Vigilance for Threat Interacts with Amygdala Responses to Subliminal Threat Cues in Specific Phobia

Background: The amygdala has been strongly implicated in the processing of threat-relevant information in specific phobia. However, there is an ongoing debate as to whether the amygdala may also be engaged outside of conscious stimulus awareness. Furthermore, considering that sustained vigilance for threat constitutes a crucial characteristic of specific phobias, we hypothesized a possible role of this symptom in modulating amygdala sensitivity to disorder-relevant cues. Using event-related functional magnetic resonance imaging, we examined responses of the amygdala to subliminal and supraliminal phobogenic stimuli in spider-phobic subjects and whether these responses might be specifically associated with disorder-related hypervigilance. Methods: Eighteen female spider-phobic subjects and 18 healthy female control subjects were exposed to pictures of spiders and phobia-irrelevant objects, presented briefly during two backward-masking conditions used to manipulate stimulus awareness. Brain activation data were analyzed as a function of subjects' perceptual performance on each single trial and were tested for correlations with different components of the phobic symptomatology, such as vigilance, as assessed by self-report scales. Results: Compared with control subjects, phobic participants showed stronger responses of both amygdalae to consciously perceived spiders versus nonspider targets, whereas during unconscious stimulus processing, enhanced activation was only apparent in the right amygdala. Moreover, the intensity of disorder-related vigilance was positively correlated with right amygdala activation specifically during the subliminal condition. Conclusions: These findings provide evidence for unconscious threat processing in specific phobia, with the magnitude of amygdala responses specifically potentiated by sustained hypervigilance for threat. Aberrations in this vigilance system may be critically involved in anxiety disorders.