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.
A central debate in philosophy and neuroscience concerns whether prefrontal cortex (PFC) activity is essential for consciousness. This synthesis of research using intracranial electrical stimulation (iES) in humans shows that stimulating only certain prefrontal regions—the orbitofrontal cortex and anterior cingulate cortex—reliably alters conscious experience. In contrast, stimulating anterolateral prefrontal sites, often emphasized by higher-order and global workspace theories, rarely produces reportable changes in consciousness. The variety of iES-elicited effects (emotions, thoughts, olfactory and visual hallucinations) shows no clear relation to the immediate environment, challenging the idea that the PFC is the basis for conscious perception. The findings suggest a specific role for orbitofrontal and anterior cingulate cortices in supporting emotional aspects of conscious experience.
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.