Melzack paradoxically showed us that the pain perception is all in our heads. Without the brain, of course, we would perceive no pain. Chronic pain is a disorder of perception and not necessarily at all correlated with the extent of tissue injury or any objective measures that we can create. Dr. Robert Bennett of the Oregon Health Sciences University wrote about the often complete absence of tissue injury in relation to chronic pain, which is diffuse and often spreads to areas well beyond the site of the original acute injury. He noted also that the ways in which acute pain are usually treated do not work for chronic pain.
University of California at San Diego neoroscientist Ramachandran showed that phantom limb pain arose from central brain maps that continued to maintain the pain present before the amputation and not from any peripheral nerve sensitization or other observable phenomena. Ramachandran found creative ways to eliminate phantom limb pain by tricking and encouraging the brain to remap itself in such a way as to eliminate the phantom limb and its pain.
Similarly, Melzack and other researchers showed why preventing post-surgery patients from feeling pain by giving them more than adequate analgesia before they feel pain actually allows them to feel less pain and use less overall analgesia. When surgical patients are allowed to feel severe pain, the brain remodels itself to feel even more pain and to exagerate the pain perceived. The consequence is that the doctor is always chasing pain that cannot be controled. When adequate analgesia is produced to prevent the experience of pain (and hypnosis, of course, or mindfulness training can be part of this prevention), then this remodeling process does not happen and paradoxically the amount of pain felt is minimized as is the patient's use of analgesia.
The presence of pain and the activation of pain circuits sensitizes, winds-up, and expands the receptive fields of CNS neurons involved in pain perception. Thus, the perception of pain also involves a dynamic process influenced by the effects of past experiences which shapes the experience of present and future 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.