Spatial sensory organization and body representation in pain perception.

Pain is a subjective experience that protects the body. This function implies a special relation between the brain mechanisms underlying pain perception and representation of the body. All sensory systems involve the body for the trivial reason that sensory receptors are located in the body. The nociceptive system of detecting noxious stimuli comprises two classes of peripheral afferents, Aδ and C nociceptors, that cover almost the entire body surface. We review evidence from experimental studies of pain in humans and other animals suggesting that Aδ skin nociceptors project to a spatially-organised, somatotopic map in the primary somatosensory cortex. While the relation between pain perception and homeostatic regulation of bodily systems is widely acknowledged, the organization of nociceptive information into spatial maps of the body has received little attention. Importantly, the somatotopic neural organization of pain systems can shed light on pain-related plasticity and pain modulation. Finally, we show that the neural coding of noxious stimuli, and consequent experience of pain, are both strongly influenced when cognitive representations of the body are activated by viewing the body, as opposed to viewing another object - an effect we term 'visual analgesia'. We argue that pain perception involves some of the representational properties of exteroceptive senses, such as vision and touch. Pain, however, has the unique feature that the content of representation is the body itself, rather than any external object of perception. We end with some suggestions regarding how linking pain to body representation could shed light on clinical conditions, notably chronic pain.