Elsevier

Cortex

Volume 119, October 2019, Pages 575-579
Cortex

Letter to the Editor
Let's share our perspectives, but only if our body postures match

https://doi.org/10.1016/j.cortex.2019.02.019Get rights and content

Introduction

Knowing where our body is in space depends on the brain's ability to process proprioceptive signals from the muscles, joints and tendons, and to integrate them with information from other sensory modalities. Previous research has shown that conflicting proprioceptive information influences the perception of our own body. For example, altering signals from the muscle spindles by simultaneous vibrations of the biceps and triceps tendons evoked a “telescoping of the arm towards the elbow” (Longo, Kammers, Gomi, Tsakiris, & Haggard, 2009). Similarly, conflicting visuo-proprioceptive signals when viewing a moving hand in a mirror gave the illusion that the other, immobilized, hand was also moving, increasing motor excitability for the motionless hand (Touzalin-Chretien, Ehrler, & Dufour, 2010).

Here we are interested in how the current position of our body in space affects visuo-spatial third-person perspective-taking. When interacting with others, we need to distinguish between our own and others' perspectives. Our position in space might play a key role for this ability. For instance, observers explicitly instructed to judge whether a glass of water is located to someone else's left or right are on average faster to perform this task when they share a same body posture (Kessler and Rutherford, 2010, Kessler and Thomson, 2010, Surtees et al., 2013b) or a same body configuration (e.g. arms crossed: Furlanetto, Gallace, Ansuini, & Becchio, 2014) than the distant person. Previous studies classified this mental process as ‘level-2 perspective taking’, which relies on embodied mental rotation of the self in order to identify how others see the world from a different perspective (Michelon & Zacks, 2006). Although, level-2 perspective-taking has been traditionally considered a rather deliberate mental simulation grounded on proprioceptive signals (Kessler and Rutherford, 2010, Kessler and Thomson, 2010, Surtees et al., 2013b) recent studies have shown that it is potentially automatic when one person is informed of the form and perspective properties of their partner's task (Elekes, Varga, & Király, 2016). By contrast, ‘level-1 perspective taking’ reflects our understanding of what someone else can see and it is generally not considered an embodied process (Kessler and Rutherford, 2010, Michelon and Zacks, 2006, Surtees et al., 2013b). Furthermore, level-1 perspective-taking has been described as an implicit process, which refers to the pre-reflective, automatic and effortless simulation of what someone else sees from their position in space (Nielsen et al., 2015, Pavlidou et al., 2018, Samson et al., 2010; for a critical perspective on this issue, however, see; Santiesteban, Catmur, Hopkins, Bird, & Heyes, 2014).

A useful measure of implicit level-1 perspective-taking in a laboratory setting is the dot-counting task (Samson et al., 2010). Participants are asked to make perceptual judgments about the number of dots visible from their egocentric viewpoint in the presence of a task-irrelevant avatar. Response times increase for trials in which the avatar “sees” a number of dots incongruent with the number of dots visible from the participants' viewpoint. This increase in response times reflects the time taken to implicitly adopt the avatar's perspective, referred to as altercentric intrusion (Samson et al., 2010). While postural effects were documented in explicit judgments about how someone else sees the environment (left/right judgment) and in explicit judgments of what is visible from someone else's position, this has not been reported for implicit level-1 perspective-taking using the dot-counting task. The present study investigates novel embodiment effects by measuring the contribution of body posture to implicit level-1 perspective-taking. We hypothesize a decrease in altercentric intrusions when participants adopt an incongruent body posture to that of the avatar compared to a congruent body posture.

Section snippets

Methods

Forty-eight healthy participants completed a modified version of the dot-counting task (Samson et al., 2010). A group of 24 participants (mean age ± SD, 24.2 ± 4.04 years) judged whether a number presented at the beginning of each trial matched the number of balls seen in a visual scene that followed (Fig. 1A). A task-irrelevant avatar oriented towards the left/right wall was shown seated in the center of a room. Participants' body posture (facing left or right) was manipulated to either match

Results

A mixed-model repeated-measures ANOVA revealed a significant interaction between Visual Stimulus and Body Posture (F1,46 = 12.0, p < .01, η2p = .21; Fig. 1C and Supplementary Fig. 2). Post-hoc analysis using Bonferroni tests revealed increased CE, namely stronger altercentric intrusions, when participants shared the posture with the avatar compared to when they were in different postures (p = .01; Supplementary Fig. 2). Altercentric intrusions were significantly stronger for the avatar than for

Discussion

Proprioception has been considered an intrinsically somatic signal, which senses the body posture and movement in space (Proske & Gandevia, 2012). Critically, proprioceptive signals are constantly integrated with visual information to build up a coherent representation of the bodily self. Here, we demonstrate that incongruent visuo-proprioceptive signals between one's own body posture and someone else's decreases the likelihood of adopting their visuo-spatial perspective. This postural effect

Acknowledgments

A.P. and C.L. are supported by the Volkswagen Foundation (grant number 89434: “Finding Perspective: Determining the embodiment of perspectival experience”). E.R.F. is supported by the UK British Academy, the UK Royal Society and European Low Gravity Association (ELGRA). M.G. is supported by a ESRC-DTC studentship. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA

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