Abstract
The evoked responses to transcranial magnetic stimulation (TMS) have been previously demonstrated to be on average greater at the beginning of a session; however the physiological reason for this remains uncertain. In order to investigate a possible hemodynamic mechanism for this phenomenon, changes in oxy-hemoglobin (HbO) following trains of single pulse TMS was investigated using near infra-red spectroscopy (NIRS). TMS was delivered in trains of two and four pulses to left pre-frontal cortex (PFC) at a typical intensity and frequency (.2Hz) used in neuroscience research. Both trains resulted in significant drops of HbO that remained after the cessation of TMS. The changes observed imply that arterial supply drops following suprathreshold TMS and oxygen consumption outstrips supply, resulting in a net drop of HbO. This study provides evidence that at typical TMS delivery frequencies, local HbO levels remain at a sustained lower level than at the beginning of the session, potentially explaining changes in sensitivity to stimulation with repeated TMS pulses.
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