Teenagers afflicted with bipolar disorder more often than not misinterpret neutral facial expressions as hostile. And when they do, they react with fear. These are the findings of a recent study conducted by researchers at the University of Cincinnati (Ohio) College of Medicine.

These results were illustrated by brain scans that showed that the amygdala - the part of the brain which processes fear -- demonstrated more activity in teens with bipolar disorder than those who did not have this condition. Melissa DelBellow, one of the researchers involved in the study, says that the evidence of this brain activity provides invaluable insight into the workings of teen behavior. It clearly shows that adolescents with bipolar disorder process emotions differently.

Scientists were already aware that the amygdale of a teen with bipolar disorder was smaller than a teen without the health condition. But they didn't know if the structure of this section of the brain would actually translate into a difference in function.

Evidence of differing brain activity in teens with bipolar disorder is sustaining these findings. Ellen Leibenluft of the National Institute of Mental Health in Maryland, used magnetic resonance imaging - better know as MRI - to examine teen brain activity.

She studied the reaction of 22 teens with bipolar disorder - average age 14 - and compared them with 21 teens without the disorder. All participants, regardless of whether they had bipolar disorder or not, reacted in a similar manner to the images of happy and fearful faces.

However, when it came to neutral faces, teens with bipolar disorder were more likely to interpret these faces as hostile expressions and feel fearful of them. When evaluated on a scale of one to five, with five being the most fearful, those with bipolar disorder rated the faces a two. Those without bipolar disorder rated the faces as a 1.4.

These findings are significant because they implicate the amygdala as having a role in bipolar disorder. This is especially true when presented with the size difference of the amygdala in teens with bipolar disorder. Some experts suggest, however, that the difference in misreading the hostility in the neutral face could be accounted for by the medication the teens were taking for this condition.