Innate and Conditioned Fear : Investigating Responses to Threat using Psychophysiology, Functional Magnetic Resonance Imaging, and Twin Methodology

Evolution has shaped systems in the human brain to respond to danger. Some of these systems are innate or hard-wired, while others are learned throughout the entire life span. One commonly studied type of learned threat, conditioned fear, is acquired from experiencing aversive consequences. When considering innate threat, processing imminent and social threat may be especially relevant for understanding the neural circuitry of human fear. Imminent fear refers to experiencing proximal encounters whereas social fear refers to exposure to an unknown conspecific. Investigating whether different brain systems process innate and conditioned threat could provide us with knowledge on how defensive systems interact and inform on development and treatment of fear-related disorders. The first aim of this thesis was to investigate whether different brain systems process innate and conditioned threat. The second aim was to investigate if neural functions supporting imminent and social threat are heritable. In Study I, skin conductance responses (SCR) to social, imminent and conditioned threat displayed in immersive virtual-reality were compared. The results showed that social threat modulated imminent threat, but neither imminent nor social threat modulated conditioned threat, indicating that distinct processes regulate imminent and social threat on one end and conditioned threat on the other. Study II compared SCR to imminent and conditioned threat displayed either in an immersive virtual-reality head-mounted display or a regular computer monitor. The findings indicated that displaying stimuli in an immersive virtual-reality head-mounted display enhanced imminent threat responses as compared to displaying them on a regular computer monitor, suggesting a modulation of SCR by display type. Conditioned SCR was similar across displays. The conclusion of the study was that imminent threat and conditioned threat are different processes as it was possible to modulate one without affecting the other. Study III was a twin study comparing the genetic influence on neural responses to imminent and social threat using functional magnetic resonance imaging (fMRI) and SCR. The results demonstrated strong genetic influence on responses in the lateral occipital cortex and the fusiform cortex to social threat whereas genetic influence on imminent threat was strong in early visual areas and occipito-parietal areas. In summary, innate and conditioned fear may depend on separate processes in the brain. Innate threat activates early visual areas, indicating a role for these areas in threat processing. Genetic influences on fMRI responses to social and imminent threat suggests that separate pathways may have evolved to process these threats