12 - The Function of Sleep

Question 1

How do you ask about the purpose for sleep?

Answer 1

It’s an evolution question, which will provide a tentative answer at best.

Often people attempt this question and then end up with contradictory results (inconsistencies).

Question 2

What are ecological circadian theories?

Answer 2

Outside world. All organisms adapted to 24 hour cycle on earth’s surface.

Question 3

Why do different animals sleep different amounts?

Answer 3

Animals have different niches, and animal sleep matches according to their niche.

Question 4

Why might a lion get a lot of sleep during the day?

Answer 4

To preserve energy for nighttime hunting, when they’re good at it AND least likely time for lion to be predated.

Lions don’t worry as much about predation during day, so sleep.

Question 5

Why might animals like sheep, giraffe etc. get short vigilant sleeps?

Answer 5

Because they need to be aware of predators

Question 6

Why is it beneficial to completely shut down consciousness during sleep?

Answer 6

Because motivations (eg. sex, food etc.) would prevent you from resting if you were conscious of them.

Butter to shut down awareness of them, than shut down survival drives.

(theoretical..)

Question 7

What is a main argument against sleep as a physiological recoverer?

Answer 7

If sleep is serving a particular essential physiological function, or many, how do you account for the fact that these actions can take less than 3 hours in horse when opossum can do it in 18 hours of sleep?

Question 8

What is an argument against sleep as an agent for learning?

Answer 8

Some animals (eg. opossum, sloths etc.) get tons of sleep, but aren’t involved in much learning/cognition. And some animals (eg. horse) get very little sleep, but perhaps have a lot more cognition/learning.

Question 9

The connections between the hippocampus and pyramidal cells are strengthened during learning (LTP). What happens when you deprive an animal of REM sleep?

What’s going on physiologically? (4)

Answer 9

Long term potentiation spikes decreases.

• REM sleep deprivation knocks down NMDA channel receptors (form of brain plasticity)

Question 10

The classic covering one cat eye plasticity experiment. How does different amounts of sleep effect this experiment?

Answer 10

• The more sleep the cats get, the greater the shift in cellular cortical organization to the open eye
• Plasticity is enhanced by sleep
• Preventing sleep prevented cortical organization and even reversed plasticity that had already occurred while cats were in light

Suggests that sleep is important for solidifying newly acquired information (eg. one eye is now important, the other is useless).

Question 11

Several parts of the brain continue to generate new neurons (eg. hippocampus), which can be acquired into new neural circuits. May be important for learning and mood regulation. How does sleep deprivation effect neurogenesis?

Answer 11

Sleep deprivation knocks down neurogenesis, suggesting that this is preventing neurogenesis mediated learning in the hippocampus.

Question 12

How do we think that REM might not be very associated with learning?

Answer 12

People who take depression treatment may have no REM sleep. There are cases of people who have not produced REM sleep for 30 years and who are very successful and have good memory.

Could be lots of other processes not controlled for outside of laboratory. Eg. some people might do EVEN better if they got REM sleep.

Eg. in rats, might see 10% difference between memory in REM/no-REM rats. This would be published saying REM is important for learning, but in reality, this doesn’t really translate to absolute performance. People can compensate in other ways.

Question 13

True or false? In humans, REM sleep is really important in emotional memory

Answer 13

TRUE

But not necessarily other types of memory..

Question 14

What is the visual texture discrimination task (VDT)? Describe sleep and improvement.

Answer 14

Rotated T or rated L appears extremely briefly as a fixation point, it is then obscured.

If you do the task correct, you then say what orientation the letter is in (eg. T vertical).

• Skill acquisition is very specific to one eye, and one visual field
• Long lasting improvement at the specific visual field that is being challenged, but no spontaneous improvement after the first 8 hours. Deterioration after around 10 hours (before improvement) if no naps/sleep is taken.
• Neural plasticity in visual cortex/visual system
• If you wait 8 hours without training, they show even more improvement. Even more improvement overnight (sleep). Deprivation of REM sleep prevents improvement.
• Robust correlation with total sleep and performance improvement
• Dramatic correlation between SWS shown during sleep at beginning of night and how much REM sleep you show late at night (increase in performance at both)

Question 15

What did the visual texture discrimination task (VDT) reveal about timing of sleep stages and learning improvement? Sleep deprivation?

Answer 15

Best improvement seen when there is high SWS early in night and high REM sleep late during sleep

Getting sleep immediately after practice is critical, and that making up lost sleep won’t really help.

Question 16

What is the effect that naps have on visual texture discrimination task (VDT)?

Answer 16

• 60 minute nap prevents deterioration in performance
• 90 minute nap increased performance without practice
• Even a 6 minute nap showed a little improvement (inconsistency that produces skeptics)

Question 17

According to Sensor et al. what is a major cause of the ‘inconsistency’ issue in lots of learning-sleep studies?

Answer 17

Subtle effects of training parameters may actually change outcomes of studies.

