ISOMETRIC FORCE-RELATED ACTIVITY IN SENSORIMOTOR CORTEX MEASURED WITHFUNCTIONAL MRI

Isometric force-related functional magnetic resonance imaging (fMRI) s ignals from primary sensorimotor cortex were investigated by imaging d uring a sustained finger flexion task at a number of force levels rela ted to maximum voluntary contraction. With increasing levels of force, there was an increase in the extent along the central sulcus from whi ch a fMRI signal could be detected and an increase in the summed signa l across voxels, but these parameters were related in such a way that the signal from each voxel was similar for each level of force. The re sults suggest that increased neuronal firing and recruitment of cortic omotor cells associated with increased voluntary isometric effort are reflected in an expansion of a relatively constant fMRI signal over a greater volume of cortex, rather than an increase in the magnitude of the response in a particular circumscribed region, possibly due to per fusion of an increase in oxygen-enriched blood over a wider region of the cortex.