Going out on a limb: Lower limb representations in individuals with and without Body Integrity Identity Disorder

The aim of this thesis was to examine lower limb perception in healthy individuals and in people with a rare disorder called Body Integrity Identity Disorder (BIID). BIID is a non-psychotic condition characterized by a strong and persistent desire to amputate/paralyze one or more healthy limbs, often the leg(s). This desire presumably arises from experiencing a mismatch between the mental image of the body and the actual physical and functional boundaries of the body itself. This mismatch might be a product of disturbed integration of lower-level sensory input (e.g. like a touch on the leg) with higher-order models of the body in the brain. Therefore, in this thesis, we used behavioural paradigms to try to tap into this internal mental representation of the legs (and the representation of the space immediately surrounding the legs and feet) with the aim to understand how and if it might be disturbed. We used a range of implicit and explicit behavioural tasks to explore the perceptual representations of the legs and feet. Specifically, I focused on three aspects of the bodily experience: 1) the configuration of the body: specifically, the integrity/shape of the underlying representation, 2) peripersonal space: i.e. the integration of visuo-tactile information in the space immediately surrounding the body, and 3) feelings of embodiment: i.e. how manipulating sensory signals about the body can temporarily ‘remove’ a body part from the bodily experience. We found that healthy participants have a distorted representation of their lower limbs that depends on which type of sensory input is probed (chapter 2). BIID participants showed similar distortions in their leg representation (chapter 5), suggesting that lower limb perception is normal in BIID. Moreover, we showed that healthy people detect tactile stimuli quicker when a visual stimulus is nearby (~ less than 75cm from the feet; chapter 3). Participants with BIID showed similar spatial boundaries around their legs (chapter 6), suggesting that although the leg feels foreign, sensory input on and about the limb is still processed normally. We also showed that people can experience dis-embodiment of their hands and feet to a similar extent using a bodily illusion. How suggestible an individual is to sensory information was also related to the illusion experience (chapter 4). BIID participants experienced a stronger feeling of ‘loss’ over their feet during this illusion, as it is in line with their desired body state. Finally, using a mental rotation task (chapter 7), we showed that the integrity of lower limb representations is robust in the face of limb loss (lower-limb amputees) or the desire to lose a limb (in BIID). Taken together, this battery of investigations tells us more about the fundamental building blocks of body representations in the general population. It also provides further insight into the underlying mechanisms of BIID, as its origin remains unknown. Further research is needed to elucidate these mechanisms, however