Deja vu from Dextromethorphan

Deja vu (from the French for "already seen") is the compelling subjective feeling that the current experience has occurred before, despite conscious awareness that it has not. In everyday life, deja vu is a relatively common phenomenon experienced by approximately 60-70% of the population, typically lasting only a few seconds. Under the influence of psychoactive substances, however, deja vu can become dramatically intensified, prolonged, and may take on qualities that go far beyond the ordinary phenomenon.

The neurological basis of deja vu is thought to involve a mismatch or anomaly in the memory encoding and retrieval systems of the medial temporal lobe, particularly the hippocampus and surrounding parahippocampal gyrus. One leading theory proposes that deja vu occurs when the brain's familiarity detection system (which signals "this has been encountered before") fires inappropriately, without the corresponding recollection system providing the specific memory. The result is a sense of familiarity without contextual detail -- a feeling of recognition detached from any specific recalled event.

Psychedelics appear to enhance deja vu through their effects on serotonergic modulation of temporal lobe processing. The 5-HT2A receptors that mediate psychedelic effects are densely expressed in the entorhinal cortex and hippocampus, regions central to memory encoding and familiarity signaling. Psychedelic activation of these receptors may cause the familiarity detection circuitry to fire with abnormally high frequency or sensitivity, producing frequent or sustained deja vu episodes.

Dissociatives can produce deja vu through a different mechanism. NMDA receptor blockade alters the normal timing and integration of neural processing, potentially desynchronizing the dual-process memory systems that normally operate in concert. The resulting temporal processing anomalies can create both deja vu (inappropriate familiarity) and its inverse, jamais vu (the feeling that familiar things are unfamiliar and strange).

At moderate to high intensity, substance-induced deja vu can be quite unsettling. The sustained and intense conviction that everything happening has happened before can trigger philosophical anxiety about free will, determinism, and the nature of time. Some users describe feeling "trapped in a loop" of recurring experience. In combination with thought loops, deja vu can contribute to deeply disorienting experiences. At its most intense, deja vu can blend with the sense of timelessness and eternal recurrence that characterizes peak psychedelic states.

Reassurance that the experience is a well-understood neurological phenomenon, that it is temporary, and that it is common during substance experiences is usually sufficient to manage any distress. Changing the environment or activity can help break the cycle.