The Impact of Real and Illusory Perturbations on the Early Trajectory Adjustments of Goal-Directed Movements

ABSTRACT

Examinations of goal-directed movements reveal a process of control that operates to make adjustments on the basis of the expected visual afference associated with the limb's movement. This experiment examined the impact of perturbations to the perceived and actual velocity of aiming movements when each was presented alone or in tandem with the other. Perturbations to perceived velocity were achieved by translating the background over which aiming movements were performed. An aiming stylus that discharged air either in the direction of the movement or in the direction opposite the movement generated the actual velocity perturbations. Kinematic analyses of the aiming movements revealed that only the actual perturbation influenced the control of early movement trajectories. The results are discussed with respect to the influence that visual information has on the control exerted against physical perturbations. Speculations are raised regarding how potential for perturbations influences the strategies adopted for minimizing their impact.

Keywords:

• feedback
• motor control
• movement
• speed accuracy

ACKNOWLEDGMENTS

This research was funded by the Natural Sciences and Engineering Council of Canada.

Notes

To dissociate movements into the planning and feedback-based subcomponents described initially in Woodworth's (1899) two-component model of goal-directed action, movement scientists have characterized certain kinematic characteristics as indicative of the end of the initial impulse or the beginning of feedback-based control: positive-to-negative zero line crossings in velocity (i.e., a movement reversal); negative-to-positive zero line crossings in acceleration (i.e., an increase an velocity following a period of slowing down); and significant deviations in negative acceleration (i.e., abrupt changes in limb braking that are not accompanied by an increase in velocity; Khan et al., 2006). Proteau and Masson's (1997) specific criteria for identifying the end of a movement's initial component was the point following peak deceleration in which the change in cursor velocity was less than 7.4 cm/s for 50 ms.

Similar evidence of early movement control based on dynamic information has come from studies that show the continuous modification of effector acceleration on the basis of the perceived velocity of a target (Brenner, Smeets, & de Lussanet, 1998; Smeets & Brenner, 1995).

Using the vertices of Müller-Lyer illusions as the intended target has been shown to be effective in altering the perception of target position and eliciting corrections in the late portions of reach trajectories (Elliott & Lee, 1995; Mendoza et al., 2006; Mendoza, Hansen, Glazebrook, Keetch, & Elliott, 2005).

The movement trajectories were also examined in the two axes perpendicular to the direction of the movement. These analyses revealed no impacts of our two manipulations. Although the lack of differences in the two axes perpendicular to the movement is important in highlighting that the performers did not rotate the stylus in their hand during air expulsion, these findings are otherwise superlative to the trajectory characteristics of the primary direction of the movement and, as such, are not reported here.