REM Sleep and Dreaming

REM Sleep and Dreaming
One of the first and most important findings in the history of research on dreams and dreaming is that which relates the phenomenon of dreaming and the physiological occurrence of rapid eye movement (REM) sleep (Dement and Kleitman, 1957). While dreaming refers to “the subjective conscious experiences that we have during sleep” (Revonsuo, 2000, p.878), REM sleep is a physiologically-defined stage of sleep. It has been established that dreaming does occur during REM sleep through the collection of dream reports from subjects awoken from REM sleep, though the same is true for non-REM sleep (NREM; Hobson, 1988). Rather than being a static process, sleep contains a number of discrete states defined by various physiological measures (Rechtschaffen and Kales, 1968).
The use of electroencephalography (EEG), electro-oculography (EOG), and electromyography (EMG) has proven useful in distinguishing between arousal states during sleep, by measuring brain activity, eye movements, and muscle activity, respectively. As we sleep, our brain passes through various stages in a cyclical manner. Some of these stages are characterized by slow brain activity and other stages occur in which the electrical activity of the brain mimics the waking brain, and can even be considered hyperactivated. This specific, hyperactive stage of sleep is known as REM sleep and has three characteristics that define it: 1) The brain is more active than while in other stages and the EEG consists of alpha and beta activity, similar to waking, 2) Muscle activity is actively inhibited within the central nervous system in order to promote paralysis, and 3) Eye-movements occur during REM sleep because the muscle paralysis does not extend to the eye muscles.
A link between REM sleep and dreaming has been established through various experimental studies (Hobson, 1988). First, it is known that people awakened from REM sleep as opposed to NREM sleep are significantly more likely to produce dream reports and these reports are likely to be more detailed and vivid than NREM dream reports. Also, evidence implicating REM sleep with dreams appears when REM sleep mechanisms malfunction. Normally during REM sleep, signals that elicit all motor output (except for eye movements) are actively inhibited. Disorders that
naturally occur in humans and lesions in other species that damage the inhibitory response can result in physically acting out dreams while asleep (Sforza, Krieger, Petiau, 1997). Further, humans can give a verbal report to substantiate the correspondence of dream actions to waking actions (Ferini-Strambi and Zucconi, 2000).
Other species cannot provide information about mental processes during sleep, so controversy surrounds the question of whether or not animals are dreaming during REM sleep. One perspective is that animals such as cats, which display threat-induced posturing and appear startled by invisible objects while in REM sleep, have a reason to produce such behavior. The reason is linked to their perception of information relevant to these displays without actual corresponding sensory information. In fact, studies using electrophysiological measures to record activity in hippocampal place cells indicate that rats which have spent a considerable amount of time during the day running through a maze show activation of the same place cells during REM sleep which were active during maze running (Louie and Wilson, 2001; Wilson and McNaughton, 1994). These data point towards the possibility that dreaming serves some type of rehearsal function, allowing animals to practice the activities performed while awake, namely running through the maze.
However, we will never know if the subjective experience of dreaming is the same for these animals as it is for humans, as we will also never truly know if another person’s subjective dream experience is similar to our own. Just as behaviorists concluded the human mind was a ‘black box’ incapable of scientific study (Watson, 1913), there is a tendency to assume that we will never be able to gain an understanding of animals’ mental states and that any attempt is simply anthropomorphism. However, the neurophysiological evidence mentioned above makes plausible the claim that during REM sleep these animals are experiencing something similar to what people call dreaming, with the caveat that the dream experience will be specific to the perceptual and cognitive abilities of the animal.
While there is a strong correlation between REM sleep and dreaming, it is also clear that dreaming can occur outside of REM sleep, and similarly, instances of REM sleep without dreaming are also feasible (Hobson, 1988; Solms, 1997). An analysis of dream content suggests that there are systematic differences between REM and NREM dream reports (Hobson, Pace-Schott, Stickgold, 2000). This data indicates that just as sleep is not a static unitary process, but rather made of discrete stages, the cognitive processes that take place throughout the sleep cycle, and that are normally uniformly called dreams, differ and can result in different classes of dreams (Fosse, Stickgold, Hobson, 2004). Dreams that occur during NREM sleep lack vivid imagery and, while they may contain themes similar to REM dreams, they often consist of a simple recurring theme.
For the purpose of this paper we will concentrate on the types of dreams that are normally reported when subjects are awakened from REM sleep. From this perspective, it is possible to make a stronger inference that certain physiological mechanisms of REM sleep influence dreaming. Specifically, activation can be
examined in forebrain areas that are more likely to be informative for a cognitive theory of dreaming, and are claimed to selectively influence dreaming without affecting REM sleep (Solms, 2000). This is not to say that we are unconscious outside of REM while sleeping and that NREM dreams are not also of potential interest, rather, it is argued that the type of consciousness that mostly occurs during REM sleep is of special interest and represents a prototypical dream. Since we currently lack the technology to achieve a highly detailed understanding of the physiological correlates of dreaming, a logical starting point is to use existing technologies to acquire data during REM sleep, in order to see how they can inform a theory of dreaming.