Concepts of pain mechanisms: the contribution of functional imaging of the human brain.

Functional imaging of the conscious human brain has a solid physiological basis in synaptically induced rCBF responses. We still do not know how these responses are generated, but recent studies have shown that the rCBF response is parametrically positively correlated with functional measures of neuronal activity. Technical advances in both fMRI and PET imaging have improved the spatial and temporal resolution of imaging methods. Further advances may be expected in the near future. Consequently, we now have an important tool to apply to the study of normal and, most importantly, pathological pain. There is a tendency to expect too much of this exciting technique, but the problems we wish to address are complex and will require considerable time, effort, and patience. We now know that the CNS adapts to both peripheral and central nervous system injury, sometimes in beneficial ways, but sometimes with reorganization that is maladaptive. An understanding of the pathophysiology of neuropathic pain is further complicated by the new knowledge, emphasized by functional brain imaging, that pain and pain modulation is mediated, not by a simple pathway with one or a few central targets, but by a network of multiple interacting modules of neuronal activity. Simplified phrenological thinking, with complete psychological functions separate and localized, is appealing, but wildly misleading. It is far more realistic and productive to apply qualitative and quantitative spatial and temporal analyses to the distributed activity of the conscious, communicating human brain. This will not be quick and easy, but there is every reason for optimism in our search for a thorough and useful understanding of both normal and pathological pain.