The human brain must constantly screen incoming stimuli for relevance. Without such screening, the brain would quickly be overwhelmed by the sheer number of stimuli we experience every day. Some of the most profound stimuli--such as other people's faces--trigger an emotional response, but there are times when fear or even happiness must be kept at bay as the brain works to solve a problem. New research has shown how the brain goes about accomplishing this task.

The brain structure known as the amygdala plays a key role in generating and perpetuating emotions such as fear by sending signals into the hypothalamus, which controls the sympathetic nervous system. The amygdala's neighbors in the brain include the anterior cingulate cortex, which neuroscientists think controls various so-called executive processes, such as deciding where attention should be focused. The rostral region of this portion of the cortex (the area on the bottom toward the front) connects directly with the amygdala, and some had speculated that the former might play a role in monitoring emotional states, allowing them to flourish when appropriate but suppressing them when not.

Designing a test of this hypothesis, however, had proved challenging. Amit Etkin of Stanford University and his colleagues turned to a classic study of conflict in the brain--the Stroop task--in which the name of a color and the color of the letters in that name either match or not. They tweaked it to create an emotional conflict: pairing faces displaying particular emotional states with words written across them that either identified that state or not, such as fear displayed on a smiling face. While being scanned by functional magnetic resonance imaging (fMRI), 19 subjects stared at 148 versions of these pairings, clicking a button to indicate whether that person was happy or afraid.

The researchers found that subjects were able to resolve conflicting images more quickly if they were preceded by other conflicting images, indicating that the brain was preparing itself for such conflict resolution. And the fMRI revealed that the emotional pictures did, in fact, activate the amygdala, but that when the face and word were discordant, the rostral anterior cingulate cortex (rACC) also lit up and inhibited the activation of this emotion-controlling structure. "For example, if someone is walking on an empty street at night and hears a loud banging sound in the near distance, the amygdala would immediately light up," explains Joy Hirsch of Columbia University, senior author of the paper presenting the result in the September 21 issue of Neuron. "Once the source is determined, the rACC determines if action is needed or not. For example, if it was a car door slamming, the rACC would shut down the amygdala."

It is possible, however, that those roles are reversed: the amygdala habitually dampening the rACC and then letting it loose when emotional activity is reduced. But the rACC always seems to activate when assessing these stimuli, and tests of skin conductance on 10 different subjects exposed to the same test showed that the sympathetic nervous system was indeed less active when the rACC was inhibiting. The finding suggests a two-stage response to perceived threats as well as what might be malfunctioning in patients suffering from post-traumatic stress disorder. It may simply be that one of the executive portions of the brain is failing to calm its emotional colleague.