Emilienne Musch

Intercepting Moving Objects During Self-Motion

It is generally assumed that in catching a fly ball, an efficient strategy for dealing with the horizontal component of the balls trajectory is for the observer to keep the angular position of the ball constant with respect to his head. That strategy is called the constant bearing angle or CBA strategy. Maintenance of angular constancy results in the simultaneous arrival of both observer and ball at the landing spot. The authors analyzed the approach behavior of 26 subjects in a ball-interception task with straight paths for both the subjects and the ball. Subjects moved at a velocity that maintained a close-to-constant horizontal angular position of the ball with respect to the end effector throughout the approach phase rather than a constant bearing angle with respect to their head. Velocity adaptations occurred as a function of the changes in the angular velocity of the ball in such a way that a positive or negative angular velocity was canceled. Thus, an actor following the CBA strategy does not need to know where and when the ball will arrive (i.e., a predictive strategy), because reliance on the CBA strategy ensures that he will make the appropriate adaptations that enable him to arrive at the right place in the right time.

Intrinsic and extrinsic factors of turning preferences in humans.

Turning behaviour in 107 adolescents was observed during walking and running under different temporal and biomechanical constraints. Participants ran and walked back and forth between two lines 9.5m apart in a neutral environment. All of the turns that the participants made to change direction between the lines were videotaped. A general preference for turning leftwards was observed with the percentage being higher in the unconstrained running condition when compared to the walking condition (71% versus 59%, respectively). This intrinsic preference was easily overruled when positional constraints on the starting position were imposed. Such positional constraints did not, however, suppress the intrinsic directional bias observed during running. It is concluded that turning preference in humans is the result of a complex interaction between intrinsic preferences and externally imposed task constraints.