Hemoglobin concentration changes in the contralateral hemisphere during and after theta burst stimulation of the human sensorimotor cortices

Using near infrared spectroscopy and repetitive transcranial magnetic stimulation (rTMS), we studied interhemispheric interactions between bilateral motor and sensory cortices in humans. RTMS consisted of a triple-pulse burst (50 Hz) repeated every 200 m for 2 s (10 bursts, 30 pulses); one kind of theta burst TMS (TBS) (Huang et al. in Neuron 45: 201-206, 2005). The hemoglobin concentration changes were recorded at the right prefrontal cortex, premotor area (PM), primary hand motor area (M1) and primary sensory area (S1) during and after TBS over the left PM, M1 and S1 or sham stimulation in eight normal volunteers. In addition, motor evoked potentials (MEPs) to TMS over the right M1 were recorded from the left first dorsal interosseous muscle after the conditioning TBS over left S1. TBS over PM induced a significant oxy-Hb decrease at the contralateral PM. TBS over M1 elicited a significant oxy-Hb decrease at the contralateral S1, and TBS over S1 significant oxy-Hb decreases at the contralateral M1 and S1. MEPs to TMS of the right M1 were significantly suppressed by the conditioning TBS over the left S1. These results suggest that there are mainly inhibitory interactions between bilateral PMs and bilateral sensorimotor cortices in humans. Those are partly compatible with the previous findings. In addition to between the primary motor cortices, bilateral connection is requisite for smooth bimanual coordination between the sensory cortices or premotor cortices.