Scientists showed various images of people linked to positive words

While images were shown, distinctive noises were played in background

When the volunteers fell into a deep sleep, the same noises were played

After waking, they had less prejudice towards women and black people

Imagine being able to 'unlearn' racism and gender prejudice during a short nap.

That's what scientists claim can happen when simple noises are played while people sleep.

In a new study, researchers were able to significantly reduce prejudice in those who underwent the a unique type of training while snoozing.

Participants were first shown female faces with words linked to maths or science.

They were also shown black faces that were linked with pleasant words.

During the tasks, two sounds were played - one that came to be strongly associated with the gender pairs and the other with the race pairs.

Participants then took a 90 minute nap. Once they entered a deep sleep, one of the sounds was played repeatedly.

Before the nap, people’s bias had fallen, but without the sounds during sleep, their level of bias had gone back to baseline after the nap.

But when participants were played the sound cues during sleep, their bias scores reduced by a further 56 per cent.

Their scores remained reduced by around 20 per cent compared to their initial baseline when the participants were tested a week later.

The findings confirm the idea that sleep provides an opportunity to access deep-rooted beliefs, such as prejudices, that we may not even know we have.

Xiaoqing Hu, who led the study at Northwestern University, said that even he was surprised by the results.

'The usual expectation is that a brief, one-time intervention is not strong enough to have a lasting influence,' he said.

'It might be better to use repeated sessions and more extensive training. But our results show how learning, even this type of learning, depends on sleep.'

Earlier studies by the Northwestern researchers have revealed memory reactivation during sleep.

Generally, participants first heard distinctive sounds during a learning session. A short period of sleep came next.

After people woke up, what they could remember was changed if learning-related sounds were presented during sleep.

'We call this Targeted Memory Reactivation, because the sounds played during sleep could produce relatively better memory for information cued during sleep compared to information not cued during sleep,' said Ken Paller, senior author of the study and professor of psychology.

'For example, we used this procedure to selectively improve spatial memory, such as learning the locations of a set of objects, and skill memory, like learning to play a melody on a keyboard.'

The current study was designed to apply the same sort of procedure to counter-stereotype training.

'This type of learning falls into the category of habit learning,' said Paller, who is also director of the Cognitive Neuroscience Program at Northwestern.

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