Radioactive 2-DG incorporation patterns in the mesial frontal cortex of task-performing monkeys

Extended practice of a motor skill is associated with reduced metabolic activity in M1

How does long-term training and the development of motor skill modify the activity of the primary motor cortex (M1)? To address this issue we trained monkeys for ~1–6 years to perform visually-guided and internally-generated sequences of reaching movements. Then, we used ¹⁴C-2-deoxyglucose (2DG) uptake and single neuron recording to measure metabolic and neuron activity in M1. After extended practice, we observed a profound reduction of metabolic activity in M1 for the performance of internally-generated compared to visually-guided tasks. In contrast, measures of neuron firing displayed little difference during the two tasks. These findings suggest that the development of skill through extended practice results in a reduction in the synaptic activity required to produce internally-generated, but not visually-guided sequences of movements. Thus, practice leading to skilled performance results in more efficient generation of neuronal activity in M1.

Activation on the medial wall during remembered sequences of reaching movements in monkeys

We used the 2-deoxyglucose (2DG) method to map activation in the motor areas on the medial wall of the hemisphere. One group of monkeys licked juice delivered at variable time intervals (LICK task). For these animals, the motor areas on the medial wall displayed restricted activation. 2DG uptake was limited largely to the face representation of the supplementary motor area (SMA). Additional labeling was present more rostrally in the banks of the cingulate sulcus. A second group of animals performed remembered sequences of reaching movements (REM task) for juice rewards. Activation related to licking also was present in this group. In addition, separate, discrete activations were found on the superior frontal gyrus and in the cingulate sulcus during the REM task. The most intense and extensive 2DG labeling was located in the dorsal bank of the cingulate sulcus, coincident with the dorsal cingulate motor area (CMAd). Weaker activations were present in the arm area of the SMA and in the pre-SMA. There was no significant 2DG incorporation in the ventral bank of the cingulate sulcus where the ventral cingulate motor area is located. Our findings suggest that the CMAd is involved more than any other medial area in the preparation for and/or execution of highly practiced, remembered sequences of movements. Overall, our results indicate that the attributes of motor control are not represented equally across the motor areas on the medial wall.