Relationship between muscle output and functional MRI-measured brain activation

The relationship between functional MRI (fMRI)-measured brain signal and mu scle force and or electromyogram (EMG) is critical in interpreting fMRI dat a and understanding the control mechanisms of voluntary motor. actions. We designed a system that could record joint force and surface EMG online with fMRI data. High-quality force and EMG, data were obtained while maintainin g the quality of the fMRI brain images. Using this system, we determined th e relationship between fMRI-measured brain activation and handgrip force an d between fMRI-measured brain signal and EMG of extrinsic finger muscles. T en volunteers participated in the experiments (only seven, subjects' data w ere analyzed due to excessive noise in the fMRI data of three subjects). Th e participants exerted 20%, 35%, 50%, 65%, and 80% of the maximal force. Du ring each contraction period, handgrip force, surface EMG of the finger fle xor and extensor muscles, and fMRI brain images were acquired. The degree o f muscle activation (force and,EMG) was directly, proportional to the ampli tude of the brain signal determined by fMRI in the entire brain and in a nu mber of motor function-related cortical fields, including primary motor, se nsory regions, supplementary motor area, premotor, prefrontal, parietal and cingulate cortices, and cerebellum. All the examined brain areas demonstra ted a similar relationship between the fMRI signal and force. A stronger fM RI signal during higher force indicates that more cortical output neurons a nd/or interneurons may participate in generating descending commands and/or processing additional sensory information. The similarity in the relations hip between muscle output and fMRI signal in the cortical regions suggests that correlated or networked activation among a number of cortical fields m ay be necessary for controlling precise static force of finger muscles.