Abstract
We have previously shown that, with the hands apart vertically, passively grasping an artificial finger induces a sense of ownership over the artificial finger and coming-together of the hands. The present study investigated this grasp illusion in the horizontal plane. Thirty healthy participants were tested in two conditions (grasp and no grasp) with their hands at different distances apart, either crossed or uncrossed. After 3 min, participants reported perceived spacing between index fingers, perceived index finger location, and, for the grasp condition, perceived ownership over the artificial finger. On average, there was no ownership at any of the hand configurations. With the hands uncrossed 7.5, 15 or 24 cm apart, there was no difference in perceived spacing between the grasp and no grasp conditions. With the hands crossed and 15 cm apart, perceived spacing between index fingers was 3.2 cm [0.7 to 5.7] (mean [95% CI]) smaller during the grasp condition compared to no grasp. Therefore, compared to when the hands are vertically separated, there is an almost complete lack of a grasp illusion in the horizontal plane which indicates the brain may process sensory inputs from the hands differently based on whether the hands are horizontally or vertically apart.
Highlights
Over the past few decades, important insights about how we sense the position of our body and what body parts belong to us have been obtained by studying disorders and experimental illusions[1,2]
In 29 healthy individuals we investigated the effect of passively grasping an artificial finger for 3 min on perceived horizontal spacing of the index fingers and perceived ownership over the artificial finger
With the hands crossed 15 cm, passively grasping an artificial finger reduced perceived horizontal spacing by 3.2 cm [0.7 to 5.7]. This effect was associated with the left index finger being perceived 1.9 cm [0.5 to 3.3] more to the left during the grasp condition compared to the no grasp condition (Fig. 3a,b); perceived location of the right index finger was not different (−0.4 cm [−1.5 to 0.6])
Summary
Over the past few decades, important insights about how we sense the position of our body and what body parts belong to us have been obtained by studying disorders and experimental illusions[1,2]. While the classic rubber hand illusion can be induced with the real and artificial hand separated vertically, it is not known whether the grasp illusion, with its static tactile and proprioceptive sensory signals, is present when the hands are in a more familiar, horizontal configuration. We hypothesized that with the hands horizontally 15 cm apart, either crossed or uncrossed, passively grasping an artificial finger would reduce perceived spacing by ~5 cm and induce a sense of ownership comparable to when the hands are vertically apart. These effects were expected to be absent with the hands spaced further apart
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