Relationship between muscle output and functional MRI-measured brain activation

Abstract

The relationship between functional MRI (fMRI)-measured brain signal and muscle force and or electromyogram (EMG) is critical in interpreting fMRI data 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 maintaining the quality of the fMRI brain images. Using this system, we determined the relationship between fMRI-measured brain activation and handgrip force and between fMRI-measured brain signal and EMG of extrinsic finger muscles. Ten volunteers participated in the experiments (only seven subjects' data were analyzed due to excessive noise in the fMRI data of three subjects). The participants exerted 20%, 35%, 50%, 65%, and 80% of the maximal force. During each contraction period, handgrip force, surface EMG of the finger flexor and extensor muscles, and fMRI brain images were acquired. The degree of muscle activation (force and EMG) was directly proportional to the amplitude of the brain signal determined by fMRI in the entire brain and in a number of motor function-related cortical fields, including primary motor, sensory regions, supplementary motor area, premotor, prefrontal, parietal and cingulate cortices, and cerebellum. All the examined brain areas demonstrated a similar relationship between the fMRI signal and force. A stronger fMRI signal during higher force indicates that more cortical output neurons and/or interneurons may participate in generating descending commands and/or processing additional sensory information. The similarity in the relationship between muscle output and fMRI signal in the cortical regions suggests that correlated or networked activation among a number of cortical fields may be necessary for controlling precise static force of finger muscles.

Force related activations in rhythmic sequence production

“…In order to control for motor timing effects, we used a rhythm production task with alternating or equal force targets combined with alternating or equal interval targets. Previous functional imaging studies have shown a positive linear relationship between tapping rate and the spread of cortical activity for movement rates between 1 Hz and 5 Hz (Rao et al, 1996), and between force level and the amplitude of the measured fMRI signal, for forces between 20% and 65% of maximum (Dai et al, 2001). We controlled for these effects of rate and force level by setting the target time interval and force for the equal conditions to be the average of the values used in the alternating conditions.…”

“…Activation of sensory motor cortex (SMC) in a finger flexion force production task was observed to be greater in repetitive squeezing than in steady contraction (Thickbroom et al, 1999). In thumb-finger squeezing movements, increases in activation with force have been noted in MI (Cramer et al, 2002), SMA, PMC and cerebellum (Dai et al, 2001).…”

Shifting of activation center in the brain during muscle fatigue: An explanation of minimal central fatigue?

“…A hypothetic answer is that the brain is a redundant organ that has multiple motor control centers with parallel projections to motoneuron pools in the spinal cord; the availability of multiple cortical centers controlling one motor task or muscle group makes "shifting of activation center" possible among these centers to compensate for fatigue. Numerous studies have shown multiple brain region activation even in controlling non-fatigue, simple motor tasks (e.g., Dai et al, 2001;Liu et al, 2003). The concept of activation shifting as a strategy to prolong a motor task under the condition of muscle fatigue has been presented for a long time with limited evidence showing rotation of motor units (Sogaard, 1995), muscles (Côté et al, 2002) and cortical neurons (Belhaj-Saïf et al, 1996).…”

A method to capture six-degrees-of-freedom mechanical measurements of isometric shoulder and elbow torques during event-related fMRI

“…Conductive strain gauges do not provide the ideal interface for monitoring torques and forces. Hydraulic solutions have been implemented (Dai et al, 2001;Liu et al, 2000;Vaillancourt et al, 2003), but there is no compact hydraulic solution for monitoring multiple DOF simultaneously. A similar method to the method described above has been implemented at the wrist, but did not suffer greatly from noise on the load cell recordings because the load cell was located further from the MR imaging region (Hidler et al, 2006).…”

“…Several groups have used one-DOF devices in order to achieve this goal (Culham et al, 2003;Dai et al, 2001;Ehrsson et al, 2003;Floyer-Lea and Matthews, 2005;Liu et al, 2002;Vaillancourt et al, 2003). We implemented a method that permits researchers to quantitatively control experiments across joints and subjects.…”