The undated photo shows two mices. (Xinhua/Credit: Hailuan Hu Lab)

WASHINGTON, July 13 (Xinhua) -- Chinese researchers may have revealed the secret of why animals increase their probability of victory after previous winnings, a phenomenon known as the "winner effect."

In a study published Thursday in the U.S. journal, Professor Hailan Hu's research group from Zhejiang University in China reported identifying for the first time a neural circuit in the brains of mice that plays a role in social dominance.

Stimulating brain cells in this circuit, known as dorsalmedial prefrontal cortex (dmPFC), significantly boosted a mouse's chance of becoming the "winner" during aggressive encounters with other mice.

"Getting to the top of social hierarchy is often not a matter of body size or brute strength, but rather determined by intrinsic mental factors such as grit, as well as extrinsic factors such as history of winning. For example, social dominance can be reinforced by a phenomenon known as the 'winner effect,'" Hu said. "However, the neural mechanism that mediates these intrinsic and extrinsic factors was poorly understood."

For the study, Hu's team performed a standard social dominance test that put male mice in a tube to face each other. Usually the subordinate animal would retreat and back out of the tube.

Then, the researchers recorded how much each one engages in certain behaviors such as push initiation, push-back, resistance, retreat, or stillness.

By monitoring individual brain cells in the dmPFC during such tests, they found a particular subset became more active during both push and resistance behaviors.

In mice with an established social rank, the researchers inhibited this subset of dominance brain cells using a drug and found within hours, these mice engaged in significantly fewer and shorter pushes and push-backs, but in more retreats.

Next the researchers used optogenetics to stimulate the dmPFC cells continuously during a social dominance encounter.

This instantaneously induced winning against previously dominant opponents with a 90 percent success rate, without affecting the motor performance or anxiety level.

"Importantly, dmPFC activation does not seem to boost dominance by enhancing basal aggression level or physical strength, but rather by initiating and maintaining more effortful behaviors during social competition," Hu said.

The findings could have important implications for treating psychiatric diseases, the researchers said.

"Considering that an excess or lack of dominance drive is associated with many personality disorders and mental problems, our results might shed light on the treatment of these psychiatric diseases," the study said.