Reward improves visual learning only after people sleep: Study

Rewarding people during a task involving visual perception may lead to performance gains only if they follow up the task with sleep, according to a study which cautions students against sacrificing sleep for late night study sessions.

In the study, published in the journal PNAS, young adults were asked to identify a letter and the orientation of a set of lines on a busy background.

Some of the participants were told to refrain from eating or drinking in the hours leading up to the task, and were then given drops of water as a reward for correct responses, the researchers including those from Brown University in the US, said.

In contrast to groups which were not rewarded during training, the findings revealed that the rewarded participants exhibited significant performance gains -- but only if they slept after the training session.

"College students work very hard, and they sometimes shorten their sleep. But they need sleep in order to retain their learning," said study co-author Yuka Sasaki, a professor of cognitive, linguistic, and psychological sciences at Brown University.

According to the researchers, reward -- or anticipation of reward -- reinforced nerve connections in the brain between areas responsible for reward and visual functions.

They said these brain circuits are more likely to reactivate during sleep to facilitate task learning.

During post-training sleep in participants who were rewarded, the brain electric activity pattern seen in electroencephalogram (EEG) recordings, revealed an increased activation in the prefrontal, reward-processing area of the brain, and decreased excitation in the untrained visual areas.

Putting this finding in context with previous studies, the researchers said the prefrontal, reward-processing area of the brain sends signals to inhibit some of the nerves in the visual processing area.

Due to this, they added, irrelevant connections are trimmed, and the most efficient circuits are preserved, improving task performance.

The researchers also assessed when the patterns of activation occurred.

They said untrained visual areas of the brain showed reduced activation during both Rapid Eye Movement (REM) and non-REM sleep.

However, the scientists said, prefrontal, reward-processing areas became active only during REM sleep -- a kind of sleep that is associated rapid eye movements, more dreaming, faster pulse and breathing.

REM sleep, they added, could be particularly important for task learning since nerve connections are reorganised and optimised during this sleep stage.

According to the researchers, this type of sleep may be linked to the activation of reward-processing areas of the brain.

Strengthening this theory, they found that the rewarded study participants displayed longer periods of REM sleep compared to those who did not receive a reward during training.

The researchers also said rewards like food and water, may have a stronger impact on neural circuits compared to comparatively abstract rewards such as money.

"Water deprivation may be fundamental. When you're really thirsty and you get water as a reward, the impact of that reward may be more prevailing to the brain," Sasaki said.