Regional cerebral blood flow (rCBF) studies of normal subjects by PET reveal cortical areas most activated during simple upper extremity motor tasks. Activity increases in the contralateral Ml and the premotor, SMA, ventral premotor, and parietal cortical areas that are linked to it during proximal and distal arm, finger, and whole-hand movements.209 Color Figure 1-8 (in separate color insert) is a fine example of the association of cortical and subcortical activations during self-paced, flexion and extension of the fingers or toes.210 In half of the subjects, the ipsilateral putamen was activated. The toe activated the dorsal putamen and the fingers localized more ventral and medial. When a subject decides which fingers to move or learns a finger tapping sequence, the caudate and putamen become active rostral to the anterior commissure, consistent with a separate cortico-basal ganglia-thalamo-cortical loop. Passive movements of the hand and foot also activate sensorimotor cortices (see Chapter 3), which is useful for studying plasticity-related motor recovery over time when subjects start with paralysis of a limb.
In a PET experiment that helps define the distributed motor system, subjects were studied under four conditions. They moved a joystick (1) after a tonal cue according to a previously trained sequence, (2) in random directions, (3) when the correct movement was specified by one of the four tones they heard, and (4) when the correct movement was the opposite of what had been specified by the tones on the previous task.209 The rCBF during these tasks was compared to simply moving the joystick forward at the same rate as the other tasks after a sound cue. The SMA had greater metabolic activity in the first two conditions. These tasks required internal generation of a movement, whereas the second two were directed by external cues. Activity within the left superior parietal cortex increased in all four conditions, suggesting that the process by which movement is selected is coded here. The bilateral premotor cortices, which are synapti-cally linked to the parietal region, were activated in all conditions as well. In the random condition, several frontal areas and the cingu-late sulcus were activated, pointing to the contribution of these areas for self-initiated acts.
Another experiment points to the contribution of parallel, cortical networks in a distributed motor system. Muscle strength increased by having individuals practice at imagining they were contracting a particular muscle.211 Indeed, when the abductor digiti quinti was exercised to increase its strength and when subjects only imagined that they practiced abduction against resistance, the abductor of the untrained hand also increased in strength. This neural, as opposed to muscular, origin for strengthening also seems to occur during physical exercise before any muscle hypertrophy is evident. Mental exercise that improves muscle strength is associated with an increase in elec-troencephalographic-derived cortical potentials.212 Positron emission tomography and other techniques suggest that this strengthening derives from the activation of central motor planning areas outside of M1. These areas increase the coordination and strength of outputs to spinal motoneurons.213 The transfer of the motor program to the nonexercised digit may have been via the corpus callosum or by bilateral activation of the SMA. It appears, from limited studies, that BA 4 makes callosal connections, but not in the hand and foot areas. The bilateral SMAs, however, are highly interconnected with each other and with contralateral BA 4.131 Increased strength, as opposed to an increase in skill, is probably not associated with a need for reorganization of movement representations for the active muscles.214
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