Plan: Dreams and philosophical realism

Brain activity

One of the central questions of sleep research is what part of the brain is driving dreams' video-auditory experience. During waking, most of the mind's internal imagery is controlled from the front of the brain by the lateral prefrontal cortex (LPFC). Reasoning, planning, and strategizing are the results of the constructive imagination conducted by the LPFC, which acts like a puppeteer assembling objects stored in memory into novel combinations. During REM sleep, however the LPFC is inactive.^[51][52] Furthermore, in people whose LPFC is damaged, dreams do not change at all, confirming that LPFC does not control dreaming.^[53]

During deep slow-wave sleep, imagery is primarily driven by the hippocampus in the process of long-term memory consolidation and predominantly includes memories of events “as they happened” without the random novel combination of objects seen in REM sleep dreams. During REM sleep, however, the communication between the neocortex and the hippocampus is disrupted by a high ACh level.^[54]

Without being driven by the LPFC (as in waking) and hippocampus (as in slow-wave sleep), it is unclear how exactly images appear in one's perception during REM sleep. A common explanation is that neuronal ensembles in the posterior cortical hot zone, primed by previous activity or current sensory or subcortical stimulation, activate spontaneously, triggered by the ponto-geniculo-occipital (PGO) waves that characterize REM sleep.^[55] In the words of Hobson & McCarley, the neocortex is making “the best of a bad job in producing even partially coherent dream imagery from the relatively noisy signals sent up from the brain stem.”^[56]

It is also commonly accepted that the intensity of dreams during REM sleep can be dialed up or down by the dopaminergic cells of the ventral tegmental area. For example, drugs that block dopaminergic activity (e.g., haloperidol) inhibit unusually frequent and vivid dreaming, while the increase of dopamine (e.g., through l-dopa) stimulates excessive vivid dreaming and nightmares.^[56]