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Sleep: Caretaker of memories, repairer of prejudice

Subliminal cues work when you're sleeping too

Learning new stuff takes sleep. Robbed of the restorative balm of slumber, research demonstrates that the human brain will quickly lose its ability to make and hold on to new memories, and learning takes a dive.

A new study, however, finds that sleep is also crucial to unlearning stuff that we may have committed to memory without a proper quality-control check.

Implicit biases -- those stereotypes about gender, race, age and ethnicity that you might never own up to but which nevertheless color your reactions to people and situations -- are laid down early, and they are surprisingly hard to break.

In a study published Thursday in Science, psychologists from Northwestern University note that although a systematic retraining session can begin to undo implicit biases, its impact is fragile and fleeting. With just a nudge -- a news report, a personal interaction that supports a long-held implicit bias or just the passage of time -- the effects of training aimed at countering such unconscious prejudice disappear.

The new research finds, however, that adding sleep to the mix -- and subtly reinforcing the retraining during subsequent sleep -- helped stamp out implicit bias robustly and enduringly in experimental subjects. When participants slept soundly after viewing a series of images designed to counter implicit bias, and got a subtle reminder of that learning during their sleep, their implicit biases were more clearly unlearned. And those prejudices stay unlearned for longer, the study found.

To demonstrate the combined power of sleep and reinforcing cues delivered during sleep, the study's authors recruited 40 white college students and measured their ingoing levels of "implicit bias" around issues of gender and race.

The participants were set before a keyboard and asked to react to paired words and images that gauged their unconscious prejudices. (Because stimuli that fit prevailing gender and racial sterotypes are more readily processed, participants with high levels of implicit bias will react quickly -- say by pressing a designated key -- to such pairings as a man's face and the word "science," or to an African American person's face and the word "bad." A person's implicit bias score will similarly increase if he or she is slow to react to a combination that's inconsistent with prevailing stereotypes, such as the pairing of a woman's face and "science," or an African American's face and "good.")

The researchers then administered a 30-minute training session aimed at countering such implicit bias. Participants were instructed to press a key when pairings of images and words went against stereotypes ("math" with a female face; "sunshine" with an African American man's face). When they responded quickly and accurately, participants heard one of two distinctive tones.

Then they slept. For 90 minutes in a darkened room equipped with a white noise generator, participants were invited to take a nap. Amidst the white noise, half were cued with the tone associated with their earlier success in countering gender bias; as the other half slept, researchers intermittently piped in the tone associated with success in countering racial stereotypes.

For the nappers who got the reinforcing audio tone that countered gender stereotypes, post-nap assessments of implicit gender bias showed that the training had stuck. Tested a week later, those who got the sleeping cues continued to score low on gender bias. But their racial biases had largely returned to baseline.

The nappers who got the audio tone associated with countering racial stereotypes also showed low post-nap levels of implicit racial bias. And a week later, those levels were still low. But their gender biases had reestablished themselves.

And the more slow-wave and rapid-eye-movement sleep a napper had during his or her period of acoustic reinforcement, the more robust was the effect of the counter-bias training.

As any psychotherapist will tell you, relearning (or unlearning) deeply held -- and often unconscious -- social beliefs and behaviors is very challenging, even for people with strong learning skills. The latest study suggests that harnessing sleep to the task -- and reinforcing such learning with associated sounds, smells or sensations introduced during sleep -- might ease the challenge of changing hearts and minds.

"Perhaps novel sleep manipulations could be adapted to aid people in changing various unwanted or maladaptive habits, such as smoking, unhealthy eating, catastrophizing, or selfishness," the authors write. 

Human behavior thrills me. Follow me on Twitter @LATMelissaHealy and "like" Los Angeles Times Science & Health on Facebook.

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