Dreaming is the most universal and most regularly occurring, as well as a perfectly natural and physiological (as opposed to pathological), altered state of consciousness. Thus, any plausible (empirical or philosophical) theory of consciousness should also describe and explain dreaming as a major state of consciousness. Most theories of consciousness, however, do not consider dreaming at all or at least do not discuss the results of dream research in any detail (Revonsuo 2006).
Dreaming presents a particularly difficult challenge for externalist, embodied, and enactive types of theories of consciousness.[1] They all anchor the existence and nature of consciousness to something in the world external to the brain, or to some kind of brain-world relations that, at least partly, reside outside the brain. By contrast, the empirical evidence from dream research shows that full-blown, complex subjective experiences similar with or identical to experiences during wakefulness (e.g., Rechtschaffen & Buchignani 1992), regularly and universally happen during rapid eye movement (REM) sleep. The conscious experiences we have during dreaming are isolated from behavioural and perceptual interactions with the environment, which refutes any theory that states that organism-environment interaction or other external relationships are constitutive of the existence of consciousness (Revonsuo 2006).
A few theories of consciousness have, however, taken dreaming as a central starting point in their conceptualization and explanation of consciousness. When dreaming is taken seriously, ideas about the nature of consciousness tend to converge on internalist theories of consciousness that take consciousness and dreaming to be varieties of the same internal phenomenon, whose main function is to simulate reality.
One of the earliest attempts to conceptualize both waking consciousness and dreaming as the expressions of the same internally-activated neural mechanism, only differently stimulated, was put forward by Llinás & Paré in 1991:
[C]onsciousness is an intrinsic property arising from the expression of existing dispositions of the brain to be active in certain ways. It is a close kin to dreaming, where sensory input by constraining the intrinsic functional states specifies, rather than informs, the brain of those properties of external reality that are important for survival. […] That consciousness is generated intrinsically is not difficult to understand when one considers the completeness of the sensory representations in our dreams. (1991, p. 531)
The argument by Llinás & Paré (1991) was mostly based on considerations of the shared neurophysiological mechanisms (in the thalamocortical system) that could act as the final common path for both dreaming and waking consciousness. Binding information together within this system intrinsically generates consciousness (“It binds, therefore I am”, Llinás 2001, p. 261); but only during wakefulness is consciousness modulated by sensory-perceptual information—in this model, wakefulness can be seen as a dream-like state (Llinás & Ribary 1994).
Although the idea that dreaming simulates waking consciousness was implicit in this neuroscientific theory, Llinás & Paré (1991) did not consider the phenomenology of dreaming and consciousness in any detail. Theoretical approaches characterizing the nature of dreaming as simulation, based on a combination of philosophical arguments and empirical facts about dreaming, started to emerge during the 1990s. In Revonsuo (1995) the idea was put forward that consciousness in general and dreaming in particular may best be characterized as a virtual reality in the brain, or a model of the world that places a (virtual) self in the centre of a (virtual) world. All experiences are virtual in the sense that they are world-models rather than the external physical world somehow directly apprehended. While the causal chains that modulate the virtual reality are different during wakefulness and dreaming, the virtual world is ontologically the same biological phenomenon: the phenomenal level of organization in the brain (Revonsuo 1995). All experiences are, according to this view, in their intrinsic phenomenal character, no different from dreams.
Metzinger (2003) took this line of thought further and analysed dreams as complex, multimodal, sequentially organized models of the world that satisfy several important constraints of consciousness. Dreams activate a global model of the world (globality), they integrate this model into a window of presence (presentationality), and this model is transparent to the experiencing subject, who takes it to be a real world and not a mere model of the world (transparency) (see also Windt & Metzinger 2007).
In Inner Presence Revonsuo (2006) presented a lengthy analysis and defence of the idea that dreams are internal virtual realities, or world-simulations, and argued that consciousness in general would be best described and explained by treating dreaming as a paradigmatic model system for consciousness. The world-simulation contains the virtual self and its sense of presence in the centre of the simulation. The virtual self is perceptually surrounded by the virtual place; the virtual place in turn contains multiple perceptual contents in the form of animate and inanimate virtual objects, including human characters. The virtual objects are bound together from phenomenal features like color, shape, and motion, but this binding in dreams does not always work coherently, thereby resulting in bizarre feature combinations and incongruous or discontinuous objects and persons in dreams (Revonsuo 2006).
Recently, Windt (2010) has formulated a definition of dreams that stems from similar basic ideas. Windt’s definition aims to capture the minimal set of phenomenological features that an experience during sleep should have in order to count as a “dream” (as opposed to other types of sleep mentation). This definition, although not explicitly applying the concept of “simulation”, is consistent with the world-simulation model of dreaming. According to Windt, dreams are Immersive Spatiotemporal Hallucinations (ISTH): there is a sense of spatial and temporal presence in dreams; there is a hallucinatory scene organized around a first-person perspective, and there is a sense of “now”, along with temporal duration. The core feature of a dream experience is, in Windt’s ISTH, the sense of immersion or presence in a spatiotemporal frame of reference. Thus, Windt’s ISTH, as well as Metzinger and Revonsuo’s earlier definitions, all involve similar ideas of dreams as involving an immersive presence of a virtual self in a virtual, spatiotemporally organized world-model or simulation.