A 1–3-Hz rhythm in deep posteromedial cortex underlies dissociative states in both mice and humans. In mice, precisely-dosed ketamine or phencyclidine induced this rhythm in layer 5 neurons of the retrosplenial cortex, which coupled with thalamus circuitry but uncoupled from most other brain regions. Rhythmic optogenetic activation of these neurons recapitulated dissociation-like behaviors. Local HCN1 pacemakers were required for ketamine to induce the rhythm and behavioral effects. In a patient with focal epilepsy, a similar localized rhythm in the homologous deep posteromedial cortex correlated with pre-seizure self-reported dissociation, and electrical stimulation of this region elicited dissociative experiences.
Electrical stimulation of the posteromedial cortex (PMC) can induce a temporary sense of self-dissociation—a distorted awareness of one's body in space and feeling like an outside observer to one's own thoughts. A patient with seizures originating in the right dorsal posterior cingulate cortex (Brodmann area 31) reported this reproducible experience at seizure onset. Stimulating the seizure zone or a homotopical region in the left PMC at 50 Hz induced a subjectively similar state. The findings suggest a causal link between the PMC and the integration of self-referential information, offering clues about the pathophysiology of self-dissociation in neuropsychiatric conditions.
Hippocampal ripples, brief synchronized electrical events in the brain, are known to support navigation in rodents and short-term memory in humans. This study shows that ripples also increase in the human hippocampus just before people recall recent or remote autobiographical events and semantic facts. Ripple patterns across different hippocampal sites were specific to the type of memory being retrieved. Over time, ripple patterns for autobiographical memories became more similar to those for semantic memory, suggesting a process of semantization. During recollection, ripple-coupled activity was widespread across the neocortex, especially in the default mode network, indicating that hippocampal ripples help coordinate communication between the hippocampus and large-scale cortical networks involved in conscious recall.
Electrical stimulation of the orbitofrontal cortex (OFC) in 22 neurosurgical patients with epilepsy produced a range of subjective experiences, mostly neutral or negative (aversive smells, tastes, sadness, anger). Negatively valenced experiences were more common with left-hemisphere stimulation, while neutral effects were strongly right-lateralized. Most elicited effects came from stimulation around the transverse orbital sulcus, with no effects from the most anterior OFC. The findings suggest a dissociation between a silent anterior OFC and a responsive middle/posterior OFC, and support a causal role for the OFC in integrating affect with multimodal sensory experience.