Activation-synthesis hypothesis

With the advancement of brain imaging technology, the sleep-waking cycle can be studied as never before. The brain can be objectively quantified and identified as being in either one of three states: awake, REM sleep, and NREM sleep due to these advanced methods of measurement. It has been shown that global deactivation of the brain from waking state to NREM sleep occurs, and a subsequent reactivation during REM sleep, to a degree greater than during waking. Consciousness and its substates, primary consciousness and secondary consciousness, play a part in identifying the state of the brain. Primary consciousness is the simple awareness of perception and emotion; that is, the awareness of the world via advanced visual and motor coordination information your brain receives. Secondary consciousness is an advanced state that includes both primary consciousness and abstract analysis, or thinking, and metacognitive components, or the awareness of being aware. Most animals show some stages of primary consciousness, but only humans have been experimentally shown to experience secondary consciousness. The cycle of waking-NREM-REM sleep is essential to mental health of mammals. It has been shown through experimentation that animals subjected to inability to enter REM sleep show an immediate attempt to quickly enter REM stages and long-term effects on motor coordination and habitual motor habits, eventually leading to the death of the animal. It has also been shown that homeothermic animals might require sleep to maintain body weight and temperature.

The waking consciousness is the awareness of the world, our bodies, and ourselves. This includes humans experiencing the awareness of being aware of ourselves, an intrinsic ability to humans. It's the ability to look in a mirror and know that you are looking at yourself, and not just another human being. Wakefulness allows the distinction between tasks and default brain states, and also distinguishes between background and foreground processing. Being awake allows the person to not only be aware of themselves and the world, but also to have conscious motor coordination and understand the difference between need and want that comes from secondary consciousness.

There is a difference between being just asleep and in a state of mind called dreaming. Sleeping can be described as the lack of conscious awareness of the outside world, meaning large portions of the brain that receive and interpret signals are deactivated during this time, while dreaming is a specific state of sleep in which enhanced brain activity has been shown to occur, theorizing the primary consciousness could be active during dreaming. Indeed, during dreams we are consciously aware of our surroundings, and assuredly have a certain perception and emotion throughout the course of the dream, suggesting that at least part of the primary consciousness is activated during the dream.

A dream has all features of primary consciousness but is produced in the brain without external stimulation. Unlike the waking state, the brain cannot recognize its own condition; that it is in the midst of the dream and is not the same as the real world. The brain has a single-minded state of primary consciousness during dreaming, which allows the brain to reach greater perception and awareness of a single scenario out of images and dreams. This is called the dream consciousness.

The four sleep stages have been identified as follows: sleep onset stage I, late-night stage II, and deep sleep stages III and IV. Deep sleep stages III and IV all occur during the first half of the night, while lighter stages I and II occur during the later half. During standard sleep laboratory measurements, the states of sleep and waking have behavioral, polygraphic, and psychological manifestation within the pontine brainstem. These states are regulated by a reciprocal relationship between two types of neuronal cells, aminergic inhibitory cells such as serotonin and norepinephrine and cholinergic excitatory cells such as acetylcholine. Changes in the sleep stages occur when the activity curves of these neurons cross. REM sleep stage I is a state of sleep just above and most closely linked to sleep onset stage I.

NREM sleep can be described as the stages of sleep that show greatly decreased brain activity. There are four different stages of NREM sleep. The brain shows dulled or limited senses of perception, though the thought process has been shown to be logical and perseverative. Episodic movements of the body occur during these stages, though they are involuntary movements.

REM sleep may be a more evolutionarily recent sleep state, and is prominent in most birds and mammals, although may exist in reptiles and other vertebrates to varying degrees. REM stands for rapid eye movement. It is generally a later sleep state following non-REM (NREM) sleep. It is regulated in part by the pontine brainstem. Infants spend most of their time in REM sleep, and rather than enter stage 1 sleep they may go directly to REM sleep. Most REM sleep occurs just above stage I of sleep, and experiences different mental abilities than during NREM sleep. …

The development of consciousness is a gradual, time-consuming and lifelong process that builds upon and uses a more primitive virtual reality generator that is more definable in our dreams. As such, the development of secondary consciousness during the lifetime requires a blank consciousness that during REM sleep creates an imaginary self that has movements and experiences emotions. This is an experimental state not associated with awareness, and this state, or protoconscious, is able to be reached during childhood. This protoconsciousness is a protoself created early in life by the brain as a building block for consciousness to develop, and provides intrinsic predictions of external inputs created by dreaming.

Hobson and McCarley originally proposed in the 1970s that the differences in the waking-NREM-REM sleep cycle was the result of interactions between aminergic REM-off cells and cholinergic REM-on cells. This was perceived as the activation-synthesis model, stating that brain activation during REM sleep results in synthesis of dream creation. Hobson's five cardinal characteristics include: intense emotions, illogical content, apparent sensory impressions, uncritical acceptance of dream events, and difficulty in being remembered.

Thanks to the development of technology since the original proposal, new experimental data has been collected and additional mechanistic details of neuronal control have been developed. It has been determined that consciousness states can be described with three values, and the AIM model is a model that uses these values for representing the similarities and differences between waking and dreaming. It is a three-dimensional state-space model that describes different states of the brain and their variance throughout the day and night. It is composed of three different values: A – activation, I – input-output gating, and M – modulation. The model is limited however, in that it cannot yet explain the regional differences in brain activity that distinguish REM sleep from waking. Other limitations include the inability to quantifiably identify and measure M in humans. …

The three-dimensional AIM model shows that during the cycle of brain states waking-NREM-REM, the brain is dynamically changing constantly, and that this state space described by the AIM has an infinite number of subregions other than the main three. It proposes that via a protoconsciousness brain activation during sleep is necessary for the development and maintenance of waking consciousness and other higher-order brain functions such as problem solving. It suggests the possibility that the state of waking consciousness is only present in humans due to the evolution of extensive cortical structures within the brain. Dreaming is a state of the brain that is similar to yet different from the waking consciousness, and interaction and correlation between the two is necessary for optimal performance from both. One study conducted measuring brain activity via EEG used Hobson's AIM model to show that quantitatively dream consciousness is remarkably similar to waking consciousness.