Stress can strengthen negative memories, a new study states.

People often remember tragic events in greater details while good memories can simply get blurred with time. A new research by Sabrina Segal and colleagues from the Arizona State University explains why this happens.

Scientists have found that a critical stress hormone helps 'fix' bad memories in brain. When the body undergoes a traumatic event, it releases two stress homrones - norepinephrine and cortisol.

Norepinephrine is structurally similar to adrenaline and helps boost heart rate and regulate fight or flight response during a stressful situation. The chemical can also act as a powerful neurotransmitter that can enhance memory.

Related studies have shown that even cortisol can help strengthen memories. However, the studies weren't conducted on humans, so researchers weren't sure whether the stress hormones played similar roles in human memory enhancement.

This is where Segal's study comes in. The team at ASU used human subjects to understand the effect of hormones on memories.

Thirty nine women were shown 144 images from the International Affective Picture Set. The images are used to elicit a range of responses from neutral to emotional reactions, according to a news release.

Before the image test, each participant was given either hydrocortisone - to simulate stress - or a placebo. Women were asked to report their feelings during the test and were asked to provide saliva samples after the exam. One week later, participants were called in again for a recall test.

The team found that "negative experiences are more readily remembered when an event is traumatic enough to release cortisol after the event, and only if norepinephrine is released during or shortly after the event," Segal said in a news release.

"This study provides a key component to better understanding how traumatic memories may be strengthened in women," Segal added, "because it suggests that if we can lower norepinephrine levels immediately following a traumatic event, we may be able to prevent this memory enhancing mechanism from occurring, regardless of how much cortisol is released following a traumatic event."

Understanding how the brain fixates over bad memories could help researchers design therapeutics that can help people suffering from Post Traumatic Stress Disorder recover.

The study is published in the journal Neuroscience.