Many sensorimotor activities have a time constraint for successful completion. In this case, any time devoted to sensory processing is at the expense of time available for motor execution. Earlier studies have explored how this competition between sensory processing and motor execution is resolved by using experimental designs that segregate the sensing and acting phase of the task. It was found that participants switch from the sensing to the acting stage such that the overall (sensorimotor) uncertainty in the outcome of the task is minimized. An unexplained observation in these studies is the substantial variability in switching times. We investigated the variability in switching time by correlating it with the underlying sensorimotor uncertainty. To this end, we used a modified version of the virtual ball catching paradigm proposed by Faisal & Wolpert (2009). Subjects were instructed to catch a ball, but as soon as they initiated their movement the ball disappeared. We extended the range of horizontal velocities and used two different start positions of the ball to quantify the dependence of sensory uncertainty on ball velocity. Velocity dependence of sensory uncertainty allowed us to manipulate sensory uncertainty and hence the sensorimotor uncertainty. We found that the variability in switching times is correlated with two factors. Firstly, variability in switching times is greater when variation in switching time has only minimal effects on sensorimotor uncertainty. Secondly, variability in switching times is greater when the sensorimotor uncertainty is larger. Our results suggest that the variability in switching time reflects an uncertainty-driven exploratory process.

Behavioural evidences suggest that sequential saccades to multiple stimuli are planned in parallel. However, it remains unclear whether such parallel programming reflects concurrent processing of goals or whether multiple motor plans coexist, unfolding subsequently during execution. Here we use midway saccades, directed at intermediate locations between two targets, as a probe to address this question in a novel double-step adaptation task. The task consisted of trials where subjects had to follow the appearance of two targets presented in succession with two sequential saccades. In some trials, the second target predictably jumped to a new location during the second saccade. Initially, the second saccade was aimed at the final target's location before the jump. As subjects adapted to the target jump, saccades were aimed to the second target's new location. We tested whether the spatial distribution of midway saccades could be explained as an interaction between two concurrent saccade goals, each directed at the two target locations, or between the initial motor plan to the first target location and a prospective motor plan directed from the initial to the final target location. A shift in the midway saccades' distribution towards the jumped location of the second target following adaptation indicated that the brain can make use of prospective motor plans to guide sequential eye movements. Furthermore, we observed that the spatiotemporal pattern of endpoints of midway saccades can be well explained by a motor addition model. These results provide strong evidence of parallel activation of prospective motor plans during sequential saccades.

While slowness of movement is an obligatory characteristic of Parkinson’s disease (PD), there are conditions in which patients move uncharacteristically fast, attributed to deficient motor inhibition. Here we investigate deficient inhibition in an optimal sensory-motor integration framework, using a game in which subjects used a paddle to catch a virtual ball. Display of the ball was extinguished as soon as the catching movement started, segregating the task into a sensing and acting phase. We analyzed the behavior of 9 PD patients (ON medication) and 10 age-matched controls (HC). The switching times (between sensing and acting phase) were compared to the predicted optimal switching time, based on the individual estimates of sensory and motor uncertainties. The comparison showed that deviation from predicted optimal switching times were similar between groups. However, PD patients showed a weaker correlation between variability in switching time and sensory-motor uncertainty, indicating a reduced propensity to generate exploratory behavior for optimizing goal-directed movements. Analysis of the movement kinematics revealed that PD patients, compared to controls, used a lower peak velocity of the paddle and intercepted the ball with greater velocity. Adjusting the trial duration to the time for the paddle to stop moving, we found that PD patients spent a smaller proportion of the trial duration for observing the ball. Altogether, the results do not show the premature movement initiation and truncated sensory processing that we predicted to ensue from deficient inhibition in PD.