Credit assignment between body and object probed by an object transportation task

Abstract

It has been proposed that learning from movement errors involves a credit assignment problem: did I misestimate properties of the object or those of my body? For example, an overestimate of arm strength and an underestimate of the weight of a coffee cup can both lead to coffee spills. Though previous studies have found signs of simultaneous learning of the object and of the body during object manipulation, there is little behavioral evidence about their quantitative relation. Here we employed a novel weight-transportation task, in which participants lift the first cup filled with liquid while assessing their learning from errors. Specifically, we examined their transfer of learning when switching to a contralateral hand, the second identical cup, or switching both hands and cups. By comparing these transfer behaviors, we found that 25% of the learning was attributed to the object (simply because of the use of the same cup) and 58% of the learning was attributed to the body (simply because of the use of the same hand). The nervous system thus seems to partition the learning of object manipulation between the object and the body.

Figure 1

Experimental setup and experimental design. (A) Experimental setup. The participant transported a cup, containing varying amount of water, to a LED target after the LED light was lit. The cup was initially placed on a platform and became unsupported once moved. A remote-controlled syringe system changed the amount of water before the trial when needed. (B) Schematic illustration of trial blocks used in Experiment 1. The four types of trial blocks (transfer, pseudo-transfer, baseline and compliance) were randomized in order. The transfer of learning was assessed when both hands and cups were switched (ΔHandΔCup condition, only condition in Experiment 1). (C) Schematic illustration of trial blocks used in Experiment 2. Experiment 2 utilized similar trial blocks as Experiment 1 with critical modifications: the transfer of learning was assessed when only hands were switched (ΔHand condition), or when only cups were switched (ΔCup condition). Squares denote trials within a trial block; color denotes the identity of cups; filled or unfilled squares denote full or emptied cups, respectively. Detailed explanations were listed in Protocols.

Figure 2

Data from a typical participant of Experiment 1. The average trajectory height is plotted as a function of movement distance. (A) In a baseline block, the left hand moves relatively straight after transporting the emptied second cup for five successive trials. (B) In the compliance block the left hand is elevated substantially when the cup is unexpectedly emptied after four full-cup trials. The trajectory appears lower in the first trial due to the aftereffect of transporting the mostly likely emptied second cup before the compliance block. (C) In the transfer block, the right hand moves relatively flat in early trials with the full first cup. The left hand transports the emptied second cup in the last trial, resulting in elevated trajectories. It indicates a partial transfer as the elevated trajectory height is still lower than the last trial in the compliance block. The gray shade denotes the range covering the duration between the trial-beginning time and the peak-velocity time, and this part of data is used to calculate the trajectory height.

Figure 3

Data from all participants of Experiment 1. The trajectory heights are plotted as a function of trial within a block; data from three block types are shown in separate panels (A–C). Lines of different colors indicate trial blocks of different sizes. A relatively stable height is established with 1 or 2 trials after a weight change. The height increases in the last trial when the second cup weight is emptied, but the increase differs between the compliance block and the transfer block.

Figure 4

The percentage of transfer of learning in Experiment 1 and Experiment 2. Transferring to a different hand and with a second cup (ΔHandΔCup), to a different hand but with the same cup (ΔHand), and to a second cup but with the same hand (ΔCup) are dramatically different. By removing the baseline level of learning estimated from ΔHandΔCup, we can estimate the learning specifically attributed to the cup (ΔHand) and the hand (ΔCup), as respectively shown in red and blue color bars. ** for P < 0.005, * for P < 0.05.