Changing the size of a mirror-reflected hand moderates the experience of embodiment but not proprioceptive drift: a repeated measures study on healthy human participants

Abstract

Mirror visual feedback is used for reducing pain and visually distorting the size of the reflection may improve efficacy. The findings of studies investigating size distortion are inconsistent. The influence of the size of the reflected hand on embodiment of the mirror reflection is not known. The aim of this study was to compare the effect of magnifying and minifying mirror reflections of the hand on embodiment measured using an eight-item questionnaire and on proprioceptive drift. During the experiment, participants (n = 45) placed their right hand behind a mirror and their left hand in front of a mirror. Participants watched a normal-sized, a magnified and a minified reflection of the left hand while performing synchronised finger movements for 3 min (adaptive phase). Measurements of embodiment were taken before (pre) and after (post) synchronous movements of the fingers of both hands (embodiment adaptive phase). Results revealed larger proprioceptive drift post-adaptive phase (p = 0.001). Participants agreed more strongly with questionnaire items associated with location, ownership and agency of the reflection of the hand post-adaptive phase (p < 0.001) and when looking at the normal-sized reflection (p < 0.001). In conclusion, irrespective of size, watching a reflection of the hand while performing synchronised movements enhances the embodiment of the reflection of the hand. Magnifying and minifying the reflection of the hand has little effect on proprioceptive drift, but it weakens the subjective embodiment experience. Such factors need to be taken into account in future studies using this technique, particularly when assessing mirror visual feedback for pain management.

Changing the size of a mirror‐reflected hand does not affect pain perception: A repeated measures study on healthy human participants

“…This was followed by an ‘adaptive phase’ to facilitate embodiment of the reflection of the hand using a method previously described by Wittkopf et al. (). Participants were asked to look at the reflection of their left hand for 30 s, followed by clenching and unclenching of both fists in synchrony with a metronome (60 beats per minute) for 60 s. This was followed by keeping both hands still for 30 s followed by 60 s of touching each fingertip with the thumb in synchrony with a metronome (60 beats per minute).…”

“…() studied 32 healthy participants and found higher heat pain thresholds only when participants reported ownership over a virtual arm. Recently, we found that an adaptive phase in which participants watch a reflection of the hand while performing synchronized finger movements with both hands over a 3‐min period enhanced the embodiment of the reflected hand (Wittkopf et al., ). In the present experiment, we investigated whether embodiment of the reflection of the hand was weakened by applying a painful stimulus to the hand hidden behind the mirror and after an adaptive phase could weaken the embodiment of the reflection of the hand.…”

The effect of visual feedback of body parts on pain perception: A systematic review of clinical and experimental studies

“…Visually distorting the size of painful body parts is another component of optimal technique that has aroused interest, despite few available studies on which to judge efficacy (Wittkopf and Johnson, ). Likewise, embodiment of the viewed body part, which describes the subjective experience of having a sense of one's own body, including a sense of ownership of body parts (Longo et al., ; de Vignemont, ), is considered an important determinant of outcome but rarely assessed in trials (McCabe, ; Foell et al., ; Wittkopf et al., ).…”

Feedback of action outcome retrospectively influences sense of agency in a continuous action task

“…Numerous studies have shown that SoA can be augmented or attenuated by internal motor signals and/or external cues [ 3 – 4 ] with tasks involving a simple action [ 5 – 6 ] and a continuous action [ 7 – 8 ]. SoA can be measured in various ways [ 5 , 9 – 10 ]. As an example of explicit measurement of SoA, after participants finished a given task, they answered to what extent they caused the external event by either verbal report or use of computer mouse [ 5 , 9 ].…”

“…SoA can be measured in various ways [ 5 , 9 – 10 ]. As an example of explicit measurement of SoA, after participants finished a given task, they answered to what extent they caused the external event by either verbal report or use of computer mouse [ 5 , 9 ]. As an example of an implicit measurement of SoA, they reproduced a delay between action and a perceived outcome [ 10 ].…”