Lower limb peripersonal space and the desire to amputate a leg

Abstract

Body integrity identity disorder (BIID) is a rare condition defined by a persistent desire to amputate or paralyze a healthy limb (usually one or both of the legs). This desire arises from experiencing a mismatch between the internal body model and the actual physical/functional boundaries of the body. People with BIID show an abnormal physiological response to stimuli approaching the affected (unwanted) but not the unaffected leg, which might suggest a retracted peripersonal space (PPS: a multisensory integration zone near the body) around the unwanted limb. Thus, using a visuo-tactile interaction task, we examined leg PPS in a group of healthy men and three men with BIID who desired unilateral leg amputation. PPS size (~ 70 cm) around the unwanted BIID legs did not differ from that of healthy controls. Although the leg feels foreign in BIID, it still seems to maintain a PPS, presumably to protect it and facilitate interactions within the surrounding environment.

Fig. 1

a Pictorial example of visuo-tactile interaction task for the right foot. b Bird’s eye view of task setup. The horizontal grey-dashed arrow indicates the visual stimulus (red circle) trajectory. The stars represent possible points (and thereby distances from the toe) in which a tactile stimulation to the toe could be given. D represents visual stimulus distance from the toe and T represents time in milliseconds from the start of the trial. Ds and Ts have been rounded to the nearest whole number

Fig. 2

Mean reaction times (represented as red opaque circles) and sigmoidal fits (red lines) for the right leg of individual control participants (represented a P for participant followed by participant number). The y-axis represents the reaction time (RT) in milliseconds and the x-axis represents the distance from the foot (in centimeters) that the visual stimulus was located when the tactile stimulus was administered

Fig. 3

Mean reaction times (represented as blue opaque circles) and sigmoidal fits (blue lines) for the left leg of individual control participants (represented a P for participant followed by participant number). The y-axis represents the reaction time (RT) in milliseconds and the x-axis represents the distance from the foot (in centimeters) that the visual stimulus was located when the tactile stimulus was administered

Fig. 4

Mean reaction times (represented as opaque circles) and sigmoidal fits (for the right legs (top row, in red) and left legs (bottom row, in blue) of individual BIID participants (participant number and affected leg as title for each plot). The y-axis represents the reaction time (RT) in milliseconds and the x-axis represents the distance from the foot (in centimeters) that the visual stimulus was located when the tactile stimulus was administered