Cerebral activation during a simple force production task: changes in the time course of the haemodynamic response

An event-related paradigm was used to investigate the fMRI signal from the primary motor cortex (MI) and the supplementary motor area (SMA) during iso lated isometric wrist extension at five different force levels. There was o nly a weak trend towards increased area of activation with increased force output, but there was a force-related increase in percentage change of sign al within voxels in MI (Kendall Tc=0.48, p<0.01), which may indicate contro l of force output by variation of neural firing rate. In SMA there was a co rrelation between peak force output and time-to-peak of the haemodynamic re sponse in SMA (Kendall Tc=0.74, p<0.0001). This unexpected finding of a tas k-related change in the shape of the haemodynamic response within a single brain area requires further investigation. It may indicate a slower rise ti me at lower perfusion rates, or may be the result of inhibitory processes i n motor control.