Helping the waiter to hold his tray: Rigid haptic linkage promotes inter-personal motor coordination

Abstract

When a glass is lifted from a tray, there is a challenge for the waiter. He must quickly compensate for the reduction in the weight of the tray to keep it balanced. This compensation is easily achieved if the waiter lifts the glass himself. Because he has, himself, initiated the action, he can predict the timing and the magnitude of the perturbation of the tray and respond (via the holding hand) accordingly. In this study, we examined coordination when either one or two people hold the tray while either one of them or a third person removes the glass. Our results show that there is exquisite coordination between the two people holding the tray. We suggest that this coordination depends upon the haptic link provided by the rigid platform that both people are holding. We conclude that the guest at a reception should not lift his drink from the waiter's tray until they have the waiter's attention but, if too thirsty to wait, should lend a hand holding the tray.

Keywords: Interpersonal coordination; collaboration; forward model; haptics; sensorimotor.

Figure 1.

Experimental paradigm. Left: general schematic of the task. One or two participants (here one with a colourful tie) held a tray while the experimenter (or participant) removed the beaker from the tray. The accelerometer and magnetometer signals from the smartphone were recorded. Magnetometer signal was used to infer the timing of the beaker’s movement. Accelerometer signal was used to infer the tray motion. Right: Experimental Conditions A through F.

Figure 2.

Tray movement analyses: (a) Magnetic signal in the axis perpendicular to the ground. Dotted lines represent individual trials. Thick lines depict the mean. (b) Same as A but for the accelerometer signal. (c) Superimposed average magnetic signal in the axis perpendicular to the ground across conditions. (d) Same as C but for the accelerometer signal. (e) Schematic representation of movement dynamics. (f) Accelerometer signal variance distributions for all the trial traces (whose mean is depicted in d). (g) Distributions of elapsed time between the positive (before 0 ms) and negative (after 0 ms) peaks of the acceleration traces. (h) Distributions of peak times. Left panel depicts maxima before 0 ms. Right panel depicts minima after 0 ms.