Introduction

Before the introduction of computerized tomographic (CT) brain imaging, studying human brain mechanisms of pain was largely limited to clinical reports and the post-mortem analysis of brain lesions. Although this approach provided important information and established the background for current investigations, these studies were usually limited by clinical descriptions of each patient's condition. Somatosensory psychophysics seldom included studies of pain and even then it was not possible to relate these observations to brain function or physiology. Because the living brain was invisible (except in the neurosurgery operating suite), research on pain mechanisms focused almost exclusively on the peripheral nervous system.

Brain CT scans introduced the opportunity to apply quantitative sensory testing to the study of living patients with visible, localized brain lesions and to begin to test hypotheses about functional localization and brain mechanisms of pain. The introduction of functional imaging by positron emission tomography (PET) and magnetic resonance imaging (MRI; fMRI) launched a new investigational paradigm into the study of pain mechanisms. Now it is possible to go well beyond the lesion analysis method and to relate human experience, in this case using somatosensory psychophysics, directly to a surrogate measure of activity in groups of neurons at the level of visible, localized brain structure. Since the early 1990s, the number and technical sophistication of functional brain imaging studies, including those related to pain, has increased at a rate that makes it almost impossible to incorporate the results into a conceptual framework.