Question 18

Density of sleep spindles increase after ____ and ___ _____ exercises

Answer 18

Motor and verbal learning exercises

Failed to see learning after motor task?

Question 19

Can interrupt SWS and REM sleep. Do you still see declarative memory performance changes or changes in consolidation of a motor task? What is the consequence of this?

Answer 19

Surprisingly no

Improvement still related to stage 2 sleep spindles though

Sleep spindles are artifacts of memory consolidation across cortex.

Question 20

What factor of sleep correlates with IQ test performance?

Answer 20

The number of sleep spindles during the night correlates with IQ

Question 21

How can you evoke sleep spindles? What is a consequence of this?

Answer 21

By making a noise

People who demonstrate more spindles to noise, wake up less from them. Is there a connection between this and spindle-IQ correlation?

Question 22

Sleep benefits people for learning when?

Answer 22

When they’re told they’re going to be retested or when they’re expecting to be retested.

It doesn’t benefit people who don’t expect it and aren’t told that they are being retested.

Suggests that sleep may only consolidate learning, when subject needs it.

Question 23

How much pause do non-grammatic (not following pattern rules of test) trials produce after adults sleep?

Answer 23

Adults had better performance after they slept, even though they weren’t aware of the increase or even what the non-grammatical pause was.

Children show the opposite.

Question 24

How is finger sequence tapping affected by sleep in adults/children?

Answer 24

Adults: Do better in remembering sequence after sleep

Children: Don’t do better right after sleep

Theory behind this is that the deterioration allows room for correcting mistakes (fluid learning, not crystal learning).

Question 25

How is performance in birds that sleep after learning part of adult song?

Answer 25

The performance deteriorates after sleep, but they eventually have better performance in the long term than non-deteriorated counterparts.

The short term performance is better if they learn in the morning

Theory behind this is that the deterioration allows room for correcting mistakes (fluid learning, not crystal learning). This model is thought to be the same in human kids.

Question 26

If you study different neurons as a rat runs through a complex environment, different cells fire at different times. What happens when you let the rat sleep?

Answer 26

Roughly same overall pattern is seen in REM sleep

• Statistically significant
• As if sleeping brain is replaying sequence of places that its visited
• Timescale: happens faster during REM than in real life.
• Also been shown in SWS
• Shown in cortex, hippocampus and striatum
• Has been seen to go backwards (don’t know what this means)
• In older rats, the performance of remembering is correlated to mimicking of waking firing during sleep (suggests that this is stabilizing or improving waking performance).

Question 27

What happens when you get someone to play video games for 12 hours then sleep?

Answer 27

Wake up during hypnagog, stage 1 and stage 3: Subjects report seeing images/elements from game in dreams or otherwise.

Brain goes through day’s experience by repeating firing patterns from during day during sleep.

Question 28

In order to solidify a memory from the hippocampus, where must signals go?

Answer 28

The cortex.

Question 29

When is the only improvement that odour has on learning observed? Why?

Answer 29

Odour during learning and SWS improves memory acquisition (at least in word pair learning task)

• Neurons from CA1 in hippocampus project to entorhinal cortex
• Entorhinal cortex projects to association neocortex

ACh projections to both these projections inhibits them when active. Suggesting that hippocampal output through this route is important for memory learning during SWS, and this doesn’t occur during REM or waking because of the high ACh levels.

Question 30

What happens to learning when ACh neurotransmission routes (nicotinic and muscarinic) are blocked during waking? Facilitated during sleep?

Answer 30

Learning improves! Probably from disinhibition of hippocampal projection to entorhinal cortex, and that projection to association cortex.

Facilitating ACh release during SWS inhibits memory formation.

Question 31

When brain is consolidating new info, it can’t rehearse old information. How does this factor into sleep?

Answer 31

Cholinergic systems drop during SWS, allowing for rehearsal of previously acquired information and inhibition for acquisition of new information.

Question 32

Describe the alternative theory that sleep is to inhibit synaptic strengthening?

Answer 32

The real point of sleep may be to reduce synaptic connectivity to allow for filtering away of noise and allowing prominent connections to survive.

Reduces energy need and makes room for new learning the next day. Long term potentiation after sleep is much more robust than LTP after the same period of waking, suggesting that there’s more synaptic flexibility for a rat coming out of sleep, than a rat that’s been awake.

Question 33

What is meant by neural replay during sleep? In which neural structures has it been observed at the level of single neurons and how might this phenomenon relate to reported experiences of dreams after intense, sustained sensory experiences in people?

Answer 33

Cell firing patterns of hippocampal pyramidal cells during the day are similarly repeated (replayed) later during REM sleep. These ‘place’ cells (bc they fire consistently to the same environment) fire faster during replay. Thought to strenthen connections between these place cells.

Question 34

How did researchers use odour cues to assess whether activation of the hippocampus during sleep can reinforce a newly acquired explicit spatial memory in people? What did they find with respect to the sleep stage during which such reinforcement can occur?

