The effect of graded aversive stimuli on limbic and visual activation

Activation studies have shown that in response to evocative visual stimuli, brain activity increases in the visual cortex and limbic areas. However, non-affective characteristics of these images, such as color composition and visual complexity, confound the interpretation of these results. To address this issue, we had subjects rate over 100 images on aversive intensity (facial mutilation, dead bodies) and semantic complexity (number of objects subjects could name). From these images, we assembled digitized image sets of non-aversive, mild and strong intensity, balanced on semantic complexity and content (human faces and figures), and adjusted for color composition. A fourth condition consisted of a fixation cross on a blank screen. Thirteen subjects underwent eight positron emission tomography scans using the [(15)O] water methodology. Measurement of skin conductance was recorded simultaneously. All picture conditions, relative to the blank screen, activated the amygdalae and bilateral orbitofrontal cortex, while we found activation trends associated with increasing aversive content in the sub-lenticular region. Skin conductance increased during all picture conditions. Relative to the non-aversive pictures, aversive image content caused modulation of occipital and occipital-temporal cortex. These results demonstrated activation of the amygdala to salient, arousing stimuli, and not just aversive stimuli. In addition, they suggest that pictorial complexity, as indexed by our semantic measure, does not account for the modulation of visual cortex by aversive, emotional stimuli.