Updated on 04 Jul. 2008

 

 

 

 

Keynote abstracts

Intercepting moving objects : does predictive information matter?
Gilles Montagne1, Antoine Morice1, Matthieu François1 and David Jacobs2
 1 Institute of Movement Sciences, University of the Mediterranean, Marseilles, France
2 Research Group on Perception and Action, Faculty of Psychology, University of Madrid, Spain.

  
During the last few decades, a number of laws of control have been proposed to account for the perceptual control of goal-directed displacements (e.g., Chardenon, Montagne, Laurent, & Bootsma, 2004; Warren, Kay, Zosh, Duchon, & Sahuc, 2001; Wilkie & Wann, 2002). In this presentation we will focus on interceptive tasks using a paradigm in which participants adjust their walking speed in order to intercept moving targets. A control law that has been shown to be used in this task is the constant bearing angle (CBA) strategy. The bearing angle is defined as the angle subtended by the current position of the target and the direction of displacement of the observer. The CBA strategy holds that walking so as to maintain the bearing angle constant leads one to intercept the target. We have previously shown that this apparently simple strategy accounts for the regulation of participants’ walking speed when task constraints are varied within trials and between trials, and also when the informational content of the visual scene is impoverished or even biased. Now the (impoverished) visual environments that were used in these previous studies consisted of a textured ground plane and the ball to be intercepted and, as such, provided few alternatives to the use of the CBA strategy.
The CBA strategy is prospective in the sense that the rate of change in bearing angle informs participants about the sufficiency of their current regulation. Participants are supposed to compensate changes in bearing angle by accelerating accordingly. This strategy gives rise to an on-line control of the action independently of the place and time of arrival of the ball. In the framework of predictive strategies, both place and time of arrival of the ball act as input variables. As an example, pre-programmed interceptive movements might be triggered when predictive information reaches a threshold value. Such strategies are likely to be used only in a limited range of interceptive tasks, for instance, when the movement time is very short and/or when a moving target is intercepted indirectly. Now, in addition to purely prospective and purely predictive strategies, hybrid strategies can also be involved in the control of interceptive tasks. As an example, predictive information can be embedded into a control law: time-to-contact information and information concerning the future interception point are involved in the required velocity strategy proposed by Peper, Bootsma, Mestre and Bakker (1994).
In this presentation we will review recent experiments designed to test the robustness of a pure prospective strategy (i.e., the CBA strategy) through the manipulation of the predictive information available while performing an interceptive task.		    These manipulations include target size falsifications (Bastin, Jacobs, Morice, Craig, & Montagne, under review) and visualization of the ball trajectory (François, Morice, Jacobs & Montagne, submitted). Visualizing the trajectory consisted of presenting the trajectory of the ball, from the starting position of the ball to the place of contact, as a white stripe through the virtual environment. When the ball trajectory was visualized, predictive information concerning the ball path curvature and the place of arrival of the ball were made explicit. Remember that CBA strategy relies on a single optical component, namely, the rate of change in the bearing angle. This means that the CBA strategy predicts that the kinematics of interception should not be affected by these manipulations (i.e., size falsification and trajectory visualization).
Target size manipulations were showed to affect the kinematics of walking marginally; conversely, visualization of the ball trajectory had large effects on the walking kinematics. Taken together, these results indicate that the CBA strategy operates in informationally poor environments and that other strategies, integrating predictive components (e.g., a required velocity strategy), in informationally rich environments. A comparison of the observed walking kinematics with quantitative predictions of the respective strategies confirmed these results. We will conclude that agents are able to take advantage of the information that happens to be available in a visual scene by selecting a suitable control law, and we will discuss the theoretical consequences of this conclusion.
References
Bastin, J., Jacobs, D.M., Morice, AHP., Craig, C., & Montagne, G. (under review). Testing the role of expansion in the prospective control of locomotion. Experimental Brain Research.
Chardenon, A., Montagne, G., Laurent, M., & Bootsma, R.J. (2004). The perceptual control of goal-directed locomotion: a common architecture for interception and navigation? Experimental Brain Research, 158, 100-108.
François, M., Morice, A.H.P., Jacobs, D.M., & Montagne, G. (submitted). Environmental constraints modify the way an interceptive action is controlled.
Peper, C.E., Bootsma, R.J., Mestre, D.R., & Bakker, F.C. (1994). Catching balls: How to get the hand to the right place at the right time. Journal or Experimental Psychology: Human Perception and Performance, 20, 591-612.
Warren, W.H., Kay, B.A., Zosh, W.D., Duchon, A.P., & Sahuc, S. (2001). Optic flow is used to control human walking. Nature Neuroscience, 4, 213-216.
Wilkie, R.M., & Wann, J.P. (2002). Driving as night falls: the contribution of retinal flow and visual direction to the control of steering. Current Biology, 12, 2014-2017.

 

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