When manually intercepting moving targets, it has been shown that the characteristics of the target motion influence how the arm reaches toward the target. In computer simulations of reaching van Donkelaar, Lee and Gellman (1992) have shown that the duration of a reaching movement is shorter when the target is moving faster, both in a full vision condition, and when vision of the hand is restricted. This relationship between interception speed and target speed was also demonstrated by Smeets and Brenner (1995) in a study where participants were required to hit targets moving across a computer screen with a handheld rod, as quickly as possible. In spite of the instruction to hit all the targets as quickly as possible, it was found that fast targets were hit with a higher velocity than slow targets. In another study, when generating reaching and grasping movements to a three-dimensional target rolling down a ramp, the speed of the reach has also been shown to increase as target speed increases, and when the target was moving very slowly (<40 mm/s), participants reached even more slowly than they did for a stationary control target. Conversely, when participants self-selected the location where they would intercept a moving target, their aiming speed decreased as the target speed increased. However, this was probably due to the observation that participants grasped the faster moving targets closer to their body. The shorter movements and related lower peak velocities that were observed when intercepting the fast targets are consistent with the predictions of Fitts’ Law.

While this relationship between the speed of the interceptive motion and the speed of the moving target exists, it has not been clearly established which characteristics of the target motion are most important. As a target moves faster between two points, two movement characteristics can change. Not only is the velocity of the faster target greater, its movement time is shorter. It is not clear whether both of these two target motion characteristics can influence the interceptive movement. One of the most commonly cited models to explain the visuomotor control of target interception has been the Tau model originally proposed by Lee (1976). While there have been many variations or updates to the model over recent years, the basic premise of this model is that a time variable (time to contact) is used to trigger the appropriate interceptive action. For example the Tau model has been shown to model actions such as jumping up to intercept a falling volleyball, or the hitting of a table tennis ball. This model does not take into account the changes in the velocity characteristics of the target, only its temporal characteristics. However, in a review by Bootsma, Fayt, Zaal and Laurent (1997) the possibility is entertained that when intercepting accelerating targets a second-order variable may be used.

In a recent study in which participants intercepted targets on a computer screen, targets moved either at a constant acceleration, a constant deceleration, or at a constant velocity. It was proposed that the response time of the interceptive movement is composed of a constant processing time, and the time required for the stimulus to travel a threshold distance, thus response time is velocity dependent. It was also suggested that some participants use an estimate of time to contact to determine when to initiate their interceptive movements. It was further shown that the interceptive movements toward slow-moving targets were produced by generating a series of sub movements that were produced to keep the displacement of the hand proportional to the target position and that this process is independent of the acceleration of the target. Thus, these authors have shown that participants are not able to utilize a higher order variable (acceleration) to control their interceptive actions, but instead, base the initiation and production of their interception movements on the temporal characteristics of the moving target.

It might not be surprising that individuals in the Lee et al. (1997) study were not able to successfully utilize target acceleration information. The general consensus in the psychophysical literature is that accelerations cannot be directly perceived from target motion. Instead it has been widely argued that target accelerations are computed indirectly from the comparisons of two instantaneous velocities acquired across a specific length of time. However, Rosenbaum noted that “the perceptual system is primarily responsive to changes in stimulation, (and) it is noteworthy that acceleration involves more changes than does constant velocity”. Thus, our visuomotor system could be viewed as one that searches for the optimal set of information that will allow it to perform the required task. Most objects in real life move with non-constant velocity profiles. Therefore it would be beneficial for the visuomotor system to be able to retrieve and use visual information about changes in object velocity to produce successful interception movements. While the majority of the evidence collectively suggests that only temporal information about target motion, or information about instantaneous velocity, is used to guide interceptive actions, in the present study, changes in the velocity characteristics of the targets were manipulated.

The purpose of the present study was to examine if the changing velocity characteristics of a target have any influence on the temporal and kinematic characteristics of manual interceptive movements. Participants intercepted targets with common movement times and magnitudes of peak velocity. However, the temporal properties of the target’s trajectory were varied across conditions. If participants are able to use information about the changing velocity of the target’s trajectory then it is expected that the kinematics of the interception movement will vary as a function of target velocity condition. A related question is how fast can that information be retrieved from the trajectory of the moving target? As well, how much of the information is needed to precisely intercept the target and when is it needed? The next two experiments were designed to investigate these issues.

For tasks where the target disappears for more than one second before contact, the interception hand trajectory cannot be controlled using continuous time to contact information, but rather the visuomotor system must rely on an internal model or representation of the future target trajectory, built during the initial planning phase. This view was experimentally supported by Whiting’s (1968) study of baseball catching. With decreased amounts of visual information, the author concluded that constant visual monitoring of the ball flight was not necessary for precise interception. Whiting showed that performance did not decrease significantly with a decreased amount of visual information about the ball trajectory. Based on these findings, one can suppose that the visuomotor system is able to create an internal model of the target trajectory and use this model to produce successful manual interception. In such a case, the performance of the interception task should suffer significantly when the visual information about the target trajectory is removed from the planning period. This hypothesis was tested by Whiting and Sharp (1974) where participants were required to catch tennis balls in a dark room. For a brief period of time the ball was visible to the participant during its flight. It was found that when the visual information about the ball trajectory was blocked until very late in the flight, the catching performance suffered, emphasizing the impact of visual information early in a target’s trajectory.