A unique representation of heat allodynia in the human brain.

Skin inflammation causes innocuous heat to become painful. This condition, called heat allodynia, is a common feature of pathological pain states. Here, we show that heat allodynia is functionally and neuroanatomically distinct from normal heat pain. We subtracted positron emission tomography scans obtained during painful heating of normal skin from scans during equally intense but normally innocuous heating of capsaicin-treated skin. This comparison reveals the specific activation of a medial thalamic pathway to the frontal lobe during heat allodynia. The results suggest that different central pathways mediate the intensity and certain qualitative aspects of pain. In making this differentiation, the brain recognizes unique physiological features of different painful conditions, thus permitting adaptive responses to different pain states.