PSY 340 Brain and Behavior
Class 24: Why Sleep? Why REM? Why Dreams? (OUTLINE)
A. Functions of Sleep
Why do we sleep? Short answer: we do not know (though we are getting closer)
- Note, too, that any theory of sleep in humans needs to consider why other animals sleep!
1. Sleep and Energy Conservation
Analogous to Sleep: Hibernation = body temp to just above freezing, brain activity almost nothing
- Small mammals
- Bears hibernate with body temperature only somewhat lower than normal (8-12º lower)
- Helps retard aging process & protects against infection & trauma.
Alternative sleep strategies in animals
2. Sleep and Memory
> long-term memories are strengthened (consolidation).
- Slow-wave sleep (SWS): Hippocampus-dependent episodic & spatial memory consolidation; verbal learning (e.g., memorizing lists of words)
- REM: Procedural (motor skills) and emotional memorySome imaging studies suggest a replaying of learning sequences from the daytime and "weeding out" or pruning
less important memory connections in brainSleeping after learning something usually results in memory gain upon awakening
Sleep may promote the development of new dendritic spines (new synapses) (see figures above)
Stage N2 sleep spindles (12-14 Hz): communication between thalamus & cerebral cortex associated with new learning. r = 0.7 with nonverbal IQ (i.e., explains 49% of variation!)
3. Sleep and Toxic Waste Removal (New, not in book)
- The brain has a system which channels cerebrospinal fluid (CSF) from tiny arteries through the actual brain tissue to veins which then remove the CSF. This has been called the glymphatic system.
- Research in 2013 found that, during sleep, the astrocytes in the brain shrink and create 60% more space through which the CSF can flow. Indeed the flow is about 10 times greater during sleep than the awake state. This permits the clearance of metabolic waste products such as amyloid-β, the waste material which accumulates in Alzheimer's disease.
B. Functions of REM Slee;p
- Longer sleep and higher %age of REM
- the human baby (REM: at birth, 50% of 22 hrs/day of sleep);
- at age 1 (30% of 17 hours/day of sleep)
- human adult (15-20% of 7-8 hours/day of sleep)
- REM may help in
- memory storage (as noted above)
- stimulating the developing nervous system of the infant & young child
- REM sleep deprivation leads to increased attempts at REM sleep subsequently
C. Biological Perspectives on Dreaming
- Complex dreaming develops as we get older (Domhoff, 2001; not in book)
- dreams of children < 5 yo are bland and static
- children < 10 yo report dreams only in about 20-30% of REM sleep
- adolescent/adult dreams are more dynamic & complex & dreams are reported in > 80% of REM sleep
- REM dreaming is different than NREM dreaming (not in book; Kandel et al., 2013)
- REM dreams: long, highly visual, often emotional, not often related to recent life experiences
- NREM dreams: short, non-visual, "conceptual" or thought-like, often related to recent life experiences
There is no single theory of dreaming which is now currently accepted by most psychologists.
Consider these five theories of what dreaming is about
Freud's Psychoanalytic Theory (1899/1900)
- Dreams as wish fulfillment
- Unconscious process constructs wish
- Censorship process distorts the wish via symbols et al.
- Each dream has two aspects
- Manifest content = what is remembered
- Latent content = underlying meaning of dream
- Dreams are interpreted within psychoanalytic therapy
J. Allan Hobson Initial theory (1997)
- Activation = during REM sleep, different brain areas, particularly the limbic system, become active under stimulation from the brain stem. These areas involve circuits related to emotion, memories, and sensations.
- Synthesis = the brain creates a story to make sense of the activity
J. Allan Hobson more recent theory (2000s)
- Brain activation in sleep allows the development & maintenance of circuits necessary for higher brain functions including consciousness
- In dreaming, we are getting ready to behave
- Dreams have more in common than not across individuals
- REM sleep = protoconscious state = provides a virtual reality model of the world that is of functional use to the development & maintenance of waking consciousnes
Neurocognitive Theory of Dreaming
- Underlying neural substrate for dreaming = Default Mode Network (DMN)
- Just as the DMN develops, so too do dreams develop as cognitive abilities mature, e,g., narrative or story abilities
- Adolescent/adult dreams reflect many of the real-life aspects of individuals, e.g., social networks or life settings
- Derived from studies of brain damaged patients
- Dreaming is a kind of quasi-hallucinatory or unusual form of thinking
- Stimulation (either internal or external) activates parietal, occipital, & temporal cortex.
- No visual information contradicts the stimulation
- Lowered prefrontal cortical activity leads to lowered censorship of stimulation (dreams can violate logic or contain contradictory materials)