Representing multiple object weights: competing priors and sensorimotor memories

Abstract

When lifting an object, individuals scale lifting forces based on long-term priors relating external object properties (such as material and size) to object weight. When experiencing objects that are poorly predicted by priors, people rapidly form and update sensorimotor memories that can be used to predict an object's atypical size-weight relation in support of predictively scaling lift forces. With extensive experience in lifting such objects, long-term priors, assessed with weight judgments, are gradually updated. The aim of the present study was to understand the formation and updating of these memory processes. Participants lifted, over multiple days, a set of black cubes with a normal size-weight mapping and green cubes with an inverse size-weight mapping. Sensorimotor memory was assessed with lifting forces, and priors associated with the black and green cubes were assessed with the size-weight illusion (SWI). Interference was observed in terms of adaptation of the SWI, indicating that priors were not independently adjusted. Half of the participants rapidly learned to scale lift forces appropriately, whereas reduced learning was observed in the others, suggesting that individual differences may be affecting sensorimotor memory abilities. A follow-up experiment showed that lifting forces are not accurately scaled to objects when concurrently performing a visuomotor association task, suggesting that sensorimotor memory formation involves cognitive resources to instantiate the mapping between object identity and weight, potentially explaining the results of experiment 1 These results provide novel insight into the formation and updating of sensorimotor memories and provide support for the independent adjustment of sensorimotor memory and priors.

Keywords: object lifting; sensorimotor integration; sensorimotor memory; weight prediction.

Fig. 1.

Experimental apparatus and stimuli. A: while seated, participants lifted and replaced 1 of 2 objects located on force sensors embedded in a platform. A data projector, located above the participant, was used to indicate which object to lift on a given trial. B: relationship between volume and mass for the 3 size-weight inverted green cubes, the 3 normally weighted black cubes, and the small and large equally weighted green and black size-weight illusion stimuli. C: 2 examples of the arbitrary visuomotor association (AVA) task instructions used in experiment 2. The instructions specify which direction the participant should translate each object in the horizontal plane after lifting it vertically. For example, in the first set of instructions, the large black cube (BL) should be moved away from the participant and to the left. In contrast, the medium-sized green cube (GM) should be moved toward the participant and to the right. D: load force functions from 2 lifts of a 7-N object. In one lift (gray curves) the initial increase in load force undershot object weight, and in the other lift (black curves) the initial increase in load force accurately reached the weight.

Fig. 2.

Perceptual results—experiment 1. A: signed percent change scores as a function of day for the black and green control cubes tested on days 0–10. The additional testing points within the control conditions are a result of testing the illusion with the alternate colored cubes. B: signed percent change scores as a function of day for the black and green experiment cubes. The height of each vertical bar and the shaded regions represent ±1 SE. Scores were calculated by determining the percent increase from the smallest to the largest numerical values and assigning a positive sign to this number if the small object was perceived as heavier and a negative sign if the larger object was perceived as heavier. The gray and black circles along the bottom indicate whether the green and black illusions were significantly different from 0 (filled circle), based on t-tests. A P value of 0.05 was considered statistically significant for all tests.

Fig. 3.

Load force rate during lifting—experiment 1. A and B: first 10 lifts of the heavy small green cubes of the green control condition (A) and the black control condition (B) on days 1, 2, 3, and 10. C: load force rate records from the first 5 trials lifting the large black cube (black traces) and the small green cubes (gray traces) on days 1, 2, 3, and 10. Each row represents data from an individual exemplar subject, identified by subject code, across days 1, 2, 3, and 10.

Fig. 4.

Initial peak in load force rate—experiment 1. All lifts of the large black cube and the small green cube are plotted separately for each participant in the black (A) and green (C) control conditions. Lifts of the large black cube (B) and the small green cube (D) of the GB6 condition are also plotted. The mean lifting forces for the BC3 and GC3 conditions were significantly different (E); however, the GC3 condition (F) demonstrated increased variability and a nonsignificant difference between lifts of the large black cube and the small green cube. Shaded areas represent ±1 SE.

Fig. 5.

Initial peak in load force rate: experiment 1, day 1. First peak in load force rate for the large black and small green cubes. The mean peak in load force rate of all subjects is shown for the first 5 and last 5 trials of each block. A significant increase in lifting force was observed for the small green cubes, indicating sensorimotor adaptation over the first day of lifting. Error bars represent ±1 SE. *P value of <0.05, corrected, considered a statistically significant difference between means.

Fig. 6.

Initial peak in load force rate: experiment 2. First peak in load force rate for the large black (A, black) and small green (B, light gray) cubes. The mean peak in load force rate of all subjects is shown for the first and last blocks for the no interference (filled) and interference (open) conditions. Error bars represent ±1 SE.