Many theories attempt to explain what causes déjà vu, but there is no definitive explanation. While some specific cases link the phenomenon to pharmaceutical drug use, schizophrenia or other neurological conditions, most cases occur in otherwise healthy individuals. Scientists generally explain these occurrences as some form of memory anomaly.
Long, uninterrupted episodes of déjà vu are a possible sign of a serious neurological condition. Patients with schizophrenia, anxiety and dissociative identity disorder report more frequent incidences of déjà vu than healthy adults. Certain dopaminergic drugs, those that affect memory and cognition, can also bring on these symptoms.
The divided attention theory states that déjà vu experiences feel familiar not because they have been experienced before, but because the brain has already committed the current situation to memory. When someone walks into a room, his brain processes and stores sensory information before he can consciously acknowledge individual objects or features. Normally this delay is so short that it is not noticed, but sometimes, it can create the sudden feeling of familiarity.
Another theory blames déjà vu on false memories. This could be something seen in a movie, read in a book or even dreamed. If the events are similar enough, the brain can mistake false memories for real ones, causing those affected to believe they have experienced them before.