Using sequential structure to improve visuomotor control

Using Sequential Structure to Improve Visuomotor Control Anna Ma-Wyatt (anna.mawyatt@adelaide.edu.au) School of Psychology, University of Adelaide, SA 5005, Australia Daniel J. Navarro (daniel.navarro@adelaide.edu.au) School of Psychology, University of Adelaide, SA 5005, Australia the lettuce before it is requested. If one does not in fact sub- sequently order lettuce, this occasionally produces errors. Taken together, these two observations (that targets can move, and that people’s prior beliefs about targets matter) suggest that when people are asked to point to targets that can move in a potentially predictable manner, people will try to anticipate where the target is likely to appear, and use this knowledge to improve the accuracy of their pointing. In this paper, we explore this topic. We report an experiment and a simple model developed to investigate the extent to which people exploit the statistical background knowledge to alter movements under time pressure. Abstract In daily life, people make rapid, goal-directed movements to interact with their environment. Since these movements are goal-directed, the outcome of the movement is important. A plan is typically formulated to make the movement using vi- sual information about target location before the movement is initiated. However, in dynamic environments people need to track the location of an object if it moves during the reach. Additionally, it would be beneficial to motor performance to learn the distribution of target locations over multiple reaches. In this paper we develop a simple model that describes how people might exploit the sequential structure over a series of trials to improve rapid visuomotor control. We then present empirical data from a sequential tracking task that investigates how people’s knowledge of the location of objects is updated over trials to improve pointing performance. The model is able to predict when people will hit and miss targets with reason- able success. Interestingly, the results suggest that not only are people able to use sequential information but also look for it even when it does not exist. Influences on Rapid Pointing Keywords: visuo-motor control; Bayesian models; sequential learning Introduction People regularly make goal-directed reaches to interact with the world, where the goal or target of these movements is usu- ally visually defined. During the course of such a reach, vi- sual information about the target location relative to the hand will necessarily change. In part, this is because the hand is moving, but it can also be because the target itself is moving. In some cases the target is essentially stationary (e.g., press- ing the car stereo button) and thus easily predictable. In oth- ers, the motor plan must be adapted during the course of the movement, because the target can move in an unpredictable fashion (e.g., swatting a mosquito). To be able to reach ac- curately and precisely, people must therefore be able to code not only the location of the goal, but also be able to respond to any changes in information about the goal that might affect the outcome of the reach. It makes intuitive sense that people develop beliefs about the probability of events occurring, and that these beliefs might be updated as the events actually occur. For example, rapid motor control in everyday life appears to show a strong effect of prior expectation. Consider the visuomotor control problems facing the “sandwich artists” at a delicatessen dur- ing the busiest time of day: most people who order sand- wiches will ask for lettuce as one of the ingredients. As a consequence, it is not uncommon for the server to reach for There are several key factors that significantly influence rapid pointing performance. The quality of visual information about target location available during the planning stage can limit pointing precision (Ma-Wyatt & McKee, 2006); simi- larly, the expected error associated with the reach influences the planning, which in turn relies on visual information about where the hand landed relative to the target (Trommersh¨auser et al, 2005). Finally, the well known speed-accuracy trade-off for controlling movements (Fitts, 1954) means that time pres- sure can play an important role: without time pressure, people have ample time to integrate feedback and refine movements, and so will need to rely less on their prior beliefs. More closely related to the current work is the finding that people make use of information differently depending on when it becomes available. In particular, the weight given to visual information is increased in the final stages of the move- ment, most likely because it tends to be the most informative about the outcome of the movement (e.g. Sober & Sabes, 2005). Moreover, there is some evidence that people can optimally integrate multiple sources of sensory input during a reach. For instance, K¨ording and Wolpert (2004) demon- strated that if there was increased uncertainty associated with new visual feedback during a reach, observers relied more heavily on prior beliefs about sensory estimates, integrating these estimates in a manner consistent with Bayesian statis- tics. In short, it may be that the visuomotor system can adapt to changing sources of information over short time scales to achieve and maintain a consistent level of motor performance. An interesting extension to this line of work is to consider how the information available to the observer changes over a time scale of multiple trials. For example, a recent exper- iment by Ma-Wyatt & McKee (2007) investigated people’s ability to could correct their reach in an online fashion. If the