week 9 - sleep and memory 2 Flashcards
What brain areas are important at different times of memory encoding and consolidation?
Encoding - hippocampus
Maturation and remote recall - Hippocampus less important. Neocortex more important
What is active systems consolidation?
- reactivation of the previous neural circuits involved in encoding of the memory
- This enables offline consolidation of memories
- Mostly based on evidence from episodic memory
- Can last days, months or years
- It involves the redistribution of memory representations away from the hippocampus and towards neocortical sites. This is for long term storage and integration with existing memories
- It implies some synapses strengthening and others weakening
- Reactivation during sleep ensures memory reactivations are not hallucinations
What are EEG signatures of non-rem sleep
Slow waves (AKA slow wave sleep)
it is abundant at the early part of the night
it shows slow neocortical oscillations
15-20hz thalamocortical spindles
100-300hz sharp wave ripples from the hippocampus
how does slow wave activity relate to learning?
association between SWA and declarative and procedural memory consolidation in the first stage of NREM sleep
Gais et al 2002
how do spindles relate to learning?
density of sleep spindles was higher after the learning task compared to the non learning task
Holtz et al
how do sharp wave ripples relate to learning?
Rats that had completed a session of learning an odour reward sensation showed an increase in the density and power of sharp wave ripples
Eschenko et al, 2008
what is the differrence between hippocampal activity during wakefulness vs. sleep?
low acetylcholine during SWS - favours replaying information within the hippocampus
High acetylcholine during wake - favours hippocampal input and encoding
This means that the excitatory cholinergic systems in the cortex modulates memory in the hippocampus
what is the evidence that replay occurs in the brain during SWS?
- mice run on a circular track during wake time
- place cell temporal firing sequences (that are active when the animal explores certain places) in hippocampus and neocortex appear during SWS
- This is associated with a sharp wave followed by a ripple (when the hippocampus reactivates the place cells, this is also when we see the sharp wave ripples)
- The replay is 5-20X fast forward
- Can be observed during quiet wake too
what is the evidence that replay occurs in the brain during SWS (from neocortical studies)
Yang et al, 2014
- animal is trained in a motor rotarod task
- animals are allowed to sleep or deprived of sleep
- throguhout the activity of neurons in motor cortex are monitored with in vivo microscopy calcium imaging
- There is a neuron in the motor cortex that is not active, then activitated during learning, and activated again in the subsequent period of NREM sleep
- During this period of reactivation, there is evidence of synaptic plasticity. So this replay in NREM sleep correlates with synaptic plasticity
What is evidence for how newly encoded memories get ‘tagged’ for sleep consolidation?
McNamara et al, 2014
object discrimination task (learning task)
Performance inmporvied in the task when optogenetics was used to enhance dopaminergic signalling
Something else about sharp wave ripples? (read the study)
What is evidence of NREM sleep activation in humans?`
ppts given memory task, accompanied with an odour
Allowed to sleep, with EEG
Tested again to see how performance has improved
Scientists presented the Odour during sleep
Presentation during NREM sleep (SWS) significantly improved performance, AND caused activity in the hippocampus (from fMRI) to increase
How are the neural oscillations temporally integrated in spatially distributed networks
There are systems-level interactions
Neocortical slow wave oscillations generate up-down states that set the temporal frame for neocortex hippocampus communication that underlies redistribution of encoded memories to long term storage sites
Thalamocortical spindles coincide with the rise phase of the slow wave. This primes the neocortex for long term storage of memory representations from hippocampal replay
Hippocampal sharp wave ripples accompany the reactivation of hippocampal neuronal ensembles. The ripples are synchornised with the spindles, the ripples go off at the bottom of the spindle
This means that the memory consolidation is temporally and spatially integrated
describe info about spindles
- arise from thalamocortical network activity
- spindle density increases after periods of intense learning
- Spindles are the sum of activity from Cortical cells, (inhibitory) TC cells and nRT cells
- This means in order to have spindles, you first need slow oscilaltions in the cortex
Why is the temporal correlation between slow oscillations in the cortex and the spindles important?
Niethas et al, 2018
- detected activity in the cortex
- also used calcium imaging to look at neurons in other layers
- there was a correlation between increased activation of cells and combination of a slow oscillation AND a spindle
read up on this study
Why is this important? It should be good for synaptic plasticity
Def need to go over this
When there i combined slow oscillation and spindle there is a suppression at synaptic level and an increase in dendrite. This allows more plasticity as it allows for more change in dendrites
What is evidence that experimental manipulation of spindle activity impacts memory retention?
Latchoumane et al, 2017
optogenic study targets gabaergic cells in the thalamus that is important for the generation of spindles
artifically increase number of spindles in the up phase of the slow oscillation, compared to the not up phase of the slow oscillation
Animals exposed to fear conditioning. Found that freezing increased only when the artifical spindles were administered in the up phase of the slow oscillation
What is evidence that sharp wave ripples are important in learning
Girardeua et al, 2009
Rats perform Y-maze navigation task
Electrodes are used to disrupt the ripples after the sharp wave in the hippocampus
No collateral change in sleep architeture
HOWEVR performance in the navigation task is decreased
How are spindles and sharp wave ripples temporally coordinated?
The slow oscillations synchronise the spindles and the sharp wave ripples
This facillitates reactivation of hippocampal memories and transfers them to the neocortedx
Up states in the slow oscillations allow plasicity at cortical synapses
These plasticity events remain supported during rem sleep (by insertation of new AMPA receptors in synapses tagged by ‘replay’)
Possible roles for REM sleep:
It is NOT associated with new spine formation
It is abundant in early life suggesting it may play a developmental role
It may participate in tagging spines for preservation or removal
It may strengthen new spines
Peever and Fuller 2017
Evidence that theta activity plays a role in consolidation during REM sleep
Using optogenetics to interfere with hippocampal theta oscillations during rem sleep disrupts fear memory
Boyce et al, 2016
This has also shown to be the case in spatial memory tasks
active systems consolidation embedded within global synaptic downscaling
what does this mean??
It also says: the contrasting synaptic homeostasis hypothesis and active systems consolidation hypothesis could explain different steps taking place during sleep
summary of sleep and memory
- there is a clear positive correlation between sleep and memory
- sleep may be required to renormalize the potential for plasticity, therefore facilitating new learning on the wake period
- sleep may allow for synaptic plasticity supporting consolidation of recently encoded memories
- replay is an essential step to transfer memories from short term hippocampal networks to long term cortical networks
- spatially distant cortical and subcortical networks display coordinated and temporally regulated activity
