Abstract
Studies on the direction of a driver’s gaze while taking a bend show that the individual looks toward the tangent-point of the inside curve. Mathematically, the direction of this point in relation to the car enables the driver to predict the curvature of the road. In the same way, when a person walking in the street turns a corner, his/her gaze anticipates the rotation of the body. A current explanation for the visuo-motor anticipation over the locomotion would be that the brain, involved in a steering behavior, executes an internal model of the trajectory that anticipates the completion of the path, and not the contrary. This paper proposes to test this hypothesis by studying the effect of an artificial manipulation of the visuo-locomotor coupling on the trajectory prediction. In this experiment, subjects remotely control a mobile robot with a pan-tilt camera. This experimental paradigm is chosen to manipulate in an easy and precise way the temporal organization of the visuo-locomotor coupling. The results show that only the visuo-locomotor coupling organized from the visual sensor to the locomotor organs enables (i) a significant smoothness of the trajectory and (ii) a velocity-curvature relationship that follows the “2/3 Power Law.” These findings are consistent with the theory of an anticipatory construction of an internal model of the trajectory. This mental representation used by the brain as a forward prediction of the formation of the path seems conditioned by the motor program. The overall results are discussed in terms of the sensorimotor scheme bases of the predictive coding.
Highlights
Various studies showed that many different human movements seem to follow a same mathematical relationship known as the “2/3 Power Law” (Lacquaniti et al, 1983; Viviani and Schneider, 1991)
A consensual explanation for the visuo-motor anticipation over the locomotion would be that the brain, involved in a steering behavior, executes an internal model of the trajectory that anticipates the completion of the path, and not the contrary (Berthoz, 1997)
The results show that the “control condition” and the “nonhuman model” of anticipation do not exhibit a significant www.frontiersin.org correlation between the tangential speeds and the curve radii (R = 0.13; NS, for the “control condition” and R = 0.16; NS, for the “non-human condition”)
Summary
Various studies showed that many different human movements seem to follow a same mathematical relationship known as the “2/3 Power Law” (Lacquaniti et al, 1983; Viviani and Schneider, 1991). Whether the action is writing (Viviani and Cenzato, 1985) or walking (Vieilledent et al, 2001), an identical constraint relationship between the velocity and the curvature of the motor trajectory is involved This law states that the angular velocity of the end effector is proportional to the two-thirds root of its curvature or, equivalently, that the instantaneous tangential velocity is proportional to the third root of the radius of curvature. What is most remarkable is the correlation between these high inflection parts and the number of occurrences of ocular fixations (Reina and Schwartz, 2003) This observation presumes that the motor control would need a superior visual feedback to compensate the higher instantaneous complexity of the geometry of the movement
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