Laboratory animal studies have focused on understanding the neural mechanisms underlying both stimulus acquisition and response expression in fear learning. Unfortunately, functional neuroimaging studies of human Pavlovian fear conditioning have largely focused on the importance of learning stimulus associations while relatively little consideration has been given to mapping response expression. Since human fear responses can be expressed in multiple forms, several unique brain regions may be involved in the generation of these individual learned responses. The current project used fMRI to systematically identify neural structures related to learned fear responses in human subjects. Implicit and explicit responses were monitored during scanning and were used to guide our imaging analysis. Several regions of interest were detected based on subjects' individual behavioral performance. Increased amygdala activity was associated with subjects that demonstrated SCR discrimination and with trials that elicited conditional SCRs shortly following CS onset. These data support the position that human amygdala activity may be closely related to the autonomic expression of conditional fear. Additionally, the pattern of results seen in the insula fit with the idea that this region plays an important role in relaying somatosensory information associated with fear to the amygdala. Using subjects' explicit responses to determine their level of awareness also produced interesting results. Differential hippocampal activity associated with awareness of the stimulus relationships was detected. Furthermore, following awareness of the stimulus relationships, CS+ evoked responses overlapped with UCS related activity observed in the primary somatosensory cortex suggesting that subjects anticipated the shock sensation. In summary, this project represents an investigation of the neural substrates underlying the expression of learned explicit and implicit responses in human fear conditioning.
