A parametric analysis of the 'rate effect' in the sensorimotor cortex: a functional magnetic resonance imaging analysis in human subjects.

We studied the effects of different movement speeds of unimanual right hand movements on functional magnetic resonance signal changes in the sensorimotor cortex using echo planar imaging (EPI). Six healthy right-handed subjects were scanned at rest and while executing a finger tapping task with their right index finger. Movement frequency was visually paced at rates ranging from 0.5 to 5 Hz, separated by 0.5 Hz steps. The blood oxygen level dependent (BOLD) response within the left sensorimotor cortex was linearly and positively related to movement frequency. However, this relation holds (r2 = 0.91) only for movement frequencies faster than 1 Hz (1.5-5 Hz). For the slower frequencies there was an initial sharp increase of the BOLD response from 0.5 to 1 Hz followed by an activity drop for 1.5 Hz. These results are compatible with the idea that two different motor control modes are operative during slow or fast movements. During slow movements a computational demanding on-line feedback control mode is operative resulting in strong BOLD signals indicating extensive neural activity. During faster movements on the other hand a program-like motor control mode is operative resulting in less demanding neural computations. The amount of neural computation for the latter control mode increases with increasing movement speed.