SHORT COMMUNICATIONS Blood oxygenation changes resulting from suprathreshold transcranial magnetic stimulation Richard H. Thomson, a Jerome J. Maller, a Zafiris J. Daskalakis, b Paul B. Fitzgerald a a Monash Alfred Psychiatry Research Centre, The Alfred and Monash University School of Psychology and Psychiatry, Victoria, Australia b Centre for Addiction and Mental Health, University of Toronto, Toronto, Ontario, Canada The hemodynamic response to low-intensity transcranial magnetic stimulation (TMS) has previously been demonstrated at motor cortex using near infrared spectroscopy (NIRS). To investigate the effect of TMS on oxy-hemoglobin (HbO) at prefrontal cortex, both subthreshold and suprathreshold TMS relative to resting motor threshold (rMT) were applied at typical intensities used in experimental settings. Although there was no significant change after 90% and 110% rMT TMS, there was a significant drop in HbO after 130% rMT TMS. This drop was maximal at approximately 8 seconds post-TMS. This study may have implications for determining appropriate TMS intensities when stimulating nonmotor areas. Ó 2011 Elsevier Inc. All rights reserved. Keywords transcranial magnetic stimulation; cortical blood flow; oxygenated hemoglobin; prefrontal cortex; brain stimulation Since its conception, transcranial magnetic stimulation (TMS) has become an important tool in neurophysiology and biological psychiatry. 1 The application of repetitive stimula- tion (rTMS) has been shown to produce lasting changes in brain activity 2 and it is increasingly being applied in thera- peutic applications, 3,4 although we lack a complete under- standing of its effects. Among the previous research examining changes in hemodynamic responses resulting from single-pulse TMS and rTMS, there is considerable inconsistency between results. The modulating parameters appear to be the measurement technique used, frequency, duration, intensity, and the location of the stimulus, for example, whether stimulation is applied to M1 or prefrontal cortex (PFC). 5-14 Near infrared spectroscopy (NIRS) is a technique that combines high- temporal resolution with a high signal to noise ratio and meets the requirements for monitoring the response to single-pulse TMS. 15 Previous studies con- ducted using NIRS to study the effects of single TMS pulses 16,17 have been reasonably consistent; however, they have only examined pulses delivered to the primary motor cortex (M1) and at low-moderate intensities, i.e., 70-140% of active motor threshold (aMT). At 140% of aMT, equiv- alent to approximately 100% of the resting motor threshold (rMT), 18 instead of the increases in oxygenated hemoglobin This work was funded by an Australian Research Council (ARC) grant (P.B.F.), National Health and Medical Research Council (NHMRC) Prac- titioner Fellowship (P.B.F.) and by National Alliance for Research on Schizophrenia and Depression (NARSAD) Young Investigator award (Z.J.D.). P.B.F. has received equipment from MagVenture A/S and BrainswayLtd. Correspondence: Professor Paul B. Fitzgerald, MAPRC, First Floor, Old Baker Building, The Alfred, Commercial Rd, Melbourne, Victoria, Australia, 3004. E-mail address: paul.fitzgerald@med.monash.edu.au Submitted July 25, 2010; revised October 15, 2010. Accepted for publication October 19, 2010. 1935-861X/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.brs.2010.10.003 Brain Stimulation (2011) 4, 165–8 www.brainstimjrnl.com