Neural correlates of skilled movement : Functional mapping of the human brain with fMRI and PET

Humans have unique abilities to perform certain types of skilled voluntary movements. In this thesis we examine the neural substrates of: (i) fine digit actions, in particular the control of fingertip forces during manipulation, and (ii) the coordination of voluntary movements of different limbs. In addition, (iii) we investigate the neural correlates of the kinesthetic perception and imagery of limb movement. Functional magnetic resonance imaging (fMRI) and positron emission tomography were used to measure the blood oxygenation level dependent contrast and regional cerebral blood flow as indexes of neuronal activity. (i) In a serie of fMRI experiments, we investigated the active cortical areas associated with the control of fingertip forces and production of hand postures with independent movements of the digits. In the fingertip force experiments the subjects used the right index finger and thumb to apply forces to a fixed object. These precision grip tasks consistently activated a set of bilateral fronto-parietal areas including the primary motor cortex (M1), the non-primary motor areas and the posterior parietal cortex (PPC). It was found that the control of small grip forces during precision grips is more dependent on non-primary fronto-parietal areas than when the force is excessively large or when a power grip is used between all digits and the palm. Specifically, we show that the bilateral ventral premotor cortex, area 44, supramarginal cortex and the right intraparietal cortex (IPS) are involved in the control of small precision grip forces. Furthermore, areas in the left PPC are involved in the control of lift forces for object displacement whereas the right posterior IPS might support the coordination of grip-lift forces during precision grips. Further, we show that M1 is particularly active during forceful gripping, but also so when holding an object close to the slip point requiring very precise force control. The SMA, CMA and left supramarginal cortex are also active in this latter task. We also demonstrate that the control of independent movements of the digits during the production of hand postures involves the SMA, the bilateral dorsal premotor cortex, postcentral cortex, cerebellum, and the left anterior IPS. In summary, we conclude that fine digit actions in humans depend on a network of bilateral fronto-parietal areas that are active in a task-dependent manner. (ii) The brain regions controlling coordinated movements of limbs were examined. A main conclusion is that coordinated movements of two limbs are controlled by the areas that control isolated movements of the same limbs. In addition, we show that two natural patterns of bimanual temporal coordination are supported by distinct regions: the left anterior cerebellar lobe (and caudal CMA and precuneus) is associated with synchronous finger tapping, whilst alternating finger tapping strongly engages bilateral fronto-parieto-temporal areas. Furthermore, the medial cerebellum is strongly activated in polyrhythmic tasks. These results are discussed in relation to the hypothesis that different brain regions support temporal and spatial inter-limb coordination. (iii) The neural correlates of the kinesthetic perception and imagery of limb movement were examined. We show that when subjects experience an illusory limb movement elicited by vibration stimuli (~80 Hz) applied to the skin over the tendon of a muscle, the contralateral M1, S1, SMA, and CMA are active. Likewise, when subjects imagine that they are executing movements of their fingers, toes and tongue, some of the corresponding gross somatotopical zones of the frontal motor areas are recruited. Thus the frontal motor areas are involved in the kinesthetic perception and imagery of limb movement, in addition to the execution of action.