Two chemical systems inside the brain regulate the waking and the dreaming experience: respectively, the "aminergic" and the "cholinergic" systems.
The fundamental thesis of the American psychiatrist Allan Hobson's book, supported by logical arguments and by
analysis of clinical cases, is that
the interplay of these chemical processes is responsible for consciousness.
Conscious states fluctuate continuously between waking and dreaming. Even at the extremes, both systems are active. Between the extremes there is a continuum of states which explains hypnosis, fantasy, concentration, etc.
The ultimate purpose of dreams is to help us learn. We dream hypothetical situations so that we will be prepared to face real situations of the same kind. When a waking situation occurs, it has probably already been played at least once in our dreams, and we know what to expect. By dreaming, we train our brain: dreams are mental gymnastics. It's like saying that, in order to see something, we must first create the vision of that something in our mind.
Dreaming works in a fashion similar to the immune system: dreams are created all the time for all possible situations, but only some will be useful in real life. An implication of the theory is that dreams may be universal ("impersonal necessities forced on brain by nature"). REM sleep provides a means to combine genetic instructions with experience. Sleep and dreaming are a survival strategy.
Whether sleeping or awake, the brain does pretty much the same thing. What makes difference is the neurotransmitters that travel through the brain. During waking states, the brain is controlled by aminergic neurotransmitters, made of molecules called "amines". During sleep, the brain is controlled by cholinergic neurotransmitters, made of a molecule called "acetylcholine". These two chemical systems are in dynamic equilibrium: if one retracts, the other advances. This means that our consciousness can fluctuate between two extremes, in which either of the chemical systems totally prevails (neither is ever completely absent). THis also means that the brain states of wake and sleep are only two extremes, between which there exists a continuum of aminergic-cholinergic interactions, and therefore a continuum of brain states. This system can be said to control the brain. It resides in the brain stem and from there it can easily control both the lower brain (senses and movement) and the upper brain (feelings and thought). When it doesn't work properly, when the balance of chemicals is altered, mental diseases like delirium occur. It is not surprising that diseases such as delirium are so similar to dreams: they are driven by exactly the same phenomenon. Hobson claims that the brain is in awake, dream or (non REM) sleep mode depending on whether amines are prevailing, cholines are prevailing or amines and cholines are "deadlocked". Three factors account for the brain behavior at any time: activation energy (amount of electrical activity), information source (internal or external) and chemical system (amines or cholines). When activation energy is high, the information source is external and the mode is aminergic: the brain is awake. As activation energy decreases, the external information source fades away and amines and cholines balance each other: the brain falls asleep. When activation energy is high, the information source is internal and the mode is cholinergic: the brain is dreaming. During an hallucination: activation energy is high, the information source is internal and the mode is aminergic. In a coma: activation energy is low, the information source is internal and the mode is cholinergic. The extremes are rare and usually traumatic. Normally, both external and internal sources contribute to the cognitive life, and both amines and cholines contribute to the brain state. The interplay of external and internal sources means that our perceptions are always mediated by our memory. Hobson thinks that our brains do not merely react (to stimuli), they also anticipate. The internal source tells us what to expect next, and thus helps us cope with the external source. Emotions are, in a sense, a measure of how well the internal source matches the external source: anxiety is caused by a major mismatch, whereas contentness is a sign of matching sources. When we dream, the spinal cord is paralyzed and the senses are disconnected. This is because of the cholinergic neurotransmitters that come from the brain stem. Hobson believes that sleep has the function to reinforce and reorganize memory: ultimately, to advance them from short-term memory to long-term memory. Amines are necessary for recording an experience, cholines consolidate memory. Hobson deduces that during REM sleep memory is consolidated. The aminergetic system is responsible for attention, focus, awareness. The cholinergetic system is responsible for the opposite process: focus on nothing, but scan everything. As for the content of dreams, Hobson thinks that they reflect a biological need to keep track of place, person (friend, foe or mate) and time. He draws the conclusion from considerations about what is typical (and bizarre) of dreams: disruptions in orientation. (This is the book's weakest part. All cognitive functions are about places, people and times, not only dreams...)
Hobson's weak point is the biorhythm: Hobson does not explain why we sleep eight hours and why REM occurs 4-5 times and lasts about 20 minutes and so forth. THere is a clock in the brain that knows when amines or cholines should prevail and for how long. What triggers the phase transitions from awake to asleep to dreaming? Hobson is moot about this.
Also, Hobson does not tackle consciousness at all. He simply defines mind ("all the information in the brain"), and then states that part of it is conscious and part of it is none.
Hobson briefly takes on emotions: emotion is a way of orienting in the world, and dreams contains the three basic states of emotion: fear of danger, aggressivity towards competitors and sexual arousal.