Predictive Action in Infancy: Tracking and Reaching for Moving Objects

Because action plans must anticipate the states of the world which will be obtained when the actions take place, effective actions depend on predictions. The present experiments begin to explore the principles underlying early-developing predictions of object motion, by focusing on 6-month-old infants' head tracking and reaching for moving objects. Infants were presented with an object that moved into reaching space on four trajectories: two linear trajectories that intersected at the center of a display and two trajectories containing a sudden turn at the point of intersection. In two studies, infants' tracking and reaching provided evidence for an extrapolation of the object motion on linear paths, in accord with the principle of inertia. This tendency was remarkably resistant to counter-evidence, for it was observed even after repeated presentations of an object that violated the principle of inertia by spontaneously stopping and then moving in a new direction. In contrast to the present findings, infants fail to extrapolate linear object motion in preferential looking experiments, suggesting that early-developing knowledge of object motion, like mature knowledge, is embedded in multiple systems of representation. 1998 Elsevier Science B.V. All rights reserved Keywords: Reaching; Infant cognition; Inertia 0010-0277/98/$19.00 1998 Elsevier Science B.V. All rights reserved PII S0010-0277(98)00029-8 COGNITION Cognition 67 (1998) 255--285 * Corresponding author

Haptically Linked Dyads Are Two Motor-Control Systems Better Than One?

We report performance of haptically linked dyads on a targetacquisition task, comparing it with that of the same individuals when they performed the task individually. In the dyad condition, a subject’s limb motion responds to the output of two motorcontrol systems—the subject’s own and his or her partner’s— which might be expected to complicate motor planning and efficient task execution. However, task completion times indicated that dyads performed significantly faster than individuals, even though dyad members exerted large task-irrelevant forces in opposition to one another, and despite many participants’ perceptions that their partner was an impediment. A much earlier study of teams using a pursuit rotor (Wegner & Zeaman, 1956) found a similar performance increment. Since that study, there has been little research on physically coupled dyads (Sallnas & Zhai, 2003; Shergill, Bays, Frith, & Wolpert

Haptic Cooperation between People, and between People and Machines

Haptic interaction between people and machines might benefit from an understanding of haptic communication between one person and another. We recently reported results showing that two people performing a physically shared dyadic task can outperform either person alone, even when the perception of each participant is that the other is a hindrance [1]. Evidently a dyad quickly negotiates a more efficient motion strategy than is available to individuals. This negotiation must take place through a haptic channel of communication, and it is apparently at a level below the awareness of the participants. Here we report results on the motion strategy that emerged. By recording forces and motions we show that the dyads specialized temporally such that one member took on early parts of the motion and the other late parts. Tests in which one participant\u27s contribution was surreptitiously replaced by a motor did not elicit a similar cooperative response from the remaining human participant, showing that the language of haptic communication between people must be rather subtle

Short Report

ages 18--24) from Northwestern University's psychology participant pool participated with informed consent. In each session, 2 randomly selected subjects stood on opposite ends of a two-handled rigid crank (Fig. 1), separated by a curtain. They were asked not to speak but were aware of each other's presence. An overhead LCD projector displayed targets and messages for each subject onto a white circular disk affixed to the crank; the messages instructed one or both subjects to grasp their handles. The task was to move a mark on the disk (a black line aligned with the position of the handle) into the target as quickly as possible and hold it there until a new target appeared (a random delay of 700-- 1,700 ms). In the dyad condition, the projector displayed corresponding targets for the 2 subjects, so that they were presented with the targets simultaneously and (because of their mechanical coupling) completed the task simultaneously. The target changed color when the handle was properly