Imagine being offered one half of a Snickers Bar that on first glance was cut into two equal halves. Unless a machine bisected it, the right half would still be just a tiny bit larger. Why is that? Roughly said, the brain’s hemispheres are responsible for different functions. The right hemisphere is specialized in spatial abilities. When asked to bisect a line, humans tend to bisect it more to the left of its veridical centre, a phenomenon that is presumed to result from the right-hemispheric dominance in spatial tasks [1,2].
This phenomenon is called “Pseudoneglect” and also occurs when you draw an imaginary line through the midst of your body, a body midline so to say. Why would I do that, you might ask? Well, similar to a GPS, the brain uses spatial coordinate frames to encode the location of objects in space. Just like a GPS, the brain needs reference frames to encode space and the body midline functions as a reference for what scientist call the egocentric reference frame . When our brain encodes the position of objects in relation to the body, it does so in an egocentric reference frame. Interestingly, the egocentric reference frame is closely related to the body schema which is a sort of body representation in the brain that (among other things) informs you about the position of your body and body parts in space .
It is well-known nowadays that the brain’s representation of the body is changed in chronic neuropathic pain and in fact, chronic pain patients often have distortions of the body schema. This is particularly the case in patients with complex regional pain syndrome (CRPS I). Imaging studies have shown that the brain’s representation of the painful hand is changed in CRPS and these alterations in the central nervous system (CNS) are presumably linked to the distortions of the body schema . But are distortions of the body schema also related to distorted spatial abilities in chronic pain?
To examine this, we asked patients with CRPS on the hand, patients with pain of other origin than CRPS on the hand and healthy subjects to visually position their body midline. The task was carried out in the light and in the dark so that in the dark, participants would have to rely on their body schema when estimating their body midline. We found that in the dark, CRPS patients positioned their body midline significantly farther away from the objective body midline. And although all other participants positioned their bodymidline slightly towards the left from the veridical centre as well (remember Pseudoneglect), the amount of leftward deviation was largest in CRPS patients. This told us 2 things: a) only CRPS patients are impaired to encode the position of objects in relation to their body midline and b) it appears that not chronic pain per se but rather the CRPS-associated CNS changes in regions responsible for the body schema underlie this distorted visuo-spatial perception. Thus, the CRPS-asscoiated CNS changes that are involved in the distortion of the body schema also underlie the CRPS-specific distortion of visuo-spatial perception. Moreover, in CRPS these CNS changes appear to cause an accentuation of the leftwards bias (Pseudoneglect) that is presumed to result from the right-hemispheric dominance in spatial tasks . Future research should focus on the close relation between the body schema and visuo-spatial perception in CRPS.
About Annika Reinersmann
Annika did her Masters of Psychology at Nijmegen University in the Netherlands. Her PhD project was a collaboration between the Department of Pain Management and the Institute of Cognitive Neuroscience, Biopsychology at Ruhr-University Bochum. Annika’s research interests are in the neural correlates of the body schema & the interaction between body schema and body image in chronic pain.
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 Galati, G., Pelle, G., Berthoz, A., & Committeri, G. (2010). Multiple reference frames used by the human brain for spatial perception and memory Exp Brain Res 206 (2), 109-120 DOI: 10.1007/s00221-010-2168-8
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 Reinersmann A, Landwehrt J, Krumova EK, Ocklenburg S, Güntürkün O, & Maier C (2012). Impaired spatial body representation in complex regional pain syndrome type 1 (CRPS I). Pain, 153 (11), 2174-81 PMID: 22841878