Ever wondered by you remember some things but not others? According to researchers, it's because the brain prioritizes rewarding memories and reinforces them by playing them over and over while you are at rest.

In a study published in the journal Neuron, Charan Ranganath, a neuroscientist from the University of California, Davis and the study's senior author, explained that rewards are helpful in remembering things because people are hardwired to desire future rewards.

According to estimates, only a small portion of detailed memories made in a day are retained. Too many details will overwhelm a person so the brain sets to work filtering information. If it's going to do so, it makes sense then for the brain to zero in on memories that may lead to future rewards.

The researchers tested this idea by enlisting volunteers and scanning their brains while they answered questions answerable with a "yes" or "no" about some objects, all of which were shown with a background image for context. Depending on the context, the researchers told the volunteers that they will either get small or big rewards for giving the right answers. The volunteers were told afterward about the rewards they got.

Once this portion of the study was completed, the researchers scanned the brains of the volunteers again while they were at rest. After being scanned, all the volunteers were given a surprise memory test.

The volunteers were not told that a memory test will be conducted but they appeared to be better at recalling objects associated with high rewards.

"Also, when an object was associated with high reward, people remembered better the particular background scene that was on the screen during scanning," said Matthias Gruber, the study's first author.

Based on the scans, the researchers found that volunteers who remembered more of the high-reward objects had increased interaction between the substantia nigra/ventral tegmental area complex and the hippocampus in the brain. As the former is involved in rewards processing and the latter in memory processing, the interaction hints at rewards stimulating the hippocampus after learning.

According to researchers, the interaction is likely to be related to dopamine release, as the brain releases the neurotransmitter when expecting a reward.

Photo: Joyce Kaes | Flickr