Optical measurement of hemodynamic changes in the contralateral motor cortex induced by transcranial magnetic stimulation

Transcranial magnetic stimulation was used to activate the left motor cortex of four volunteers. The consequent hemodynamic changes in the right motor cortex were measured with optical imaging. Different stimulus intensities were applied, and a sham measurement was made in which the coil was tilted 90 degrees from its tangential orientation, so that it the induced electric field is not sufficient to activate the cortex. The TMSevoked changes were compared with an ipsilateral finger tapping task. The primary somatosensory cortex (S1) was localised based on the N20m response evoked by median nerve stimulation. It was found that magnetic stimulation induces a decrease in the 830 nm signal in the contralateral motor cortex. For one subject, the optical data was coregistered with anatomical MRI, and the optical activation was found strongest in the hand area of the motor cortex.