Referred sensations elicited by video-mediated mirroring of hands

Abstract

Humans readily perceive ownership of a limb even when it is artificially induced as in the case of using a mirror reflection. However, mirror reflections are very constrained perceptions which do not allow transformations and varied contexts as often occurs in real life. The extent to which perceived limb ownership occurs with video-mediated manipulations is not known, particularly given the perception would no longer be a precise copy (reflection) of a person's own limb. The present study directly compared referred sensations of the limbs with the use of a mirror reflection to those obtained with a new video-mediated setup to assess perceived ownership. Manipulations that could not be performed with a standard mirror reflection, such as reversal of the spatial positions of the limbs, were also investigated to examine how far the perceived ownership effects could be pushed. Across a series of experiments, data on the quality, intensity and location of referred sensations were collected and analyzed together with measures of hand ownership and participants' experience of the two setups. Results reveal that participants felt referred sensations in both the optical and the video-mediated setup, and that video-mediated manipulations of hand-position reversals produced equal to stronger effects of ownership compared with the mirror reflection. These findings open up new possibilities for scientific experimentation and therapy that are discussed in the paper.

Figure 1. Experimental setup.

For the conditions ART and advART a monitor (A) is used which displays the reflected hand(s) to the participants. Left and right boxes (B) block visibility of the hands for the participants and capture the hand movement with cameras. The box on the right hand side is also used to hide the left hand (without capturing) for the OMB condition, in which the participants place the hand into the opening (C) where a cardboard cover (D) blocks the view to the hand. A viewing slit (E) allows participants a clear view of the reflection of a hand without seeing the actual hand. Brushing stimulation (X) takes place under the cardboard cover (D).

Figure 2. The video-mediated system as used for the advART condition.

In this (advART) as well as in the other two condition the participant sits in front of the system and observes the mirrored images of his hand(s) either on the screen or in the optical mirror while an assistant is brushing the participant’s right hand. In the advART condition, shown in this picture, the participant sees the mirror image of his actually brushed right hand on the left side of the screen while the mirror image of his left hand is shown on the right side of the screen.

Figure 3. OMB condition.

In the OMB condition the optical reflection (mirroring) of the right hand, which is stimulated with brushing strokes, is observed through the viewing slit. The left hand is hidden in the neighboring box (behind a curtain out of the view from the participant).

Figure 4. ART (A) and advART (B) conditions.

In both the ART (A) and the advART (B) conditions the participant’s right hand is stimulated with a brush. This stimulation is not directly visible to the participant. A mirrored image of the right hand is presented on the left side of the screen. In the advART (B) in addition to the mirrored right hand on the left side also the mirrored left hand is shown on the screen.

Figure 5. Perceived ownership of displayed limbs on the screen (ART) and on the mirror (OMB).

In this histogram the perceived ownership rating of the displayed hand is shown for the two setups. The majority of participants perceived the hand shown in both setups to be their hand and rated it a 7 on a 7 point scale.

Figure 6. Comparison of reported referred sensation (RS).

The classification of intensities of RS sensation in the study by Takasugi et al. (A) and for our own study (B) using a similar OMB setup. The figure shows that the number of people who experienced RS is similar in both studies (40% and 43% respectively).

Figure 7. Comparison of RS intensities (Mean and Std-Error) between the three conditions.

The figure shows the average intensities of RS felt by participants in the OMB (M = 1.5), ART (M = 1.45) and advART (M = 1.76) condition. (participants who did not report on experiencing RS in any condition were excluded).

Figure 8. Ownership comparison with other experiments.

The two leftmost columns show the ownership ratings obtained with our ART and OMB setups. All other columns show ratings found in the related literature: 1a) RHI in box with laser-brush (subjects who felt laser)*; 1b) RHI in optical mirror box with laser-brush (subjects who felt laser)*; 1c) RHI in optical mirror box with laser-brush (subjects who did not feel laser)*; (as in [25]) 2a) Virtual Reality RHI (Camera & Projector)*; 2b) Mixed Reality RHI (Camera & Projector, projection brushed)*; (as in 3) Virtual Reality RHI (mirrored LCD) (as in [23]). *approximated values.