Two positron emission tomography (PET) studies were performed on 18 normal volunteers to investigate regional cortical and subcortical activation induced by the repetitive lifting-holding-replacing of an object using a precision grip between the index finger and thumb. A three-dimensional PET was performed using H215O as a tracer. The first study was aimed to investigate the brain areas involved in a grasp-lift-hold-replace action and the effect of object weight. Data were obtained for 10 subjects under three object weight conditions (4, 200, and 600 g) and a rest condition. Grip and lift forces on a similar object and the activity of selected muscles in the hand, arm, and shoulder were also recorded in separate lifting trials. An external auditory beep sounded every 3.5 seconds to initiate the lifting action. The subjects repeated the lifting action 20-22 times during each PET scanning. A comparison between all movement conditions and the rest condition revealed significant activation of the primary motor (M1), primary sensory (S1), dorsocaudal premotor (PM), caudal supplementary motor (SMA), and cingulate motor (CMA) cortices contralateral to the hand used. On the ipsilateral side, activation of the M1, caudal SMA, and inferior parietal (BA40) was found. In the subcortical areas, the hemispheres of the bilateral cerebellum, left basal ganglia, and thalamus were activated. Behavioral adaptation to a heavier object weight was revealed in a nearly proportional increase of both grip and lift forces, and a higher level of hand and arm muscle activities. An increase in the rCBF associated with these changes was noted in several cortical and subcortical areas. Consistent object-weight-dependent activation, however, was observed only in the M1/S1 contralateral to the hand used and to some extent in the ipsilateral cerebellum.
The second study was aimed at investigating the brain areas involved in each of the preparatory and execution phases for lifting of an object. In addition, the difference in brain activation between right (dominant) and left (non-dominant) hands was also investigated. Eight subjects were scanned under four conditions. The first condition was preparing and executing the task by the right hand, and the second was preparing only. The third condition was preparing and executing the task by the left hand, and the fourth was resting as a control condition. The subjects were cued for preparation for grasp by a tape-recorded voice saying “prepare, ” which was followed by a beep at some interval (4-8 seconds). For the execute condition, the subjects grasped and lifted the object at the beep sound, while for the prepare-only condition, the subjects stopped the preparatory state. This was repeated 10 or 11 times during each PET scanning. For the prepare and execute condition, results were similar to the activated areas for lifting action in the first study. BA40, however, was activated in the left side this time. For the prepare-only condition, after subtracting the resting condition, left SMA and S1 were the activated areas, and for the execute-only condition (the prepare-execute condition minus the prepare-only condition), left M1 and thalamus, and right cerebellum hemisphere were activated. When the task was performed by the left hand, there was nearly a mirror image of the activated areas seen in the right-hand lifting condition. These findings provided fundamental data for understanding the neural network governing control of fingers in grip-lift-replacement actions.
