Entering a dark, unfamiliar room can fill some people with dread, while others step in without thinking twice. Scientists wanted to know if a brain structure called the amygdala could help explain why.

Earlier work by a team at Caltech showed that the amygdala plays an important role in fear responses to clear threats–like a snake or a charging tiger–but it’s not know if it also plays a role in these more ambiguous but potentially scary situations.

In a paper recently published, California Institute of Technology researchers Laura Harrison and Ralph Adolphs examined whether amygdala lesions caused people to evaluate stimuli on the basis of a specific positivity bias—in this case, a face-approach bias.

Previous studies have shown that monkeys with amygdala lesions display a tendency to actually approach stimuli that are normally considered threatening.

Amygdala Damage

The researchers asked control subjects and three subjects with rare bilateral amygdala damage to indicate the degree to which they found pictures of people with or without obscured (and hence ambiguous) central facial features either trustworthy or threatening.

Participants with amygdala damage exhibited a greater tendency than did control participants to rate obscured faces as more approachable than whole faces. The increase in trust ratings and decrease in threat ratings in the obscured-face condition, compared with the whole-face condition, was greater for the patients than for the control subjects.

According to the researchers, the findings suggest that amygdala lesions lead to a global tendency to evaluate stimuli positively, even when they can’t be seen.

The results also indicate that the amygdala plays a significant role in detecting threats—critical information in the ongoing effort to treat anxiety and similar life-altering conditions.

Laura A. Harrison, Rene Hurlemann, and Ralph Adolphs An Enhanced Default Approach Bias Following Amygdala Lesions in Humans Psychological Science October 2015 26: 1543-1555, 2015 doi:10.1177/0956797615583804

Top Illustration: Thomas Hawk/Flickr

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