Unilateral left prefrontal transcranial magnetic stimulation (TMS) produces intensity-dependent bilateral effects as measured by interleaved BOLD fMRI

Transcranial magnetic stimulation (TMS) administered over the prefrontal co rtex has been shown to subtly influence neuropsychological tasks, and has a ntidepressant effects when applied daily for several weeks. Prefrontal TMS does not, however, produce an immediate easily observable effect, making it hard to determine if one has stimulated the cortex. Most prefrontal TMS st udies have stimulated using intensity relative to the more easily determine d motor threshold (MT) over motor cortex. Five healthy adults were studied in a 1.5 T MRI scanner during short trains of 1 Hz TMS delivered with a figure eight MR compatible TMS coil followed by rest epochs. In a randomized manner, left prefrontal TMS was delivered a t 80%, 100% and 120% of MT interleaved with BOLD fMRI acquisition. Compared to rest, all TMS epochs activated auditory cortex, with 80% MT hav ing no other areas of significant activation. 100% MT showed contralateral activation and 120% MT showed bilateral prefrontal activation. Higher inten sity TMS, compared to lower, in general produced more activity both under t he coil and contralaterally. Higher prefrontal TMS stimulation intensity produces greater local and cont ralateral activation. Importantly, unilateral prefrontal TMS produces bilat eral effects, and TMS at 80 % MT produces only minimal prefrontal cortex ac tivation. (C) 2001 Society of Biological Psychiatry.