Activation-Synthesis Hypothesis
Category: Sleep Science
How the Theory Emerged and What It Claims
In 1977, the Harvard psychiatrist J. Allan Hobson and the neuroscientist Robert McCarley published a hypothesis about how dreams are generated. Against Freud's then-influential view that dreams are the fulfillment of unconscious wishes, the two set out to explain dreaming as a neurophysiological process.
The core of the hypothesis is a two-stage process. In the first stage, activation, giant cells in the pons of the brainstem fire spontaneously during REM sleep and send random neural signals upward. In the second stage, synthesis, the forebrain gives meaning to that random input and assembles it into a coherent narrative. The strangeness and illogic of dreams are explained as the product of the brain's effort to turn signals that carry no message of their own into a story that holds together.
The Neurological Mechanism in Detail
During REM sleep the brain undergoes a characteristic neurochemical shift. The noradrenergic and serotonergic systems that are busy during waking are suppressed, while the cholinergic system takes over. This state of aminergic suppression and cholinergic dominance is thought to produce the peculiar qualities of dreams. The prefrontal cortex, which handles logical checking and self-criticism, stays down-regulated, so limbic activation comes to the fore and distortions of time and space and amplified emotion arise easily. Even if synthesis is the work of the forebrain as a whole, the part that would proofread the result remains asleep, which makes the roughness of dream plots easier to understand.
PGO waves (ponto-geniculo-occipital waves) travel from the brainstem to the visual cortex and are held to take part in generating the visual imagery of dreams. As these waves stimulate the visual cortex, vivid visual experience arises with no input from outside. The rapid eye movements themselves are linked to PGO waves, and some hold that the direction one is looking within a dream sometimes matches the direction of the eye movements. That correspondence, however, remains unsettled among researchers.
Criticism and the Later Development of the Theory
The activation-synthesis hypothesis provoked fierce debate from the moment it appeared. A central criticism is that dream content is not entirely random. It is pointed out that dreams reflect a person's preoccupations, emotional problems and daytime experiences (day residue), which the synthesis of purely random signals cannot fully account for. The fact that dreams are also reported from NREM sleep points to the narrow reach of a theory confined to REM.
In response to such criticism, Hobson himself developed the theory further. A 2000 review in Behavioral and Brain Sciences set it out as the AIM model (Activation-Input-Modulation), describing states of consciousness in a three-dimensional space: level of brain activation (A), source of information input (I: external or internal) and the ratio of neuromodulators (M: aminergic or cholinergic). The idea behind the model goes back to the early 1990s. Within this framework, dreaming is located at high activation, internal input and cholinergic dominance.
How It Relates to Dream Interpretation
At first glance the activation-synthesis hypothesis looks opposed to dream interpretation, which searches dreams for meaning. Yet once you focus on the synthesis stage, the two are not necessarily in conflict. When the brain works random signals into a narrative, it draws on personal memories, emotions and concerns as raw material. The raw material of a dream may be random, but personal psychology is deeply involved in the processing.
Read this way, dream interpretation becomes an inquiry into which materials the brain selected during synthesis and how it combined them. Why did the brain reach for those particular memories out of countless others, and why did it feel that this combination meant something? Such questions leave room to explore the psychological meaning of dreams even inside a neuroscientific framework. Recording dreams immediately on waking and watching for patterns in the material that gets chosen can be a useful clue to understanding yourself.
Was this article helpful?