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In his 1895 “Project for a Scientific Psychology,” Sigmund Freud clearly stated his goal: to integrate the workings of the mind with the workings of the brain. But in his day, too little neurobiology was known to make his goal attainable and he tried instead to understand such fascinating normal phenomena as dreaming in exclusively psychodynamic terms. A century later, Freud's brilliant but entirely speculative dream theory is in need of radical revision, if not complete overhaul, because dreams, as well as other unusual states of consciousness, can finally be approached from the solid foundation of modern neuroscience. In other words, the goals of Freud's Project are at last within our grasp. Ironically, an obstacle to progress is the tenacious adherence of orthodox psychoanalysis to a theory that even by the standards of its originator, is now clearly outmoded.
In this paper, recent positron emission tomography (PET) imaging and brain lesion studies in humans are integrated with new basic research findings at the cellular level in animals to explain how the formal cognitive features of dreaming may be the combined product of a shift in the neuromodulatory balance of the brain and a related redistribution of regional blood flow. The human PET data indicate a preferential activation in rapid eye movement (REM) sleep of the pontomesencephalic brainstem and of limbic and paralimbic cortical structures involved in the emotional and mnemonic aspects of cognition. The pontine brainstem mechanisms controlling the neuromodulatory balance of the brain in rats and cats include noradrenergic and serotonergic influences which enhance waking and impede REM via anticholinergic mechanisms, and cholinergic mechanisms which are essential to REM sleep and only come into full play when the serotonergic and noradrenergic systems are inhibited. In REM, the brain thus becomes activated but processes its internally generated data in a manner quite different from that of waking.