Table 1.

Contemporary theories of dreaming

	Psychodynamic (Freud, Solms)	Activation-Input-Modulation [AIM] Model (Hobson)	Neurocognitive (Foulkes, Domhoff)
General	Dreams represent fulfillment of unconscious wishes related to egoistic (often infantile sexual) impulses [5]. Latent unconscious content is disguised via censorship creating the bizarre manifest dream content [5]. More recently, the drive for dreaming has been associated with dopaminergic systems and “appetitive interests” [132].	Our conscious state is determined by three factors: (a) Activation - total and regional brain activity levels, (b) Input - activation generated internally or externally, (c) Modulation - the ratio of aminergic to cholinergic neuromodulators. REM sleep and dreaming are characterized by high levels of activation, internal input, and cholinergic modulation [11].	Dreaming is what occurs when the mature brain is adequately activated, disconnected from external stimuli and without self-reflection. Once instigated, dreaming actively draws on memory schemas, general knowledge, and episodic information to produce simulations of the world [13, 14].
Dream amnesia	Since unconscious wishes are noxious to our consciousness, they are actively repressed via censorship processes [5]. Dream amnesia is anything but arbitrary: “our memory reproduces the dream not only incompletely but also untruthfully, in a falsifying manner”[5].	Dream amnesia largely stems from a state-change. Aminergic de-modulation and deactivation of dorsolateral prefrontal cortex in REM sleep create a brain state which is not favorable for subsequent memory [11]. This also explains why we forget moments of brief awakenings during sleep.	Dream amnesia is primarily related to a cognitive state and lack of context. To remember, we need an external narrative to which internal events can be tied [14, 21]. Dream amnesia cannot be explained by a state- change since dreaming can occur at any state (NREM sleep and wake).
Signal propagation in dreaming	“Top-down”: dreams originate from psychic motives which are later instantiated as sensory percepts: “a thought... is objectified in the dream, and represented as a scene”[5].	“Bottom-up”: dreams originate from activation of sensory cortex by the brainstem (e.g. PGO waves), later to be interpreted and synthesized by mnemonic and high-order modules [11, 47].	“Top-down”: Dreams originate in abstract knowledge and figurative thought which are processed back into “imaginal copies” of perceptual phenomena [14].
Is REM sleep a good model for dreaming?	No. REM sleep and dreaming can occur one without the other [23, 154], for example in neurological patients. Dream- like experiences are related to forebrain mechanisms rather than to REM sleep generators in brainstem [22, 23].	Yes. Because REM sleep provides the most favorable brain conditions for dreaming, we can focus on its neurophysiology in our attempt to model the neuronal basis of dreaming [4, 47].	No. Dream-like experiences can occur also in NREM sleep, sleep onset, and wakefulness [13, 155]. Children studies show that REM sleep may be an important condition for dreaming but not sufficient [13, 21].
Is dreaming largely similar to waking consciousness?	No. The apparent (manifest) aspect of dreams is bizarre and includes nonsensical changes in time and place, as well as incongruities of plot, character, and action [5]. This is because the true (latent) dream content is disguised by the censor [5]. Dreaming may be closely akin to mental illness [5, 156].	No. Dreaming is altogether comparable to delirium (acute confusional state) that can occur upon alcohol withdrawal [3]; REM sleep shares its physiological substrate with psycho-pathological conditions such as schizophrenia (limbic hyper-activation and frontal hypo-activation) [157, 158].	Yes. Dreams are “a remarkably faithful replica of waking life” [159]. Dreams are largely coherent and internally plausible narrative sequences rather than the stereotypical illogical sequences of bizarre images. Content analysis indicate a strong continuity between dream content and waking life[13]. Evidence linking dreams to psychosis is limited[155]: REM sleep deprivation does not alter schizophrenic pathology, aminergic agonists suppress REM sleep with no psychopathological effects.
Neurochemistry of dreaming	Dreaming is driven by the ‘wanting’ dopaminergic system: evidence from prefrontal leucotomies & effects of l-DOPA on dreaming [23, 156].	Primarily a cholinergic role for REM sleep and dreaming [4, 11]. Administration of cholinergic agonists (e.g. pilocarpine) can induce an artificial REM sleep period associated with dream reports [160].	Dreaming is unlikely to be driven by a specific chemical or brain region. It is most likely related to a complex neurochemical mixture where serotonin, norepinephrine, and histamine are absent while both acetylcholine and dopamine are present [13, 155].
The function of dreaming	According to Freud, dreams preserve sleep in the face of unconscious needs for excitement [5]. More recently (Solms): “the biological function of dreaming remains unknown” [23].	Dreams may serve a creative function by providing a virtual reality model (protoconsciousness). The brain is preparing itself for integrative functions including learning and secondary consciousness [4].	Dreams probably have no function, but they do have coherence and meaning, which is often conflated with function[13]: “dreaming is a spandrel of the mind, a by- product of the evolution of sleep and consciousness.” [13]
What is the psychological meaning of dreams?	This theory emphasizes dream content: individual dreams carry meaningful information about the dreamer. This theory lacks in power with regards to explaining dreams shared by all people [5] (e.g. flying, teeth falling).	Dreaming is an attempt to best interpret activating signals in a coherent manner, and contents of individual dreams are nearly random. Nevertheless, the process of interpretation may carry some psychological meaning[11].	This theory emphasizes dream form: dreams are based on stored memory representations and therefore reflect individual ways of abstracting knowledge, but specific dreams are not traceable back to particular episodes in our life[14].
Are dreams directly related to previous experience?	Yes. Dream content is related to daytime experience (“Day’s residue”) that triggers the emergence of related memories. “All the material making up the content of a dream is in some way derived from experience” [5]	No. Dream content is largely unrelated to the preceding day’s experiences[45] and in general does not accurately represent episodic memories which are available during wakefulness[42, 45].	No. Familiar settings and people are sometimes incorporated into dreams but dreams are not a recollection of everyday life [14].