Statistical decision theory suggests that choosing an ideal action requires taking several factors into account: (1) prior knowledge of the probability of various world states, (2) sensory information concerning the world state, (3) the probability of outcomes given a choice of action, and (4) the loss or gain associated with those outcomes. In previous work, we found that, in many circumstances, humans act like ideal decision makers in planning a reaching movement. They select a movement aim point that maximizes expected gain, thus taking into account outcome uncertainty (motor noise) and the consequences of their actions. Here, we ask whether humans can optimally combine prior knowledge and uncertain sensory information in planning a reach. Subjects rapidly pointed at unseen targets, indicated with dots drawn from a distribution centered on the invisible target location. Target location had a prior distribution, the form of which was known to the subject. We varied the number of dots and hence target spatial uncertainty. An analysis of the sources of uncertainty impacting performance in this task indicated that the optimal strategy was to aim between the mean of the prior (the screen center) and the mean stimulus location (centroid of the dot cloud). With increased target location uncertainty, the aim point should have moved closer to the prior. Subjects used near-optimal strategies, combining stimulus uncertainty and prior information appropriately. Observer behavior was well modeled as having three additional sources of inefficiency originating in the motor system, calculation of centroid location, and calculation of aim points.

Hudson TE, Maloney LT, Landy MS. Movement planning with probabilistic target information. J Neurophysiol 98: 3034 -3046, 2007. First published September 26, 2007; doi:10.1152/jn.00858.2007. We examined how subjects plan speeded reaching movements when the precise target of the movement is not known at movement onset. Before each reach, subjects were given only a probability distribution on possible target positions. Only after completing part of the movement did the actual target appear. In separate experiments we varied the location of the mode and the scale of the prior distribution for possible targets. In both cases we found that subjects made use of prior probability information when planning reaches. We also devised two tests (Composite Benefit and Row Dominance tests) to determine whether subjects' performance met necessary conditions for optimality (defined as maximizing expected gain). We could not reject the hypothesis of optimality in the experiment where we varied the mode of the prior, but departures from optimality were found in response to changes in the scale of prior distributions. I N T R O D U C T I O NPerformance in speeded-reaching tasks is often assessed by examining movements toward a spatial target at a known position in space. The target is visible before the start of movement and the key task for the motor system is to plan the most effective movement possible to reach the target (e.g., Körding and Wolpert 2004;Sabes and Jordan 1997;Todorov and Jordan 2002; Trommershäuser et al. 2003a,b). Other researchers have demonstrated that the motor system can update a planned movement in response to unanticipated changes in position, velocity, and visual properties of a fixed target (Brenner and Smeets 2004;Elliott et al. 1999;Komilis et al. 1993;Pélisson et al. 1986;Saunders and Knill 2004;Schmidt 2002). In all of these studies, a specific target is visible before movement onset even if the subject is fully aware it may unpredictably change location during the actual movement.It is conceptually difficult to separate movement planning from movement execution in such tasks because the movement plan (including possible compensation for changes in target location) would likely be fully formed before movement onset (e.g., Bédard and Proteau 2004;Gribble et al. 2003;Heath et al. 2004;Rabin and Gordon 2004;Saunders and Knill 2004;Torres and Zipser 2004;Vindras and Viviani 2002). There are, however, natural movements for which there is substantial initial uncertainty concerning the final spatial goal of the movement, and the initial part of the movement must therefore be planned relative to the uncertainty of the goal information available before movement. In water polo, for example, an attacker must often plan and initiate a shot on the goal while a defender is simultaneously attempting to block the shot. Neither attacker nor defender can anticipate with certainty the actions of the other at movement onset and each can potentially react to the other's movement during the brief duration of the attack. T...

Motor control requires the generation of a precise temporal sequence of control signals sent to the skeletal musculature. We describe an experiment that, for good performance, requires human subjects to plan movements taking into account uncertainty in their movement duration and the increase in that uncertainty with increasing movement duration. We do this by rewarding movements performed within a specified time window, and penalizing slower movements in some conditions and faster movements in others. Our results indicate that subjects compensated for their natural duration-dependent temporal uncertainty as well as an overall increase in temporal uncertainty that was imposed experimentally. Their compensation for temporal uncertainty, both the natural duration-dependent and imposed overall components, was nearly optimal in the sense of maximizing expected gain in the task. The motor system is able to model its temporal uncertainty and compensate for that uncertainty so as to optimize the consequences of movement.

ObjectiveThe King–Devick (KD) test, which is based on rapid number naming speed, is a performance measure that adds vision and eye movement assessments to sideline concussion testing. We performed a laboratory‐based study to characterize ocular motor behavior during the KD test in a patient cohort with chronic concussion to identify features associated with prolonged KD reading times.MethodsTwenty‐five patients with a concussion history (mean age: 31) were compared to control participants with no concussion history (n = 42, mean age: 32). Participants performed a computerized KD test under infrared‐based video‐oculography.ResultsAverage intersaccadic intervals for task‐specific saccades were significantly longer among concussed patients compared to controls (324.4 ± 85.6 msec vs. 286.1 ± 49.7 msec, P = 0.027). Digitized KD reading times were prolonged in concussed participants versus controls (53.43 ± 14.04 sec vs. 43.80 ± 8.55 sec, P = 0.004) and were highly correlated with intersaccadic intervals. Concussion was also associated with a greater number of saccades during number reading and larger average deviations of saccade endpoint distances from the centers of the to‐be‐read numbers (1.22 ± 0.29° vs. 0.98 ± 0.27°, P = 0.002). There were no differences in saccade peak velocity, duration, or amplitude.InterpretationProlonged intersaccadic intervals, greater numbers of saccades, and larger deviations of saccade endpoints underlie prolonged KD reading times in chronic concussion. The KD test relies upon a diffuse neurocognitive network that mediates the fine control of efferent visual function. One sequela of chronic concussion may be disruption of this system, which may produce deficits in spatial target selection and planning of eye movements.

Objective Concussion is a major public health problem and considerable efforts are focused on sideline-based diagnostic testing to guide return-to-play decision-making and clinical care. The King–Devick (K–D) test, a sensitive sideline performance measure for concussion detection, reveals slowed reading times in acutely concussed subjects, as compared to healthy controls; however, the normal behavior of eye movements during the task and deficits underlying the slowing have not been defined. Methods Twelve healthy control subjects underwent quantitative eye tracking during digitized K–D testing. Results The total K–D reading time was 51.24 (±9.7) seconds. A total of 145 saccades (±15) per subject were generated, with average peak velocity 299.5°/s and average amplitude 8.2°. The average inter-saccadic interval was 248.4 ms. Task-specific horizontal and oblique saccades per subject numbered, respectively, 102 (±10) and 17 (±4). Subjects with the fewest saccades tended to blink more, resulting in a larger amount of missing data; whereas, subjects with the most saccades tended to make extra saccades during line transitions. Conclusions Establishment of normal and objective ocular motor behavior during the K–D test is a critical first step towards defining the range of deficits underlying abnormal testing in concussion. Further, it sets the groundwork for exploration of K–D correlations with cognitive dysfunction and saccadic paradigms that may reflect specific neuroanatomic deficits in the concussed brain.