pre-stimulation relative to subjects receiving genuine tDCS who are not able to identify the solution. Moreover, we determined whether active tDCS was associated with improved visuospatial task performance. Following Chi and Snyder (2012), participants received cathodal stimulation of the left ATL and anodal stimulation targeting right ATL, or sham tDCS, during one session of 10 min duration. Unexpectedly, groups did not differ on the 9-dot problem with only 1 of 24 subjects receiving active tDCS and 1 of 26 participants assigned to the sham group identifying the solution to the problem. Active-tDCS and sham-tDCS group differences on tests of visuospatial skills at baseline and following tDCS session did not approach significance. We conducted a second study that closely adhered to Chi and Snyder's (2012) original protocol. In the second study, active-tDCS and sham groups did not differ on the 9-dot problem with 2 of 13 subjects in each group successfully identifying the solution to the problem. Contrary to expectation, active and sham groups in both studies did not differ on the 9-dot problem. Given the striking increase in the use of neuromodulation technologies, investigators should report failures to replicate which will enhance our understanding of the limitations of non-invasive brain stimulation. © 2026 Elsevier Ltd