The Brain’s Continuum
Dreaming, as my theory suggests, occupies one end of a continuum of mental functioning, at the opposite end of which lies focused waking thought. Biological research suggests that the underlying activity of the brain occurs on a similar continuum.
Two separate bodies of research are relevant. First, modern brain imaging techniques have revealed much about “rapid eye movement” (REM) sleep, a state that facilitates dreaming. For example, the dorso-lateral prefrontal cortex (DLPFC), a brain area crucial for focused thought and planning, is relatively inactive in REM sleep. This is consistent with the relatively poor focused thought and planning in dreams. And, equally clearly, areas such as the amygdala that are associated with strong emotional reactions are highly active in REM sleep. This is consistent with the critical role of emotion in dreams.
The second body of research centers on hundreds of patients with brain damage. These studies show that lesions in the large DLPFC areas have very little effect on dreaming, exactly as the imaging results above would indicate. The studies also mention several brain areas where the damage stops dreaming altogether. However, such areas do not correspond directly to those that the imaging techniques found were most active in REM sleep.
My own tentative explanation for this is that looking at the effects of damage to one brain area or another simply cannot tell us the whole story. Why? Because, I believe, the most essential biological elements of dreaming occur at a deeper level, involving altered patterns of connectivity among neurons. While the patterns are not easy to study directly, since several billion neurons are implicated, studies of chemical changes in the dreaming brain offer some useful hints about what might be happening.
Consider norepinephrine, a chemical that tends to narrow down connectivity among neurons so that only certain brain areas are activated. REM sleep is marked by low levels of norepinephrine—the perfect setup for wide-ranging activation of brain areas. And such broad activation might well be responsible for the tendency of dreams to bring together seemingly unrelated material—associating, say, one’s friend with a wolf. I suspect we will find that activation of brain areas becomes more and more wide-ranging, and connections among neurons more and more disparate, as we move along the continuum from focused thought to dreaming.