Anchoring the Self to the Body in Bilateral Vestibular Failure

Abstract

Recent findings suggest that vestibular information plays a significant role in anchoring the self to the body. Out-of-body experiences of neurological origin are frequently associated with vestibular sensations, and galvanic vestibular stimulation in healthy participants anchors the self to the body. Here, we provide the first objective measures of anchoring the self to the body in chronic bilateral vestibular failure (BVF). We compared 23 patients with idiopathic BVF to 23 healthy participants in a series of experiments addressing several aspects of visuo-spatial perspective taking and embodiment. In Experiment 1, participants were involved in a virtual "dot-counting task" from their own perspective or the perspective of a distant avatar, to measure implicit and explicit perspective taking, respectively. In both groups, response times increased similarly when the avatar's and participant's viewpoint differed, for both implicit and explicit perspective taking. In Experiment 2, participants named ambiguous letters (such as "b" or "q") traced on their forehead that could be perceived from an internal or external perspective. The frequency of perceiving ambiguous letters from an internal perspective was similar in both groups. In Experiment 3, participants completed a questionnaire measuring the experienced self/body and self/environment "closeness". Both groups reported a similar embodied experience. Altogether, our data show that idiopathic BVF does not change implicit and explicit perspective taking nor subjective anchoring of the self to the body. Our negative findings offer insight into the multisensory mechanisms of embodiment. Only acute peripheral vestibular disorders and neurological disorders in vestibular brain areas (characterized by strong multisensory conflicts) may evoke disembodied experiences.

Fig 1. Methods for visuo-spatial perspective-taking tasks (Experiment 1).

(A) Examples of visual stimuli used for the tasks of implicit perspective taking (IPT), explicit perspective taking (EPT) task, and visuo-spatial control (VSC) task. Visual stimuli presented a congruent or an incongruent viewpoint of the avatar with the participant’s viewpoint. (B) Participants indicated whether the number of balls seen from their viewpoint (IPT and VSC tasks) matched (i.e., matching trials) or did not match (i.e., mismatching trials) the number presented in the instruction.

Fig 2. Results for the visuo-spatial perspective-taking tasks (Experiment 1; Response times).

Histograms represent the effect of the within-subject factor Viewpoint, which was significant for the implicit perspective-taking (IPT) task (p<0.05) and the explicit perspective-taking (EPT) task (p<0.05), but not for the visuo-spatial control (VSC) task (n.s.: not significant). Data for patients and controls are shown separately for illustration purposes only. Vertical bars represent the standard error of the mean.

Fig 3. Results for the visuo-spatial perspective-taking tasks (Experiment 1; Congruency effects).

Histograms represent the average congruency effect (incongruent viewpoint minus congruent viewpoint) calculated for the implicit perspective-taking (IPT) task, explicit perspective-taking (EPT) task, and visuo-spatial control (VSC) task for patients and controls. * indicates significant differences with respect to zero (t-test). Vertical bars represent the standard error of the mean.

Fig 4. Methods for the graphaesthesia task (Experiment 2).

(A) The experimenter drew letters on the participants’ forehead and neck by using a cotton bud while participants kept their eyes closed. (B) Letters included ambiguous, non-symmetrical letters (b, d, p, q) and non-ambiguous, symmetrical letters (w, n, o, v) that were all drawn in the canonical direction of writing or in the reverse direction, starting from the opposite end of the letter. All letters were drawn by using a single and continuous hand motion.

Fig 5. Results from the graphaesthesia task (Experiment 2).

Histograms on the left represent the main effect of the within-subject factor Site of stimulation and histograms on the right represent the main effect of the between-subject factor Group. Data from patients and controls are shown separately for illustration purposes only. Histograms represent the mean of the percentage of trials perceived from a first-person perspective and vertical bars represent the standard error of the mean.

Fig 6. Subjective measures of self-body anchoring (Experiment 3).

Pictorial descriptions used to measure the closeness between the self, body, environment and others. Seven pairs of circles shown at the bottom of the figure were presented to participants, who had to indicate which one better represented the perceived degree of “closeness” between two items, such as the self and body. Colored symbols represent the mean of self-reports from patients and controls and horizontal bars represent the standard error of the mean.