"The picture of the dreaming brain which emerges from recent neuroscientific research may therefore be summarized as follows: the process of dreaming is initiated by an arousal stimulus. If this stimulus is sufficiently intense or persistent to activate the motivational mechanisms of the brain (or if it attracts the interest of these mechanisms for some other reason), the dream process proper begins. The functioning of the motivational systems of the brain is normally channelled toward goal-directed action but access to the motor systems is blocked during sleep. The purposive action which would be the normal outcome of motivated interest is thereby rendered impossible during sleep. As a result (and quite possibly in order to protect sleep), the process of activation assumes a regressive course. This appears to involve a two-stage process. First, the higher parts of the perceptual systems (which serve memory and abstract thinking) are activated; then the lower parts (which serve concrete imagery) are activated. As a result of this regressive process, the dreamer does not actually engage in motivated activity during sleep, but rather imagines himself to be doing so. Due to inactivation during sleep of the reflective systems in the frontal part of the limbic brain, the imagined scene is uncritically accepted, and the dreamer mistakes it for a real perception.
There is a great deal about the dreaming brain that we still do not understand. It is also evident that we have not yet discovered the neurological correlates of some crucial components of the ‘dream-work’ as Freud understood it. The function of ‘censorship’ is the most glaring example of this kind. However, we are beginning to understand something about the neurological correlates of that function, and we know at least that the structures which are most likely to be implicated (Solms 1998) are indeed highly active during dreaming sleep (Braun et al. 1997, 1998).
Hopefully it is apparent to the reader from this brief overview that the picture of the dreaming brain which has begun to emerge from the most recent neuroscientific researches is broadly compatible with the psychological theory that Freud advanced. In fact, aspects of Freud's account of the dreaming mind are so consistent with the currently available neuroscientific data that I personally think we would be well advised to use Freud's model as a guide for the next phase of our neuroscientific investigations. Unlike the research effort of the past few decades, the next stage in our search for the brain mechanisms of dreaming (if it is to succeed) must take as its starting point the new perspective we have gained on the role of REM sleep. REM sleep, which has hitherto diverted our attention away from the neuropsychological mechanisms of dreaming, should simply be added to the various ‘somatic sources’ of dreams that Freud discussed in chapters 1 and 5 of his book (Freud 1900a). The major focus of our future research efforts should then be directed towards elucidating the brain correlates of the mechanisms that Freud discussed in his chapters 6 and 7—the mechanisms of the dream-work proper: