Motor decision-making under uncertainty and time pressure.

Purposeful movement often requires selection of a particular action from a range of alternatives, but how does the brain represent potential actions so that they can be compared for selection, and how are motor commands generated if movement is initiated before the final goal is identified? According to one hypothesis, the brain averages partially prepared motor plans to generate movement when there is goal uncertainty. This is consistent with the idea that motor decision-making unfolds through competition between internal representations of alternative actions. An alternative hypothesis holds that only one movement, which is optimized for task performance, is prepared for execution at any time. Under this conception, decisions about the best motor goal given current information are completed upstream from neural circuits that perform motor planning. To distinguish between these hypotheses, we modified an experiment (Alhussein L, Smith MA. eLife 10: e67019, 2021) in which participants had to start reaching toward targets associated with opposite curl force fields before knowing the correct target to reach. Crucially, we forced the participants to initiate movement immediately after target presentation (i.e., mean reaction times ∼250 ms) so that they had limited opportunity to deliberate between the available alternatives. We found that the reaching dynamics reflected only those learned for the selected reach direction, rather than a combination of those for the alternative targets presented, irrespective of the time available to initiate movement. The data are consistent with the conclusion that reaching dynamics were specified downstream of action selection under the target uncertainty conditions of this study.NEW & NOTEWORTHY Here we found no evidence of "motor averaging" of reach dynamics for multiple potential actions when people had to respond as quickly as possible to uncertain target location cues. People exerted forces appropriate for the specific reach direction they selected irrespective of movement initiation time, suggesting that reaching dynamics were specified downstream of action selection.