What déjà vu and the "dreamy state" tell us about episodic memory networks

Illusions of inappropriate familiarity with the current experience or hallucinatory recall of memories are reported in temporal lobe seizures. Pathophysiological hypotheses have been proposed, involving temporal limbic regions (Hughlings-Jackson), temporal neocortex ("interpretive cortex", Penfield), or both (Bancaud). Recent data acquired from presurgical investigations using intracerebral electrode recordings, demonstrate a critical role for the sub- and para-hippocampal cortices. From this, a novel hypothesis of cortico-limbic networks emerged: déjà-vu results from an abnormal synchronization between rhinal cortices and hippocampus, and reminiscences ("dreamy state") from activation of the associational function of the hippocampus in re-assembling elements of the past experience networks. "Experiential" phenomena are better scrutinized during direct cortical stimulation than during spontaneous occurrence, because it allows precise spatiotemporal correlations to be made between the illusion/hallucination and the electrical discharge features and localization. Therefore, we present a summary of the stimulation data published since Penfield's seminal studies, review the anatomical and physiological correlations of stimulation findings, and question their functional significance. We reappraise the distinct and coactive roles of the various regions involved in perception-memory processes including the hippocampus, rhinal cortices, temporal neocortex and constituent elements of the ventral stream. Additionally, we draw insights from what is known about the perception-cognition continuum underlying the construction of episodic memories. Finally, we compare the results from cortical stimulation in the epileptogenic zone with the use of stimulation for memory enhancement and explore what this reveals about the mechanisms of stimulation.