Keeping in touch with one's self: multisensory mechanisms of self-consciousness

Abstract

Background: The spatial unity between self and body can be disrupted by employing conflicting visual-somatosensory bodily input, thereby bringing neurological observations on bodily self-consciousness under scientific scrutiny. Here we designed a novel paradigm linking the study of bodily self-consciousness to the spatial representation of visuo-tactile stimuli by measuring crossmodal congruency effects (CCEs) for the full body.

Methodology/principal findings: We measured full body CCEs by attaching four vibrator-light pairs to the trunks (backs) of subjects who viewed their bodies from behind via a camera and a head mounted display (HMD). Subjects made speeded elevation (up/down) judgments of the tactile stimuli while ignoring light stimuli. To modulate self-identification for the seen body subjects were stroked on their backs with a stick and the felt stroking was either synchronous or asynchronous with the stroking that could be seen via the HMD. We found that (1) tactile stimuli were mislocalized towards the seen body (2) CCEs were modulated systematically during visual-somatosensory conflict when subjects viewed their body but not when they viewed a body-sized object, i.e. CCEs were larger during synchronous than during asynchronous stroking of the body and (3) these changes in the mapping of tactile stimuli were induced in the same experimental condition in which predictable changes in bodily self-consciousness occurred.

Conclusions/significance: These data reveal that systematic alterations in the mapping of tactile stimuli occur in a full body illusion and thus establish CCE magnitude as an online performance proxy for subjective changes in global bodily self-consciousness.

Figure 1. Experimental set-up for different conditions.

Subject stood two metres in front of a camera with a 3D-encoder. Four light-vibration devices were fixed to the subject's back, the upper two at the inner edges of the shoulder blades and the lower two 9 cm below. In the object control conditions the lights were attached to a white rectangular metal panel. The small inset windows represent what the subject viewed via the head mounted device. 1. (Upper row) left panel: ‘body visible’ condition; right panel: ‘body not visible’ condition. 2. (Middle row) left panel: synchronous stroking condition; right panel: asynchronous stroking. 3. (Bottom row) - Object control – left panel: synchronous stroking; right panel: asynchronous stroking.

Figure 2. CCE in study 1 – ‘body visible’ and ‘body not visible’ conditions.

Mean congruency effects in reaction time (RT) in milliseconds (RT in incongruent trials minus RT in congruent trials) in Study 1 for ‘body visible’ and ‘body not visible’ conditions. Error bars show standard errors of the mean.

Figure 3. CCE in study 1 – synchronous and asynchronous stroking conditions.

Mean congruency effects in reaction time in milliseconds (RT in incongruent trials minus RT in congruent trials) in Study 1 for synchronous and asynchronous conditions.

Figure 4. CCE in study 2 – synchronous and asynchronous stroking conditions.

Mean congruency effects in reaction time in milliseconds (RT in incongruent trials minus RT in congruent trials) in Study 2 for synchronous and asynchronous conditions. Error bars show standard errors of the mean.

Figure 5. Drift and questionnaire scores in study 2.

1. (Left inset) Drift measured in cm for synchronous and asynchronous conditions on the posterior-anterior axis (Study 2). 2. (Right inset) Score on the “self-identification questionnaire” (Study 2) as adapted from .

Figure 6. CCE in study 3 – Object control.

Mean congruency effects in reaction time in milliseconds (RT in incongruent trials minus RT in congruent trials) in the object control study (Study 3) for synchronous and asynchronous conditions.