A new study looks into the mechanism behind sleep-dependent neuroplasticity in the process of assimilating new visual information.
An important part of how we relate to the world is perceptual learning, which refers to our ability to "make sense" of various stimuli - visual, auditory, or related to taste, smell, and touch - through repeated exposure to them.
Perceptual learning improves the way in which we relate to stimuli, helping us to unpick ambiguous ones. Research had already shown that for consolidating perceptual learning, immersion in slow-wave - or non-rapid eye movement (NREM) - sleep is required.
A previous study concluded that "perceptual memory consolidation requires top-down cortico-cortical input during NREM sleep," which means that the route of information transmitted from one cortical area to another is crucial to fully assimilating perceptual learning undergone throughout the day.
Now, new research from the University of Michigan in Ann Arbor is observing how new visual experiences are consolidated as memories during NREM slow-wave sleep.
Led by principal investigator Dr. Sara Aton, scientists used mouse models to understand the neural mechanism that underlies this process of consolidation. Their findings were published in the Proceedings of the National Academy of Sciences.
Deep sleep consolidates visual learning
Dr. Aton explains that new visual stimuli are transmitted via the retina to a region of the brain called the thalamus, which then relays that information to the cerebral cortex, which is known to play a role in memory formation.
During wakefulness, the neurons that communicate that visual information between the thalamus and the cerebral cortex ensure a steady flow of electrical impulses. During NREM deep sleep, on the other hand, the neurons "burst," meaning that spikes of activity are registered.
Dr. Aton explains that after bursting, the neurons pause in a rhythmic then in a synchronized pattern. The team also noted that the cortex fires information back at the thalamus, so that the information is fed back and forth in a circular fashion.
In a previous study by Dr. Aton and team, they experimented with the impact of sleep on the brain mechanism behind processing and consolidating new visual information.
Working with mice, they exposed the animals to novel visual stimuli and then allowed them to sleep. After sleep, the scientists noted, the neurons in the cerebral cortex became more active when exposed to the same visual stimuli.
At the same time, if the rodents experienced sleep deprivation, the cortical neurons were unable to form new connections and consolidate the new information.
But in the new study, Dr. Aton explains that she and her team were interested in finding out what would happen if they performed a reverse experiment. She explains, "We wondered what would happen if we just disrupted that pattern of [brain] activity without waking up these animals at all?"
Sleep-dependent plasticity mechanism
In the recent study, the researchers inhibited neurons from the visual cortex - that is, the part of the cerebral cortex directly implicated in processing visual stimuli - in order to disrupt the feedback pattern between the thalamus and the cortex.
This was done as the mice were either naturally asleep or naturally awake. When asleep, the disruption did not awake the animals.
But during NREM slow-wave sleep, it distorted the normal rhythm of communication between the visual cortex and the thalamus. This means that neuroplasticity, or the ability to form fresh neural connections to accommodate and consolidate novel information, is affected in the cortex, and the mice are consequently unable to cement visual learning.
"The big finding in our study," says Dr. Aton, "is that if you disrupt communication from the cortex to the thalamus during slow-wave sleep, it will completely disrupt that slow-wave rhythm and the plasticity in the visual cortex."
At the same time, the researchers noted that disrupting the thalamus-cortex feedback loop during a state of wakefulness or during other sleep states, such as in rapid eye movement sleep, had no impact on the plasticity of the visual cortex.
"But if you disrupt these oscillatory patterns during slow-wave sleep, you see a deficit. What we're thinking is you need these big waves of activity occurring in order to have that benefit of sleep [on visual memory consolidation]."
Dr. Sara Aton
To investigate the importance of the "big waves of activity" - the "burst and pause" neural patterns during slow-wave sleep - lead study author Jaclyn Durkin, who is a doctoral student in Dr. Aton's laboratory, monitored neural activity in the mice's visual cortex and lateral geniculate nucleus, which is a part of the thalamus specifically involved in relaying visual information.
Durkin monitored neural activity in these two key regions as she exposed the animals to a series of visual stimuli. "In these mice," explains Durkin, "during visual experience, we saw immediate changes in the neurons in the thalamus, but nothing going on in the visual cortex."
She adds, "These waves during subsequent sleep are apparently able to transfer information from the thalamus to the cortex, and that information reflects what that animal has just been looking at."
The next step from here, the researchers say, will be to explore what kinds of information can be transmitted to the cortex by the thalamus via this mechanism.
Another important area for investigation will be to see how visual perception and memory are impacted by sleep-dependent plasticity.