A growing body of literature suggests that body awareness disorders such as BIID occur as a result of abnormal activity in the right parietal lobe, which is known to be essential for constructing a mental representation of the body. Specifically, this body image is constructed in the superior parietal lobule (SPL), which performs a function referred to as multisensory integration, whereby different types of sensory information entering the brain are brought together. Thus, information from the visual parts of the brain and the primary
somatosensory cortex, which processes tactile sensations and proprioceptive information relating to the position of the body within space, is sent to the superior parietal lobule. There, it is combined with information from the motor cortex, which controls movement, and all is processed further to generate an internal model of the body. If these processes are perturbed, the body image is compromised.
Paul McGeoch of the
Brain and Perceptual Process Laboratory at UCSD and his colleagues therefore postulated that the desire to have an otherwise healthy limb amputated occurs as a result of abnormal activity in the right superior parietal lobule, and recruited four male BIID sufferers (or apotemnophiles) from internet support groups to test their hypothesis. Three of these expressed a desire to have their left leg amputated, while the fourth wanted both legs removed. For their study, the researchers simply tapped the participants’ feet with a bundle of fibre-optic filaments, and at the same time, recorded the electrical activity of their brains using magnetoencephalography (MEG). Their responses to the tactile stimulation were compared to those of four controls.
In all four controls, tapping either foot caused an almost instantaneous activation of the right SPL. In the three apotemnophiles who wanted one leg amputated, tapping the unaffected foot evoked a response in the right SPL, but tapping the affected one did not, and in the fourth apotemnophile, who sought amputation of both legs, neither foot evoked a response. These findings confirm the researchers’ hypothesis that BIID arises as a result of abnormal function in the right parietal lobe. The brain does not register the limb as a part of the body, and contains no representation of it, so it is not incorporated into the body image. As a result, the apotemnophile has no sense of ownership over the limb, and feels strongly that it does not “belong” to him. It feels extraneous or redundant, so he wishes to have it removed.