Fitts' law is not continuous in reciprocal aiming

It takes longer to accomplish difficult tasks than easy ones. In the context of motor behaviour, Fitts' famous law states that the time needed to successfully execute an aiming movement increases linearly with task difficulty. While Fitts' explicit formulation has met criticism, the relation between task difficulty and movement time is invariantly portrayed as continuous. Here, we demonstrate that Fitts' law is discontinuous in reciprocal aiming owing to a transition in operative motor control mechanisms with increasing task difficulty. In particular, rhythmic movements are implemented in easy tasks and discrete movements in difficult ones. How movement time increases with task difficulty differs in both movement types. It appears, therefore, that the human nervous system abruptly engages a different control mechanism when task difficulty increases.

Electromyographic Control of Movement Time in a Rapid Aiming Movement

One of the major issues to emerge from research on human-limb movement is the manner in which the central nervous system regulates electromyographic (EMG) activity to produce movements that differ in duration and distance. Different models of control predict different relations between EMC characteristics and movement kinematics, particularly with regard to the role of EMC burst duration and movement time. However, models have been evaluated with means averaged over individuals and across large numbers of practice trials. The goal of this study was to assess how well individual subjects' data conform to the predictions of the control models. Participants ( n = 4) performed an elbow flexion and extension task over 45° in movement times between 90 and 260 msec. EMG amplitude and EMG burst duration from the right elbow flexors were correlated with movement time for each individual. As expected, movement time was positively correlated with EMG burst duration and negatively correlated with EMG amplitude, with wider ranges in the EMG burst duration–movement time correlations across participants. Data from all participants supported predictions of the impulse-timing control model, but the slopes of the studied relations varied across participants.

Effects of Practice and Unpredictable Distractors on Planning and Executing Aiming after Stroke

The primary purpose of this study was to examine practice effects on the planning and execution of an aiming movement after right versus left stroke. A secondary purpose was to investigate the effects of a distractor that appeared randomly on motor performance after stroke. Right-hand dominant individuals, 15 with right stroke (right-sided brain damage), 16 with left stroke, and 30 without stroke, performed aiming movements to targets. Those with stroke used the ipsilesional upper extremity (UE). Right and left comparison groups used the right and left UE, respectively. Reaction time (RT) and movement time (MT) were collected to represent movement planning and execution, respectively. Individuals with right stroke improved RT with practice. Individuals with left stroke did not improve RT with practice and made more errors than their comparison group. Those with left stroke achieved faster MT with practice, but MT remained slower than their comparison group. There were no effects of the distractor on RT or MT. Adults with left stroke have persistent deficits in movement planning and execution. Further studies are needed to determine how the performance of older adults, with or without stroke, is affected by an unpredictable visual distractor.

The Control of Sequential Goal-Directed Movement: Learning to Use Feedback or Central Planning?

Fifteen participants practiced a two-target sequential aiming movement with either full vision of the movement environment, vision during flight, or vision while in contact with the first target. After 100 acquisition trials, participants performed a retention test in their own condition and then were transferred to each of the other two vision conditions. Both performance and kinematic data indicated that rather than becoming less dependent on visual information with practice, subjects learned to adjust their movement trajectories to use the visual information available in their particular vision condition. Although transfer to a degraded vision condition disrupted performance, when vision was augmented participants quickly adjusted their aiming trajectories to use the added information. The findings suggest that at least part of learning involves the development of rapid and efficient procedures for processing afferent information, including visual response-produced feedback.

Effect of Size and Frame of Visual Field on the Accuracy of an Aiming Movement

The aim of this study was to analyse the effects of manipulating the size and contour of the visual field on the accuracy of an aiming task. Subjects were required to perform pointing movements without seeing their moving hand. The target was displayed in either a wide structured visual field (control condition), a narrow visual field with orthogonal frame, or a narrow visual field with circular frame. The visual information surrounding the target was always provided prior to movement onset, but during the execution of the movement on only half of the trials. Overall, the results showed that undershooting was a common performance characteristic in all of the conditions. In comparison to the control performance, an increase of the degree of undershoot was found when the target was displayed inside a narrower visual field. An additional radial error was found when the contour of the visual scene was circular, but only when the visual context was available during the movement. The same pattern of results was observed for variable error. However, angular errors were not found to vary over the different conditions. Overall, the findings suggested that the visual context contributed to the assessment of the target locations, and the subsequent motor programming. Furthermore, visual information aided the on-line control of the unseen hand, but the extent of this was dependent on the size and shape of the frame denoting the visual scene. Finally, in the absence of any unexpected perturbation, the en-route amendment of the arm trajectory, based on visual information processing, seemed to be more related to distance than azimuth control.