Bilateral responses of prefrontal and motor cortices to repetitive Transcranial Magnetic Stimulation as measured by functional Near Infrared Spectroscopy

Simultaneously acquiring cortical functional Near Infrared Spectroscopy (fNIRS) during repeated Transcranial Magnetic Stimulation (rTMS) offers the possibility of directly investigating the effects of rTMS on brain regions without quantifiable behavioral changes. In this study, the left motor cortex and subsequently the left prefrontal cortex were stimulated at 1 Hz while fNIRS data was simultaneously acquired. Changes in hemodynamic signals were measured on both ipsilateral and contralateral sides. In each cortex, a significantly larger decrease in the concentration of oxygenated hemoglobin and a smaller increase in the concentration of deoxygenated hemoglobin during the stimulation periods were observed in both the motor and prefrontal cortices. The ipsilateral and contralateral changes showed high temporal consistency. Same experiment was repeated for each subject 2 or 3 days later. The hemodynamic responses associated with the stimulation showed good reproducibility in two sessions. To our knowledge, this is the first report of simultaneous fNIRS measurement of ipsilateral and contralateral changes of either the motor or prefrontal cortex during rTMS stimulation.