Laboratory animal and human subject studies report that the amygdala is a critical brain structure that supports the acquisition and expression of conditional fear. Recent functional neuroimaging studies in humans have reported that activity in this region is closely related to the behavioral expression of conditional skin conductance responses (SCR). However, SCR waveforms following conditional stimulus (CS) presentation contain both early period and late period responses that may differ with respect to underlying central processes. It is not known whether amygdala activity corresponds to the expression of early conditonal responses (CRs) that occur shortly following CS onset or late CRs that closely precede UCS onset. The present study used event-related functional magnetic resonance imaging and concurrent skin conductance measurements to determine whether amygdala activity is more closely related to the expression of early or late period CRs. Increased amygdala activity was detected during the formation of early, but not late period CRs. Additionally, this pattern of amygdala activity did not dissipate, but persisted into late stages of the experiment. These findings are consistent with the idea that amygdala responding is critically involved in the generation of CRs formed shortly following CS onset.
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