157 Central Neuroplasticity and Pathological Pain RONALD MELZACK, TERENCE J. CODERRE, a JOEL KATZ, b AND ANTHONY L. VACCARINO c Department of Psychology, McGill University, Montreal, Quebec H3A 1B1, Canada ABSTRACT: The traditional specificity theory of pain perception holds that pain involves a direct transmission system from somatic receptors to the brain. The amount of pain perceived, moreover, is assumed to be directly proportional to the extent of injury. Recent research, however, indicates far more complex mechanisms. Clinical and experimental evidence shows that noxious stimuli may sensitize central neural structures involved in pain perception. Salient clinical examples of these effects include amputees with pains in a phantom limb that are similar or identical to those felt in the limb before it was ampu- tated, and patients after surgery who have benefited from preemptive analgesia which blocks the surgery-induced afferent barrage and/or its central conse- quences. Experimental evidence of these changes is illustrated by the develop- ment of sensitization, wind-up, or expansion of receptive fields of CNS neurons, as well as by the enhancement of flexion reflexes and the persistence of pain or hyperalgesia after inputs from injured tissues are blocked. It is clear from the material presented that the perception of pain does not simply involve a moment-to-moment analysis of afferent noxious input, but rather involves a dynamic process that is influenced by the effects of past experiences. Sensory stimuli act on neural systems that have been modified by past inputs, and the behavioral output is significantly influenced by the “memory” of these prior events. An increased understanding of the central changes induced by periph- eral injury or noxious stimulation should lead to new and improved clinical treatment for the relief and prevention of pathological pain. INTRODUCTION The relationship between central neuroplasticity and chronic, pathological pain is not a simple one. Phantom limbs, for example, reveal the complexities of the relationship. It is well known that, after a hand is amputated, punctate stimulation of the skin of the forearm produces sensations not only at the stimulated sites, but also in specific parts of the phantom hand. 1–3 An excellent somatotopic map of the phan- tom hand is revealed on the forearm which is reasonably assumed to reflect neuro- plastic changes in representations of the hand and forearm in the central nervous system from spinal cord to cerebral cortex. These changes in somatotopic maps at both the forearm and brain are found within hours after amputation, suggesting that they are due to the removal of inhibition of existing neurons (rather than growth of a Current address: Department of Anesthesia, McGill University, Montreal, Quebec H3A 1B1, Canada. b Current address: Department of Anesthesia, Toronto General Hospital, Toronto, Ontario M5G 2C4, Canada. c Current address: Department of Psychology, University of New Orleans, New Orleans, LA 70148.