Attentional Modulation of Brain Responses to Phobia-Relevant Stimuli: An Event-Related fMRI Study

Using event-related functional magnetic resonance imaging (fMRI), brain activation to phobia-related pictures (P, spiders) and neutral pictures (N, mushrooms) was measured in spider phobic subjects and healthy controls under two task conditions. In the direct task, subjects were asked to identify the object (spider or mushroom) depicted in the background of the pictures. In the highly demanding indirect task subjects had to match geometric figures in the foreground of the pictures. In controls, there was no increased brain activation to P vs. N under any task condition. In contrast, phobics showed greater responses to P vs. N in the amygdala regardless of task, while the insula and anterior cingulate gyrus (ACC) were more strongly activated only during the direct task. The pronounced amygdalar activation during the demanding indirect task suggests an important role of the amygdala for the rather automatic processing of phobogenic threats. In contrast, activation of areas such as the insula and ACC requires focused attention and evaluation related to the threatening stimuli. Therefore, our findings propose specific neural correlates for the detection vs. the detailed processing of phobia-relevant threats.