Alternative Explanation for fMRI Findings in Social Phobia

To the Editor: In the December 2010 issue of the Journal, Karina S. Blair, Ph.D., and colleagues (1) analyzed the differences in processing unintentional and intentional social norm transgressions comparing patients with generalized social phobia and healthy volunteers. They found that patients with generalized social phobia showed an increased response to unintentional transgressions in the medial prefrontal cortex. The investigators also described increased responses in the amygdala and the insula, arguing that this must be because of a greater general responsiveness to social stimuli in social phobia.

In this study, the authors extensively described how they presented the stimuli and how they analyzed the fMRI results. For example, they excluded alternative explanations of their findings, such as the influence of low mood as measured on the Inventory of Depressive Symptomatology–Self Report scale. However, the authors gave only one example of the stimuli they used, namely, choking and coughing up food (unintentional) and spitting out bad-tasting food (intentional). This example is very likely to be perceived as disgusting, and this makes one wonder whether all of the harmless social norm violations used in this study were disgusting.

There are harmless non-disgusting transgressions, such as wearing two different shoes, which can be done intentionally or by accident. It is unclear from the information provided if these types of stimuli were included in the study, and if so, how many of the 26 stem stories described non-disgusting transgressions. In general, the stimuli used in fMRI studies should be made public, or an extensive description should be offered in order to make replication of the study possible.

However, in this particular study, there is another reason to describe the stimuli more precisely, as an alternative explanation for the findings is possible. Disgust stimuli can activate the insula (2), and there is evidence for increased activation of the insula in response to disgust faces in social phobia (3). Therefore, on the basis of the information provided, increased sensitivity to disgust could explain the difference in insula responses between patients with social phobia and healthy comparison subjects.