Neural activity in monkey motor cortex (M1) and dorsal premotor cortex (PMd) can reflect a chosen movement well before that movement begins. The pattern of neural activity then changes profoundly just before movement onset. We considered the prediction, derived from formal considerations, that the transition from preparation to movement might be accompanied by a large overall change in the neural state that reflects when movement is made rather than which movement is made. Specifically, we examined ‘components’ of the population response: time-varying patterns of activity from which each neuron’s response is approximately composed. Amid the response complexity of individual M1 and PMd neurons, we identified robust response components that were ‘condition-invariant’: their magnitude and time course were nearly identical regardless of reach direction or path. These condition-invariant response components occupied dimensions orthogonal to those occupied by the ‘tuned’ response components. The largest condition-invariant component was much larger than any of the tuned components; i.e., it explained more of the structure in individual-neuron responses. This condition-invariant response component underwent a rapid change before movement onset. The timing of that change predicted most of the trial-by-trial variance in reaction time. Thus, although individual M1 and PMd neurons essentially always reflected which movement was made, the largest component of the population response reflected movement timing rather than movement type.
Significance Statement: The activity of neurons often conveys information about externally observable variables, such as the location of a nearby object or the direction of a reach made to that object. Yet neural signals can also relate to ‘internal’ factors: the thoughts and computations that link perception to action. We characterized a neural signal that occurs during the transition from preparing a reaching movement to actually reaching. This neural signal conveys remarkably accurate information about when the reach will occur, but carries essentially no information about what that reach will be. The identity of the reach itself is carried by other signals. Thus, the brain appears to employ distinct signals to convey what should be done and when it should be done.
The authors report no conflict of interest.
Grossman Charitable Trust, National Science Foundation, Swartz Foundation, Burroughs Wellcome Fund, NIH Director’s Office, DARPA REPAIR, Searle Scholars Foundation, Sloan Research Foundation, McKnight Foundation, Esther A. & Joseph Klingenstein Fund, and the Simons Foundation.