Idiosyncratic representation of peripersonal space depends on the success of one's own motor actions, but also the successful actions of others!

Abstract

Peripersonal space is a multisensory representation of the environment around the body in relation to the motor system, underlying the interactions with the physical and social world. Although changing body properties and social context have been shown to alter the functional processing of space, little is known about how changing the value of objects influences the representation of peripersonal space. In two experiments, we tested the effect of modifying the spatial distribution of reward-yielding targets on manual reaching actions and peripersonal space representation. Before and after performing a target-selection task consisting of manually selecting a set of targets on a touch-screen table, participants performed a two-alternative forced-choice reachability-judgment task. In the target-selection task, half of the targets were associated with a reward (change of colour from grey to green, providing 1 point), the other half being associated with no reward (change of colour from grey to red, providing no point). In Experiment 1, the target-selection task was performed individually with the aim of maximizing the point count, and the distribution of the reward-yielding targets was either 50%, 25% or 75% in the proximal and distal spaces. In Experiment 2, the target-selection task was performed in a social context involving cooperation between two participants to maximize the point count, and the distribution of the reward-yielding targets was 50% in the proximal and distal spaces. Results showed that changing the distribution of the reward-yielding targets or introducing the social context modified concurrently the amplitude of self-generated manual reaching actions and the representation of peripersonal space. Moreover, a decrease of the amplitude of manual reaching actions caused a reduction of peripersonal space when resulting from the distribution of reward-yielding targets, while this effect was not observed in a social interaction context. In that case, the decreased amplitude of manual reaching actions was accompanied by an increase of peripersonal space representation, which was not due to the mere presence of a confederate (control experiment). We conclude that reward-dependent modulation of objects values in the environment modifies the representation of peripersonal space, when resulting from either self-generated motor actions or observation of motor actions performed by a confederate.

Fig 1

Upper part: Sketch of the apparatus and stimuli display in the target-selection and the reachability-judgment tasks of Exp. 1. Left: The probability of selecting a reward-yielding target was 50%, 25% or 75% in the proximal and distal spaces. Centre: In the target-selection task, the 32 targets were directly visible on the touch-screen table. In the reachability-judgment task, the 31 targets were visible through the mirror as displayed by the video projector. Right: the position of each target displayed (32 out of 42 possible locations separated by 12.65 x 8.30 cm) was randomly computed from the centre of the cell ± 60 pixels). Lower part: Timeline for the tasks.

Fig 2

(A) Posttest-Pretest boundary of peripersonal space relating to arm length (cm) as a function of the group in Exp. 1 (Far group, Control group, Near group), in Exp. 2 (social) and in the control experiment. Stars indicate significant difference between the pretest and the posttest. (B) Individual variation of performance between the pretest and the posttest (negative and positive signs indicate respectively reduction and expansion of peripersonal space).

Fig 3

(A) Density maps of the target selected in the target-selection task by all the participants across the 400 manual reaching actions. Blue colour indicates infrequent selection; Red colour indicates frequent selection. (B) Mean amplitude of manual reaching actions in the first and last blocks for the Far group, Control group and Near group (Exp. 1). Stars indicate significant differences. (C) Linear regression (individual data) between the number of reward-yielding targets selected in the distal space and the change of reachability threshold (upper-left), between the number of reward-yielding targets selected in the proximal space and the change of reachability threshold (lower-left), and between the change of movement amplitude (mm) and the change of reachability threshold (right). The linear regression coefficients are indicated in the upper-left part of each plot.

Fig 4

(A) Mean amplitude of manual reaching actions in the target-selection task across the 400 target selections (Exp. 1) for the Far group, Control group and Near group. (B) Mean percentage of target selected in the proximal space (row 1, 2, 3) across the 40 blocks. The three groups diverged from the 3^rd block. (C) Mean number and percentage of reward-yielding targets selected in the Far group, Control group and Near group.

Fig 5

Upper part: Sketch of the apparatus and stimuli display in the target-selection and the reachability-judgment tasks (Exp. 2). Left: The probability of selecting a reward-yielding target was 50% in the proximal and distal spaces. Right: Two participants were facing each other (position A and B). In the target-selection task, the 32 targets were directly visible on the touch-screen table. In the reachability-judgment task, the 31 targets were visible through the mirror projecting the image displayed by the video projector on the touch-screen table. Lower part: Time sequence of the tasks presentation.

Fig 6

(A) Density map of the targets selected in the target-selection task by all participants across the 408 manual reaching actions in the social experiment. Blue colour indicates infrequent selection; Red colour indicates frequent selection. (B) Mean amplitude of manual reaching actions in the first and last blocks for the participants in position A and position B (Exp. 2). Stars indicate significant differences.

Fig 7

(A) Mean amplitude of manual reaching actions in the target-selection task across the 408 target selections (Exp. 2) for the participants at position A (left y axis) and position B (right y axis). (B) Mean percentage of targets selected in the proximal space (row 1, 2, 3) across the 34 blocks. The performance was not influenced by the participants' position (A, B). (C) Mean number and percentage of reward-yielding targets selected for the participants at position A and position B.

Fig 8

(A) Density map of the targets selected in the target-selection task by all participants across the 408 manual reaching actions in the control experiment. Blue colour indicates infrequent selection; Red colour indicates frequent selection. (B) Mean amplitude of manual reaching actions across the 408 target selections. (C) Mean percentage of targets selected in the proximal space (row 1, 2, 3) across the 34 blocks.