Answer 34

People played concentration (a match up game) and gave people either an odor or no odor simultaneously. They also did it for a finger tapping class. They then gave the same odor later on in various sleep stages. Improvement was only seen in people given odor during concentration and SWS. No improvement in any other groups.

Question 35

What is meant by an ecological/circadian hypothesis that accounts for why animals sleep?

Answer 35

• Theories focusing on the role of environmental factors in determining characteristics of sleep for a species
• These emphasize the role of sleep as one aspect of the circadian cycles that organize an animal’s physiology and behaviour to suit the particular ecological niche it occupies.
• The wide diversity of sleep durations and proportions of sleep types shown by different species, as well as the relation between sleep duration and life habits are the key arguments supporting this approach

Question 36

Which striking differences in durations and types of sleep shown by different species are supportive of ecological/circadian hypothesis theories?

Answer 36

• A particularly striking example of ecological pressures shaping sleep patterns is the unihemispheric sleep and reduced or absent REM sleep that characterizes sleep in cetaceans, some amphibious species and birds
• Predators sleep longer
• Prey have short, vigilant bouts of sleep

Question 37

What are some of the arguments against ‘pure’ ecological/circadian explanations for why animals sleep as they do?

Answer 37

If one goal of sleep is to increase our safety by avoiding the inhospitable nocturnal environment, why do we lose consciousness and awareness of the external environment during this rest phase? From a safety perspective, it seems counter-productive to lose awareness of the environment and thereby increase our vulnerability to potential external threats.

A potential answer might be that it might be easier for both humans and lions to block the stimuli that evoke drives (i.e., lose conscious awareness) rather than to organize regulatory mechanisms so that they completely ignore provocative stimuli for part of each day, while maintaining a strong tendency to respond at other times.

Question 38

Does the evidence for widely differing patterns and durations of sleep among animal species strike a fatal blow against hypotheses that propose that sleep serves an essential physiological function for animals?

Answer 38

Not necessarily, evidence for differences in the way species fulfill other goals that are unarguably related to physiological needs (eg. varying water consumption)

If sleep serves a common and essential physiological function, why do some mammals sleep for 3 h and others for 20 h daily? If sleep, for example, serves the function of storing and integrating new information into our brains, how do we account for the very long sleep durations of sloths and opossums?

The evidence for diversity of sleep patterns and durations across species and the flexible adaptation of sleep to different ecological niches does not demonstrate that it serves no essential physiological need. As with food or water, there may be many different ways to satisfy a common need. In addition, one has to consider that sleep may both satisfy a hypothetical common need and serve additional functions related to each species’ ecological adaptations.

Question 39

What role in memory consolidation was proposed for REM sleep based on early research in laboratory species and in humans?

Answer 39

• One proposal is that REM sleep is involved in consolidating procedural learning (implicit learning tasks, such as motor skills) while NREM sleep is involved in consolidating declarative learning (explicit tasks, such as learning lists of words or associations)
• But human studies involving declarative learning also showed effects of REM sleep deprivation
• Others have proposed that REM sleep may be particularly important for consolidating acquisition of new skills and information, while other sleep stages may play a larger role in refining already acquired skills
• Evidence seemed to reveal that REM consolidated memories through NMDA-dependent LTP in the hippocampus

Question 40

What is long-term potentiation (LTP) and how is it studied in the hippocampus?

Answer 40

One model system that is often used to study the fundamental properties of plasticity in neural transmission in mammalian brains is long-term potentiation (LTP), particularly in the hippocampus. In this model, an electrode placed in the perforant path entering the hippocampus is used to generate a brief test stimulus that evokes a population spike, reflecting the extent of activation of hippocampal neurons. If a high-frequency burst of stimulation is provided through this pathway and the original weak stimulus is then retested, it results in a large increase in the size of the evoked population spike that lasts for many hours or even days. LTP has been studied for decades as a model of experience-induced enhancement of neural transmission and synaptic plasticity

Question 41

What arguments have been raised against REM sleep playing a significant role in memory enhancement? How valid are these arguments?

Answer 41

• People taking classes of antidepressant drugs (tricyclics, in particular) that profoundly suppress REM sleep are not reported to show deficits in learning, and may be highly successful professionals
• In addition, the timing of REM sleep windows after learning varied considerably across studies. This variation may be related to particular training parameters used in different studies, or it may indicate an unreliable phenomenon that is not closely tied to actual learning processes

Question 42

Why were kittens (and not cats) used in the plasticity/ocular dominance studies?

Answer 42

In many mammalian species, sleep durations are much longer in developing than in mature individuals, suggesting that infancy may be a particularly favourable time during which to study the role of sleep in brain plasticity.

Question 43

What are the effects of REM sleep deprivation on hippocampal LTP? What physiological changes observed after REM sleep deprivation may be responsible for these effects?

Answer 43

Reduced LTP

Perhaps from reduced NMDA synapses