Distinct patterns of electrical activity in the sleeping brain may influence whether we remember or forget what we learned the previous day, according to a new study by UC San Francisco researchers.
The scientists were able to influence how well rats learned a new skill by tweaking these brainwaves while animals slept, suggesting potential future applications in boosting human memory or forgetting traumatic experiences, the researchers say.
In the new study, published online October 3 in the journal Cell, a research team led by Karunesh Ganguly, MD, PhD, an associate professor of neurology and member of the UCSF Weill Institute for Neurosciences, used a technique called optogenetics to dampen specific types of brain activity in sleeping rats at will.
This allowed the researchers to determine that two distinct types of slow brain waves seen during sleep, called slow oscillations and delta waves, respectively strengthened or weakened the firing of specific brain cells involved in a newly learned skill — in this case how to operate a water spout that the rats could control with their brains via a neural implant.
“We were astonished to find that we could make learning better or worse by dampening these distinct types of brain waves during sleep,” Ganguly said.
“In particular, delta waves are a big part of sleep, but they have been less studied, and nobody had ascribed a role to them. We believe these two types of slow waves compete during sleep to determine whether new information is consolidated and stored, or else forgotten.”
“Linking a specific type of brain wave to forgetting is a new concept,” Ganguly added. “More studies have been done on strengthening of memories, fewer on forgetting, and they tend to be studied in isolation from one another. What our data indicate is that there is a constant competition between the two — it’s the balance between them that determines what we remember.”
Some Sleep to Remember, Others to Forget
Over the past two decades the centuries-old human hunch that sleep plays a role in the formation of memories has been increasingly supported by scientific studies.
Animal studies show that the same neurons involved in forming the initial memory of a new task or experience are reactivated during sleep to consolidate these memory traces in the brain.
Many scientists believe that forgetting is also an important function of sleep — perhaps as a way of uncluttering the mind by eliminating unimportant information.
Slow oscillations and delta waves are hallmarks of so-called non-REM sleep, which — in humans, at least — makes up half or more of a night’s sleep.
There is evidence that these non-REM sleep stages play a role in consolidating various kinds of memory, including the learning of motor skills.
In humans, researchers have found that time spent in the early stages of non-REM sleep is associated with better learning of a simple piano riff, for instance.
Ganguly’s team began studying the role of sleep in learning as part of their ongoing efforts to develop neural implants that would allow people with paralysis to more reliably control robotic limbs with their brain.
In early experiments in laboratory animals, he had noted that the biggest improvements in the animals’ ability to operate these brain-computer interfaces occurred when they slept between training sessions.
“We realized that we needed to understand how learning and forgetting occur during sleep to understand how to truly integrate artificial systems into the brain,” Ganguly said.
Brain Waves Compete to Determine Learning During Sleep
In the new study, a dozen rats were implanted with electrodes that monitor firing among a small group of selected neurons in their brains’ motor cortex, which is involved in conceiving and executing voluntary movements.
Producing a particular pattern of neural firing allowed the rats to control a water-dispensing tube in their cages.
In essence, the rats were performing a kind of biofeedback — each rat learned how to fire a small ensemble of neurons together in a unique new pattern in order to move the spigot and get the water.
Ganguly’s team observed the same unique new firing pattern replaying in animals’ brains as they slept.
The strength of this reactivation during sleep determined how well rats were able to control the water spout the next day.
But the researchers wanted to go further — to understand how the brain controls whether rats learn or forget while they slumber.
To manipulate the effect of brain waves during non-REM sleep, the researchers genetically modified rat neurons to express a light-sensitive optogenetic control switch, allowing the team to use lasers and fiber optics to instantaneously dampen brain activity associated with the transmission of specific brain waves.
With precise, millisecond timing of the laser, the scientists in separate experiments specifically dampened either slow oscillating waves or delta waves in a tiny patch of the brain around the new memory circuit.
Disruption of delta waves strengthened reactivation of the task-associated neural activity during sleep and was associated with better performance upon waking.
Conversely, disruption of slow oscillations resulted in poor performance upon waking. “Slow oscillations seem to be protecting new patterns of neural firing after learning, while delta waves tend to erase them and promote forgetting,” Ganguly said.
Over the past two decades the centuries-old human hunch that sleep plays a role in the formation of memories has been increasingly supported by scientific studies.
