Human cortical muscle coherence is directly related to specific motor parameters

Cortical oscillations have been the target of many recent investigations, b ecause it has been proposed that they could function to solve the "binding" problem. In the motor cortex, oscillatory activity has been reported at a variety of frequencies between similar to4 and similar to 60 Hz. Previous r esearch has shown that 15-30 Hz oscillatory activity in the primary motor c ortex is coherent or phase locked to activity in contralateral hand and for earm muscles during isometric contractions. However, the function of this o scillatory activity remains unclear. Is it simply an epiphenomenon or is it related to specific motor parameters? In this study, we investigated task- dependent modulation in coherence between motor cortex and hand muscles dur ing precision grip tasks. Twelve right-handed subjects used index finger an d thumb to grip two levers that were under robotic control. Each lever was fitted with a sensitive force gauge. Subjects received visual feedback of l ever force levels and were instructed to keep them within target boxes thro ughout each trial. Surface EMGs were recorded from four hand and forearm mu scles, and magnetoencephalography (MEG) was recorded using a 306 channel ne uromagnetometer. All subjects showed significant levels of coherence (0.086 -0.599) between MEG and muscle in the 15-30 Hz range. Coherence was signifi cantly smaller when the task was performed under an isometric condition (le vers fixed) compared with a compliant condition in which subjects moved the levers against a spring-like load. Furthermore, there was a positive, sign ificant relationship between the level of coherence and the degree of lever compliance. These results argue in favor of coherence between cortex and m uscle being related to specific parameters of hand motor function.