Side effects of transcranial magnetic stimulation biased task performance in a cognitive neuroscience study

Transcranial magnetic stimulation (TMS) is increasingly used as a research tool for functional brain mapping in cognitive neuroscience. Despite being mostly tolerable, side effects of TMS could influence task performance in behavioural TMS studies. In order to test this issue, healthy subjects assessed the discomfort caused by the stimulation during a verbal working memory task. We investigated the relation between subjective disturbance and task performance. Subjects were stimulated during the delay period of a delayed-match-to-sample task above cortical areas that had been identified before to be involved in working memory. Task performance and subjective disturbance due to side effects were monitored. The subjects' grade of discomfort correlated with the error rates: the higher the discomfort, the more errors were made. Conclusively, TMS side effects may bias task performance in cognitive neuroscience studies and may thereby lead to misinterpretation of results. We emphasize the importance of controlling side effects of the stimulation as a source of biasing effects in TMS studies.