element, the unpleasantness of pain), and pain that is distinctly located also activates primary and secondary somatosensory cortex. Spinal cord fibers dedicated to carrying A-delta fiber pain signals, and others that carry both A-delta and C fiber pain signals to the thalamus have been identified. Other spinal cord fibers, known as wide dynamic range neurons, respond to A-delta and C fibers, but also to the much larger, more heavily myelinated A-beta fibers that carry touch, pressure and vibration signals. Ronald Melzack and Patrick Wall introduced their gate control theory in the 1965 Science article "Pain Mechanisms: A New Theory". The authors proposed that the thin C and A-delta (pain) and large diameter A-beta (touch, pressure, vibration) nerve fibers carry information from the site of injury to two destinations in the dorsal horn of the spinal cord, and that A-beta fiber signals acting on inhibitory cells in the dorsal horn can reduce the intensity of pain signals sent to the brain. Three dimensions of pain In 1968 Ronald Melzack and Kenneth Casey described chronic pain in terms of its three dimensions: "sensory-discriminative" (sense of the intensity, location, quality and duration of the pain), "affective-motivational" (unpleasantness and urge to escape the unpleasantness), and "cognitive-evaluative" (cognitions such as appraisal, cultural values, distraction and hypnotic suggestion). They theorized that pain intensity (the sensory discriminative dimension) and unpleasantness (the affective-motivational dimension) are not simply determined by the magnitude of the painful stimulus, but "higher" cognitive activities can influence perceived intensity and unpleasantness. Cognitive activities may affect both sensory and affective experience or they may modify primarily the affective-motivational dimension. Thus, excitement in games or war appears to block both the sensory-discriminative and affective-motivational dimensions of pain, while suggestion and placebos may modulate only the affective-motivational dimension and leave the sensory-discriminative dimension relatively undisturbed. (p. 432) The paper ends with a call to action: "Pain can be treated not only by trying to cut down the sensory input by anesthetic block, surgical intervention and the like, but also by influencing the motivational-affective and cognitive factors as well." (p. 435) Evolutionary and behavioral role Pain is part of the body's defense system, producing a reflexive retraction from the painful stimulus, and tendencies to protect the affected body part while it heals, and avoid that harmful situation in the future. It is an important part of animal life, vital to healthy survival. People with congenital insensitivity to pain