Neural Networks of Unconscious Processes: A Systematic Review of Functional Connectivity in Dreams and Free Association

Although previous neurophysiological studies validated aspects of unconscious processing, a detailed characterization of specific neural circuits required modern functional neuroimaging techniques. This systematic review examines the shared neural substrates between dreaming and free association of ideas, two paradigmatic manifestations of unconscious processing. A systematic search was conducted in PubMed, Scopus, and Web of Science (last 25 years) using terms related to functional neuroimaging, REM (rapid eye movement) sleep/NREM (non‑rapid eye movement) sleep, and free association. A total of 28 studies meeting specific inclusion criteria were included: use of advanced neuroimaging techniques (functional magnetic resonance imaging (fMRI), electroencephalography (EEG), intracranial electroencephalography (iEEG), and positron emission tomography (PET)), research in humans on unconscious processes, and explicit linkage to functional neuroanatomy. Bias assessment was performed using the Newcastle-Ottawa scale. Both processes converge in the activation of the default mode network (DMN), specifically the medial prefrontal cortex, posterior cingulate cortex, and precuneus. During REM sleep, hyperactivation of the amygdalo-hippocampal complex is observed with functional disconnection of the dorsolateral prefrontal cortex (DLPFC). Free association shows similar patterns: activation of the anterior temporal lobe and angular gyrus with reduced prefrontal modulation. Functional microstates in REM reveal oscillations between tonic (residual sensory processing) and phasic (environmental isolation) periods. Thalamo-cortical connectivity modulates narrative content generation in both states. The findings empirically validate psychoanalytic concepts through the identification of shared neural circuits. Prefrontal deactivation facilitates the expression of associations embedded in deep semantic networks, released from executive control. These results establish neurobiological foundations for contemporary neuropsychoanalysis and suggest specific therapeutic targets for interventions in unconscious processing disorders.