Motor cortex fine-tunes preparatory activity to cope with uncertainty

Voluntary movement is often prepared by motor cortex before its execution. Motor cortical neurons exhibit
preparatory-activity, a neural correlate of upcoming movement [1]. Animals adopt different behavioral
strategies in response to uncertainties existing in nature such as environmental cue informing ‘go’ [2].
However, how motor cortical circuits cope with such uncertainty while benefiting from explicit information
is less understood. Here, we analyzed firing activity of anterolateral motor cortex (ALM) while mice
performed one of the two different auditory delayed-response tasks: random or fixed delay task. In random
delay task, delay length was randomly determined thus mice could not predict when go cue would be given.
In fixed delay task, delay length was fixed across trials. We sought out preparation subspace (P) to infer
putative preparatory-activity. In both tasks, we found that preparatory-activity on P increased during delay
and mechanistic of upcoming movement (e.g., RT) was predicted by neuronal state on P (data not shown
here). In random delay task, preparatory-activity on P quickly reached a state to make movement right after
delay started and persisted until go cue. Unexpectedly, progression of preparatory-activity on P made RT
longer in fixed delay task while resulting in more accurate decisions. In fact, there were positive correlations
between behavioral accuracy and RT only in the fixed delay task. Both neuronal and behavioral data
supported that preparation process was altered to increase decision accuracy by sacrificing RT to receive
more rewards. We further tested why increasing decision accuracy resulted in longer RT. We found that as
preparatory-activity was altered in direction of increasing accuracy, it required a larger cost in state transition
which result in longer RT. In conclusion, preparatory-activity in motor cortex is altered depending on the
presence of time uncertainty by fine-tuning preparatory-activity in a way to respond to temporal
predictability.