Generation of Rapid Sequences by Motor Cortex

Rapid execution of motor sequences is believed to depend upon the fusing of movement elements into cohesive units that are executed holistically. We sought to determine the contribution of motor cortex activity to this ability. Two monkeys performed highly practiced two-reach sequences, interleaved with matched reaches performed alone or separated by a delay. We partitioned neural population activity into components pertaining to preparation, initiation, and execution. The hypothesis that movement elements fuse makes specific predictions regarding all three forms of activity. We observed none of these predicted effects. Instead, two-reach sequences involved the same set of neural events as individual reaches, but with a remarkable temporal compression: preparation for the second reach occurred as the first was in flight. Thus, at the level of motor cortex, skillfully executing a rapid sequence depends not on fusing elements, but on the ability to perform two computations at the same time.