Sleep vital to associating emotion with memory: study

Children and their teacher play a sleeping game at a kindergarten in Weinan City, northwest China's Shaanxi Province, March 21, 2017. (Xinhua/Cui Zhengbo)

CHICAGO, Feb. 23 (Xinhua) -- A study of the University of Michigan suggests that groups of neurons activated during prior learning keep humming, tattooing memories into brain when one slips into sleep.

Focusing on a specific set of neurons in the primary visual cortex, the researchers created a visual memory test. They showed a group of mice a neutral image, and expressed genes in the visual cortex neurons activated by the image.

To verify that these neurons registered the neutral image, the researchers tested whether they could instigate the memory of the image stimulus by selectively activating the neurons without showing them the image. When they activated the neurons and paired that activation with a mild foot shock, they found that their subjects would subsequently be afraid of visual stimuli that looked similar to the image those cells encode. They found the reverse also to be true: after pairing the visual stimulus with a foot shock, their subjects would subsequently respond with fear to reactivating the neurons.

"Basically, the precept of the visual stimulus and the precept of this completely artificial activation of the neurons generated the same response," said Sara Aton, senior author of the study and a professor in the UM Department of Molecular, Cellular and Developmental Biology.

The researchers found that when they disrupted sleep after they showed the subjects an image and had given them a mild foot shock, there was no fear associated with the visual stimulus. Those with un-manipulated sleep learned to fear the specific visual stimulus that had been paired with the foot shock.

"We found that these mice actually became afraid of every visual stimulus we showed them," Aton said. "From the time they go to the chamber where the visual stimuli are presented, they seem to know there's a reason to feel fear, but they don't know what specifically they're afraid of."

This likely shows that, in order for them to make an accurate fear association with a visual stimulus, they have to have sleep-associated reactivation of the neurons encoding that stimulus in the sensory cortex. This allows a memory specific to that visual cue to be generated. The researchers think that at the same time, that sensory cortical area must communicate with other brain structures, to marry the sensory aspect of the memory to the emotional aspect.

Aton said the findings could have implications for how anxiety and post-traumatic stress disorder are understood.

"To me this is kind of a clue that says, if you're linking fear to some very specific event during sleep, sleep disruption may affect this process. In the absence of sleep, the brain seems to manage processing the fact that you are afraid, but you may be unable to link that to what specifically you should be afraid of," Aton said. "That specification process may be one that goes awry with PTSD or generalized anxiety."

The study, posted on UM's website on Monday, has been published in Nature Communications.