The spoon task: a trajectory-constrained speed-accuracy trade-off task in children with dystonia

Previous studies have shown that in children with dystonia motor speed is significantly reduced in order to account for increased variability, and the nature of the relationship between speed and accuracy is altered during simple point-to-point movements. Deriving a speed-accuracy trade-off function is a preferred metric for execution assessment. Most speed-accuracy tasks are characterized with Fitts’ Law. Although this is a useful approach, it has mostly been applied in terms of the end-state goal. However, tasks involved in daily life rarely match this scenario because the quality of the motor performance goes beyond the accuracy on the final target. Therefore, there is the need to quantitatively assess the motor performance in tasks requiring accurate execution throughout the entire movement trajectory. Furthermore, the motor assessment has to be suitable for both healthy children and children with dystonia, involving simple and easily performed tasks. For this reason, we have defined a task mimicking components of self-feeding, where subjects were asked to transport a marble in a spoon back and forth between two targets, as fast as possible and without letting the marble fall. The result of the last requirement is that the accuracy of the performance was constrained along the whole trajectory. To test if the Fitts’ Law holds for this task, five different Indexes of Difficulty (ID) were tested by simply changing the spoon size. During the performance of the task, spoon and upper-limb kinematics were recorded in children with dystonia and age-matched healthy controls. First we tested whether the task follows Fitts’ Law by studying the linear regression between Movement Time and ID. A significant regression coefficient was found, meaning that the task effectively imposes a speed-accuracy trade-off that applies to an entire trajectory. Further analyses were mainly aimed at characterizing the dystonic motor behavior in terms of kinematic trajectories, velocity and accuracy profiles, and motor variability. As expected, subjects with showed less smooth trajectories, reduced velocity and acceleration peaks, and higher variability of movement. We also investigated the Index of Performance (IP), the correlation between IP and variability, and the correlation between IP and Severity Rating Score in children with dystonia. Results revealed a significantly lower IP for children with dystonia, and a significant correlation between IP and variability, suggesting that the IP for this task is an efficient parameter to quantitatively assess the motor performance in children with movement disorders.