Hockenbury Discovering Psychology 5th txtbk

148 CHAPTER 4 Consciousness and Its VariationsFOCUS ON NEUROSCIENCEThe Dreaming Brain: Turning REM On and OffNeuroscientists are making sense out of the fact that our dreamsoften don’t make sense. Compared to the awake brain, or eventhe brain in NREM sleep, the dreaming brain undergoes distinctchanges. About every 90 minutes, key brain areas are ramped up,scaled back, or blocked from sending or receiving input duringREM sleep. Levels of certain neurotransmitters surge while otherneurotransmitters recede. As the internal dynamics of the brainfluctuate, they are reflected in the psychological and emotionalaspects of the dream itself (Fuller & others, 2006; Nofzinger,2006; Pace-Schott, 2005).REM-off and REM-on NeuronsEarlier on page 145, Figure 4.3 depicted the 90-minute cycles ofNREM and REM sleep over a typical night. Although the figuregives the impression of distinct shifts from one NREM stage toanother, and from NREM to REM sleep, it’s not quite like shiftinggears. Instead, each 90-minute cycle of NREM and REM sleep reflectsthe gradually changing balance of REM-off and REM-onneuronal activity.REM-off neurons produce the neurotransmitters norepinephrineand serotonin, which suppress REM sleep. In contrast, REM-onneurons produce the neurotransmitter acetylcholine, which promotesREM sleep. When the activity of REM-on neurons and acetylcholinelevels reach a certain threshold, the characteristic signs ofREM sleep emerge—increased brain activity, rapid eye movements,and suppressed voluntary muscle movements. As acetylcholine levelscontinue to rise, these REM-related characteristics intensify.When acetylcholine and REM activity peaks, REM-off neuronalactivity picks up, increasing serotonin and norepinephrine levels.Eventually, REM sleep is suppressed and the characteristic featuresof slow-wave NREM sleep reemerge—reduced brain and physiologicalactivity, movement capabilities, and the vague, ruminatingthoughts of sleep thinking (McCarley, 2007).Adjusting the Brain for REM SleepPET scan and other neuroimaging studies have revealed how thebrain’s activity during REM sleep is distinctly different from its(a) REM sleep comparedto wakefulness(b) REM sleep comparedto slow-wave sleepactivity as compared to wakefulness (PET scan a) and NREMslow-wave sleep (PET scan b). To help orient you, the top of eachPET scan corresponds to the front of the brain and the bottomto the back of the brain. The PET scans are color-coded: Bluishpurpleindicates areas of decreased brain activity and yellow-redindicates areas of increased brain activity.Compared to wakefulness, PET scan (a) reveals that REM sleepinvolves decreased activity in the frontal lobes, which are involvedin rational thinking. Also decreased is the activity of the primaryvisual cortex, which normally processes external visual stimuli. Ineffect, the dreamer is cut off from the reality-testing functions ofthe frontal lobes—a fact that no doubt contributes to the weirdnessof some dreams. The yellow-red areas in PET scan (a) indicateincreased activity in association areas of the visual cortex. This brainactivation gives rise to the visual images occurring in a dream.Compared to slow-wave sleep, the yellow-red areas in PETscan (b) indicate that REM sleep is characterized by a sharpincrease in limbic system brain areas associated with emotion,motivation, and memory. The activation of limbic system brainareas reflects the dream’s emotional qualities, which can sometimesbe intense (Braun & others, 1998; Nofzinger, 2005b).Sleep and Memory Formation: Let Me Sleep on It!Sleep plays a critical role in strengthening new memories and in integrating newmemories with existing memories (Ellenbogen & others, 2007; Walker, 2005).Interestingly, different sleep states and stages seem to contribute to forming differentkinds of memories. For example, research suggests that NREM slow-wavesleep contributes to forming new episodic memories, which are memories of personallyexperienced events (Rasch & Born, 2008). In contrast, REM sleep andNREM stage 2 sleep seem to help consolidate new procedural memories,which involve learning a new skill or task until it can be performed automatically(Stickgold & Walker, 2007; Walker & Stickgold, 2006).Sleep researchers are finding that the strengthening and enhancement of newmemories during sleep is a very active process. That active process seems to worklike this: New memories formed during the day are reactivated during the90-minute cycles of sleep that occur throughout the night. This process of repeatedlyreactivating these newly encoded memories during sleep strengthens the neuronalconnections that contribute to forming long-term memories. But along with helpingsolidify new memories, sleep is also critical to integrating the new memories into existingnetworks of memories (Rasch & Born, 2008; Stickgold & Walker, 2007).