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OBJECTIVE: The central nervous system representation of social phobia (social anxiety disorder) is largely unknown. The aim of this study was to examine brain activity during symptom provocation in social phobics. METHOD: Positron emission tomography with the use of (15)O water was used to measure regional cerebral blood flow (rCBF) in 18 subjects with DSM-IV-defined social phobia and a nonphobic comparison group while they were speaking in front of an audience and in private. Heart rate and subjective anxiety were also recorded. RESULTS: During public versus private speaking, subjective anxiety increased more in the social phobics than in the comparison group. Increased anxiety was accompanied by enhanced rCBF in the amygdaloid complex in the social phobics relative to the comparison subjects. Cortically, brain blood flow decreased in the social phobics and increased in the comparison subjects more during public than private speaking in the orbitofrontal and insular cortices as well as in the temporal pole and increased less in the social phobics than in the comparison group in the parietal and secondary visual cortices. Furthermore, rCBF increased in the comparison group, but not in the social phobics, in the perirhinal and retrosplenial cortices. CONCLUSIONS: An rCBF pattern of relatively increased cortical rather than subcortical perfusion was observed in the nonphobic subjects, indicating that cortical evaluative processes were taxed by public performance. In contrast, the social phobia symptom profile was associated with increased subcortical activity. Thus, the functional neuroanatomy of social phobia involves the activation of a phylogenetically older danger-recognition system.