The Body and beyond the Body
Authors:   , Smit Miranda 
Published: November 2023

In the first part of my thesis no hand lateralization effect was found, neither did it matter whether the illusion was applied to the left or the right hand. I believe that feelings of body ownership may not be as lateralized as current literature indicates and that hand representations in the brain might be equal for the left and right hand during the rubber hand illusion. Next, results indicated that actual touch to induce ownership over a fake hand was not necessary. I therefore suggest that our brain uses bottom–up multisensory information, as well as top-down predictions about anticipated sensory input to represent our hand. I furthermore showed that synchronous multisensory stimulation as opposed to asynchronous stimulation is able to shift the perceived action space towards the fake hand. Critically, the subjective experience of the illusion did not correlate with this shift in space, however proprioceptive drift did. Thus, it seems likely that multisensory integration of bodily information drives this shift in PPS and not feelings of ownership per se. Lastly, results showed that we perceive our hands as highly distorted; an overestimation of width and an underestimation of length. The overall shape of the hand was perceived wider than it is long. What was surprising is that this shape was robust to most sensory modulations. However, when the finger moved, perceived distortions became slightly more apparent. Overall, I conclude that implicit representation of our hand relies on a stored body-model, which seems unaffected by the modulation of sensory input.

In the second part I found that patients with somatosensory deficits perceive their hand as (normally) distorted as healthy individuals do; wide short hands. A few patients perceived their affected hand as disproportionally large, which I believe is linked to diminished body awareness, and sensorimotor deficits. As expected, most patients had a veridical percept of their conscious hand as assessed with the template matching task. What I found surprising was that patients in the severe group perceived the overall shape as less veridical than patients who are moderately affected and HC's. In fact, as opposed to the somatosensory representation, they perceived the overall shape to be longer than wide. I suggest that this is a result of a visual correction of the distorted cortical somatosensory representation, this correction becomes redundant when somatosensory information is absent since there is nothing to rescale. Next, I found that a patient without primary somatosensory (and visual) deficits still can disown his body. I believe that there was an overreliance on visual input instead of combining visual and somatosensory input, indicating suboptimal multisensory integration. Additionally, I found that exercises that encompasses visual input about the body from a third person perspective (i.e., through a mirror) combined with tactile stimulation improved his body awareness. Finally, treating patients with visuospatial neglect repeatedly with tDCS did not improve their lateralized deficits. Health and safety issues are paramount in this trial and I suggest that with the current parameters performing large randomized controlled trials are difficult to achieve.