LONDON It often seems a thin line between love and hate, and now scientists think they know why.
Brain scans of people shown images of individuals they hated revealed a pattern of brain activity that partly occurs in areas also activated by romantic love, Semir Zeki and John Paul Romaya of University College London reported on Wednesday.
"This linkage may account for why love and hate are so closely linked to each other in life," the researchers wrote in the Public Library of Science journal PLoS One.
"Our results show that there is a unique pattern of activity in the brain in the context of hate."
In their study, the researchers showed 17 men and women pictures of someone the volunteers said they hated along with three familiar, neutral faces. The hated individuals were all former lovers or work rivals, except for one famous politician.
The brain scans identified a pattern of activity in different areas of the brain the researchers called a "hate circuit" that switched on when people saw faces they despised, the researchers said.
"As far as we can determine it is unique to the sentiment of hate even though individual sites within it have been shown to be active in other conditions that are related to hate," the researchers wrote.
The so-called hate circuit includes structures in the cortex and the sub-cortex and represented a pattern distinct from emotions such as fear, threat and danger, Zeki said in a telephone interview.
One part of the brain that switched on was an area considered critical in predicting other people's actions, something that is likely key when confronting a hated person, the researchers said.
The brain activity also occurred in the putamen and insula, two areas activated when people viewed the face of a loved person. Scientists have linked the regions to aggressive action and distressing situations, Zeki said.
But there were important differences as well. A bigger part of the cerebral cortex -- an area linked to judgment and reasoning -- de-activates with love compared to hate.
While both emotions are all-consuming passions, it may be that people in love are often less critical and judgmental about their partner but need to maintain their focus when dealing with a hated rival, the researchers said.
"It is more likely that in the context of hate the hater may want to exercise judgment in calculating moves to (cause) harm," Zeki said in a statement.
(Reporting by Michael Kahn; editing by Will Dunham and Michael Roddy)