Shared contributions of the head and torso to spatial reference frames across spatial judgments

Abstract

Egocentric frames of reference take the body as the point of origin of a spatial coordinate system. Bodies, however, are not points, but extended objects, with distinct parts that can move independently of one another. We recently developed a novel paradigm to probe the use of different body parts in simple spatial judgments, what we called the misalignment paradigm . In this study, we applied the misalignment paradigm in a perspective-taking task to investigate whether the weightings given to different body parts are shared across different spatial judgments involving different spatial axes. Participants saw birds-eye images of a person with their head rotated 45° relative to the torso. On each trial, a ball appeared and participants made judgments either of whether the ball was to the person's left or right, or whether the ball was in front of the person or behind them. By analysing the pattern of responses with respect to both head and torso, we quantified the contribution of each body part to the reference frames underlying each judgment. For both judgment types we found clear contributions of both head and torso, with more weight being given on average to the torso. Individual differences in the use of the two body parts were correlated across judgment types indicating the use of a shared set of weightings used across spatial axes and judgments. Moreover, retesting of participants several months later showed high stability of these weightings, suggesting that they are stable characteristics of people.

The vestibular system modulates the contributions of head and torso to egocentric spatial judgements

“…Stimuli for the Misalignment Paradigm were similar to our previous studies (Alsmith et al 2017;Longo et al 2020) and are shown in Fig. 1A.…”

“…The analysis was similar to that used in our previous study (Alsmith et al 2017;Longo et al 2020). Best-fitting cumulative Gaussian functions were fit using maximum-likelihood estimation for each participant in each condition using the Palamedes toolbox for MATLAB (Mathworks, Natick, MA, Prins and Kingdom (2009), available online at: http:// www.…”

“…The Misalignment Paradigm suggests that egocentric spatial judgments involve the use of reference frames centred both on the head and torso, with differences between people in the use of these body parts as anchor (Alsmith et al 2017 ; Longo et al 2020 ). Although the factors that may drive individual differences in the use of these body parts or in the use of a weighted combination of head and torso are not yet entirely clear, we have recently shown an high stability across time (Longo et al 2020 ). Participants re-tested on the Misalignment Paradigm several months after the original test showed a strong correlation between the two testing sessions in their use of head vs. torso references.…”

“…We have recently developed a Misalignment Paradigm which isolates the respective contribution of the head and torso to egocentric spatial judgments (Alsmith et al 2017 ; Longo et al 2020 ). This paradigm is essentially an experimentalization of Peacocke’s ( 1992 ) Buckingham Palace thought experiment, described above.…”

“…Our results suggested that both head and torso contribute to egocentric spatial judgments, though with greater weight given to the torso in most participants. Interestingly, individual differences in the weighting of the two body parts were correlated across different spatial axes and stable over time (Longo et al 2020 ).

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