Unilateral repetitive transcranial magnetic stimulation differentially affects younger and older adults completing a verbal working memory task

Functional neuroimaging studies have found that lateralization of activity in the dorsolateral prefrontal cortex (dlPFC) is reduced with aging. In the present study, repetitive transcranial magnetic stimulation (rTMS) was used to disrupt dlPFC activity in order to test the relationship of dlPFC laterality and age in verbal working memory (WM). Young (n = 36) and older (n = 35) subjects received 1 Hz-rTMS (sham or active) to left or right dlPFC and WM performance was assessed pre- and post-stimulation via the n-back task. Significant increases in WM accuracy were observed following rTMS in the right dlPFC and sham conditions, but not with the left dlPFC stimulation. This was accompanied by a decrease in left P1 latency was also observed following left dlPFC stimulation. In contrast, older adults did not show a disruption in WM performance following rTMS in any of the stimulation conditions and exhibited increased left P3 amplitude following left stimulation. Our results show that changes in prefrontal laterality are evident with increasing age (left stimulation affects younger adults while older adults are not affected by stimulation) and this change is associated with specific neurophysiologic measures.