Oscillatory motor cortex-muscle coupling during painful laser and nonpainful tactile stimulation

Noxious stimulation activates-in addition to the brain structures related to sensory, emotional, and cognitive components of pain-also the brain's motor system. Effect of noxious input on the primary motor (MI) cortex remains, however, poorly understood. To characterize this effect in more detail, we quantified the ongoing oscillatory communication between the MI cortex and hand muscles during selectively noxious laser stimulation. The subjects maintained an isometric contraction of finger muscles while receiving the laser stimuli to the dorsum of the hand. Tactile stimuli with well-known effects on the MI cortex reactivity served as control stimuli. Cortex-muscle coherence was computed between magnetoencephalographic (MEG) signals from the contralateral MI and electromyographic (EMG) signals from the hand muscles. Statistically significant coherence at ∼20 Hz was found in 6 out of 7 subjects. The coherence increased phasically after both types of stimuli but significantly later after laser than tactile stimuli (mean ± SEM peak latencies 1.05 ± 0.12 s vs. 0.58 ± 0.06 s; P < 0.05), and the coherence increase lasted longer after laser than tactile stimuli (0.87 ± 0.09 s vs. 0.50 ± 0.06 s, P < 0.05). The observed coherence increase could be related to stabilization of the motor-cortex control after sensory input. Our findings add to the clinically interesting evidence about the cortical pain-motor system interaction.