No effect of transcranial direct-current stimulation to dorsolateral prefrontal cortex on naturalistic prospective memory in healthy young and older adults

Transcranial direct current stimulation (tDCS), a form of non-invasive brain stimulation, has been shown to enhance working memory and multitasking abilities. Because of the substantial overlap in both cognitive processes (maintenance, monitoring) and neural substrates (dorsolateral prefrontal cortex, DLPFC) that support both working memory and prospective memory – the ability to remember to perform intended actions at appropriate moments in the future (taking medications, turning off appliances) – we tested whether tDCS would also enhance young and older adults’ prospective memory. If tDCS enhances DLPFC activation, then it should benefit prospective memory performance, particularly for tasks that rely on controlled monitoring processes for prospective memory cue detection. Healthy young and older adults played the Virtual Week game while they received either a session of active tDCS to DLPFC and then, after at least 48 h, a session of placebo-controlled, sham stimulation, or vice versa. The Virtual Week game is a reliable measure of naturalistic prospective memory that includes assessments of five different types of prospective memory tasks that vary in cue-type (time vs. event) and task-regularity and, thus, the extent to which task performance should rely on controlled monitoring processes and DLPFC activation. Active tDCS had no effect on prospective memory performance relative to sham tDCS for either age group or any task type. In contrast, there were reliable practice effects across test sessions regardless of whether active tDCS was applied during the first or second session. A single session of tDCS to DLPFC did not enhance young or older adults’ prospective memory performance, but practice did.