Further analysis showed that in order to protect learning, slow oscillations had to occur at the same time as a third, well-studied brain wave phenomenon, called sleep spindles.
A sleep spindle is a high-frequency, short-duration burst of activity that originates in a region called the thalamus and then propagates to other parts of the brain. They have been linked to memory consolidation, and a lack of normal sleep spindles is associated with brain maladies including schizophrenia and developmental delay, and also with aging.
“Our work shows that there is a strong drive to forget during sleep,” Ganguly said. “Very brief pairings of sleep spindles and slow oscillations can overcome delta wave-driven forgetting and preserve learning, but the balance is very delicate. Even small disturbances in these events lead to forgetting.”
It’s not yet known what tips the scales between delta wave-driven forgetting and slow oscillation-driven learning, but it’s clear that better understanding the process could have profound impacts on the study of human learning and memory, Ganguly said.
“Sleep is truly driving profound changes in the brain. Understanding these changes will be critical for brain integration of artificial interfaces and may one day allow us to modify neural circuits to aid in movement rehabilitation, such as after stroke, where previous studies have shown that sleep plays an important role in successful recovery.”
Funding: The study was funded by the Department of Veterans Affairs, the National Institutes of Health, the National Research Foundation of Korea, and the Burroughs Wellcome Fund. Ganguly designed the study with postdoctoral fellows Jaekyung Kim and Tanuj Gulati, who conducted the experiments.
Karunesh Ganguly, MD, PhD, is corresponding author for the new study. He is an associate professor of neurology at UCSF, a member of the UCSF Weill Institute for Neurosciences, and affiliated with the Neurology and Rehabilitation Service of the San Francisco Veterans Affairs Medical Center. Jaekyung Kim, PhD, a postdoctoral researcher in the Ganguly lab, was the study’s lead author. Tanuj Gulati, PhD, now at Cedars-Sinai Medical Center, was a co-author.
Subpopulation of neurons that were activated during learning, is reactivated during retrieval [1–6] and that activation of the specific neuronal ensemble is required and sufficient to retrieve that memory [7–10]. These findings indicate that memories are stored in cell ensemble activated during learning.
Acquisition of a new memory is susceptible to modification by the simultaneous and artificial activation of a specific neural ensemble corresponding to that pre-stored memory, generating synthetic or false memories [4, 11]. Retrieval of two independent memories by natural cue or optogenetic stimulation associates distinct events [12, 13].
Coincident activation of neurons results in a strengthening in synaptic efficacy such as long-term potentiation (LTP) [14, 15]. LTP at appropriate synapses are both necessary and sufficient for information storage [16, 17], and it also contributes to associative learning or memory update [18–20].
Previous gain-of-function studies using an optogenetic technique showed that manipulation of the hippocampal dentate gyrus (DG) [4, 8, 21–24] or CA1 [12] cell ensembles is important for memory reactivation and to generate synthetic or false memory by linking between stored information and sensory input by artificial activation of cell ensembles. However, gain-of-function study manipulating hippocampal CA3 cell ensembles has not been reported.
Pyramidal cells in the CA3 region of the hippocampus make synapses with each other via recurrent collaterals [25, 26]. This recurrent excitatory circuit has a key role in retrieving whole pattern from degraded cue, a process called pattern completion [27–29].
The hippocampal CA3 is reported to have an important role in the associative learning [30, 31], and this recurrent network is thought to form the associative memory within one brain region [30]. However, there is no experimental evidence demonstrating that CA3 region is important for the incorporation of previously stored information within one brain region to generate associative memories.
We hypothesized that the coincident firing of cell ensembles of the hippocampal CA3, which have recurrent network within one brain region, integrates distinct events. Also, we tested whether the synchronous activation of CA3 induces LTP in CA3-CA3 synapses. Here we showed that the synchronous activation of ensembles in CA3 associates distinct events. Also, in vivo electrophysiological recording showed that 20-Hz optical stimulation of ChR2-mCherry expressing CA3 neurons, which is the same stimulation protocol used in behavioral experiment, induces LTP in CA3-CA3 synapses.
This results of electrophysiology potentially suggest that the artificial association of memory events might be induced by the strengthening of synaptic efficacy between CA3 ensembles via recurrent circuit.
