Functional Imaging Correlates of Pain Perception

Pain is a complex experience characterized by sensory,emotional, and cognitive aspects, which is likely to resultfrom the activity of large populations of central nervoussystem (CNS) cells. A crucial challenge in pain research istherefore to identify how the joint involvement of the differentnociceptive networks, that have been described byanatomical and electrophysiological techniques in experimentalanimals [1], gives rise to pain perception andmodulation.To this end, it is necessary to monitor functional activityof large neuronal arrays, in parallel with psychophysicalself-reports. This has been accomplished in humansusing different kinds of functional imaging techniques,among which positron emission tomography (PET) andfunctional magnetic resonance imaging (fMRI). Theyallow an indirect estimation of neural activity by investigatinglocal metabolic or hemodynamic changes, whichare closely related to the electrical activity of nerve cellsunder physiological circumstances [2]. In many studies, ablock experimental design was adopted: namely, brainactivity was compared among short periods of time (typically60 seconds in PET studies), each characterized bycontinuous (or repeated) innocuous or noxious stimulation.A single perceptual judgement was obtained at theend of each block, thus preventing appreciation of timerelatedchanges in nociceptive activity. Using blockdesigns makes it difficult to discriminate between brainactivity related to differences in the perceived pain intensity,to changes in the level of arousal during noxiousinput, or to anticipation of pain. The fMRI technique hasa better temporal resolution (usually on the order ofseconds) and it is therefore more suitable than PET totrace changes in functional activity of discrete brain areasover time. This can be done using brief stimuli (from hundredsmilliseconds to 2 seconds) in the so-called eventrelatedparadigms [3], using short epochs of 20-30 s, orduring time-varying pain induced by prolonged chemicalstimulation [4].Recent imaging studies have demonstrated the involvementof specific cortical and subcortical systems in differentaspects of the conscious experience of pain (seereview in [5]).