Previous gain-of-function studies using an optogenetic technique showed that manipulation of the hippocampal dentate gyrus or CA1 cell ensembles is important for memory reactivation and to generate synthetic or false memory. However, gain-of-function study manipulating CA3 cell ensembles has not been reported. The CA3 area of the hippocampus comprises a recurrent excitatory circuit, which is thought to be important for the generation of associations among the stored information within one brain region. We investigated whether the coincident firing of cell ensembles in one brain region, hippocampal CA3, associates distinct events. CA3 cell ensembles responding to context exploration and during contextual fear conditioning were labeled with channelrhodopsin-2 (ChR2)-mCherry.
The synchronous activation of these ensembles induced freezing behavior in mice in a neutral context, in which a foot shock had never been delivered. The recall of this artificial associative fear memory was context specific. In vivo electrophysiological recordings showed that 20-Hz optical stimulation of ChR2-mCherry-expressing CA3 neurons, which is the same stimulation protocol used in behavioral experiment, induced long-term potentiation at CA3-CA3 synapses.
Altogether, these results demonstrate that the synchronous activation of ensembles in one brain region, CA3 of the hippocampus, is sufficient for the association of distinct events. The results of our electrophysiology potentially suggest that this artificial association of memory events might be induced by the strengthening of synaptic efficacy between CA3 ensembles via recurrent circuit.
Electronic supplementary material
The online version of this article (10.1186/s13041-018-0424-1) contains supplementary material, which is available to authorized users.
Introduction
Subpopulation of neurons that were activated during learning, is reactivated during retrieval [1–6] and that activation of the specific neuronal ensemble is required and sufficient to retrieve that memory [7–10]. These findings indicate that memories are stored in cell ensemble activated during learning.
Acquisition of a new memory is susceptible to modification by the simultaneous and artificial activation of a specific neural ensemble corresponding to that pre-stored memory, generating synthetic or false memories [4, 11]. Retrieval of two independent memories by natural cue or optogenetic stimulation associates distinct events [12, 13].
Coincident activation of neurons results in a strengthening in synaptic efficacy such as long-term potentiation (LTP) [14, 15]. LTP at appropriate synapses are both necessary and sufficient for information storage [16, 17], and it also contributes to associative learning or memory update [18–20].
Previous gain-of-function studies using an optogenetic technique showed that manipulation of the hippocampal dentate gyrus (DG) [4, 8, 21–24] or CA1 [12] cell ensembles is important for memory reactivation and to generate synthetic or false memory by linking between stored information and sensory input by artificial activation of cell ensembles. However, gain-of-function study manipulating hippocampal CA3 cell ensembles has not been reported.
Pyramidal cells in the CA3 region of the hippocampus make synapses with each other via recurrent collaterals [25, 26]. This recurrent excitatory circuit has a key role in retrieving whole pattern from degraded cue, a process called pattern completion [27–29]. The hippocampal CA3 is reported to have an important role in the associative learning [30, 31], and this recurrent network is thought to form the associative memory within one brain region [30]. However, there is no experimental evidence demonstrating that CA3 region is important for the incorporation of previously stored information within one brain region to generate associative memories.
We hypothesized that the coincident firing of cell ensembles of the hippocampal CA3, which have recurrent network within one brain region, integrates distinct events. Also, we tested whether the synchronous activation of CA3 induces LTP in CA3-CA3 synapses. Here we showed that the synchronous activation of ensembles in CA3 associates distinct events. Also, in vivo electrophysiological recording showed that 20-Hz optical stimulation of ChR2-mCherry expressing CA3 neurons, which is the same stimulation protocol used in behavioral experiment, induces LTP in CA3-CA3 synapses. This results of electrophysiology potentially suggest that the artificial association of memory events might be induced by the strengthening of synaptic efficacy between CA3 ensembles via recurrent circuit.
Source:
UCSF
Media Contacts:
Nicholas Weiler – UCSF
Image Source:
The image is in the public domain.
Original Research: Closed access
“Competing Roles of Slow Oscillations and Delta Waves in Memory Consolidation versus Forgetting”. Jaekyung Kim, Tanuj Gulati, Karunesh Ganguly.
Cell doi:10.1016/j.cell.2019.08.040.
