Déjà vu (DV) is an interesting and relatable phenomenon since it occurs in around 60-80% of the population. There are multiple explanations for DV ranging from small seizures in the temporal lobes (an area involved in memory encoding) to what's know as the "tape recording theory".
The tape recording theory works by explaining that DV is the result of two memory-related processes that normally work together becoming asynchronous. When the memory encoding process is activated at the same time as the memory retrieval process, a sense of familiarity is induced causing DV.
A third mechanism for DV concerns the dopaminergic system. Dopamine is a neurotransmitter known to be involved in in expectation-reward systems (reinforcement). Dopamine has also been shown to be involved in schizophrenia , accounting for some of the "positive" (hallucinations and delusions) symptoms of schizophrenia. One theory for schizophrenia is that it's a result of an excess of dopamine. The link between dopamine and DV comes from studies showing that when patients use antipsychotic medications shown to reduce dopamine levels, DV symptoms increased.
In 2009 Kovacs et al did a neuroimaging study on a 22-year-old woman born with right-sided spastic hemiparesis (weakness on one side of the body). The woman had neuro-psychological batteries which caused deep brain stimulation (DBS) implanted in her left internal globus pallidium (GPi). The researchers performed neuropsychological tests and Single-photon emission computed tomography (SPECT) scans when this region was stimulated. SPECT is a form of neuroimaging similar to PET (positron-emission tomography) in which gamma rays from a radioactive isotope is detected as it travels through the brain tissues of interest.
It's important to note that this is a case study with a neuroatypical patient. The main problem with case studies is generalization. You can't necessarily apply the findings from a single data point to the general population. This is especially true working with somebody that is neuroatypical. That said, I'm reminded of the story the famous neuroscientist V.S. Ramachandran sometimes tells:
"If I showed you a talking pig, you wouldn't say "Hey! Why don't you have more talking pigs, you can't generalize this finding to the entire pig population!" No, you would ask, why is that pig talking?"
The researchers discovered two different states when the tissues were stimulated, a standby state for DV (SSDV) caused when the electrodes were first turned on and an expected regular DV state that occurred when the stimulation was increased. During SSDV, the subject felt a sense of familiarity but did not experience hallucinations or express an ability to predict the future.
Normal DV symptoms were reported with increasing habituation (a decrease in the response to a stimulus). The subject experienced DV only when her eyes were open (visual stimulation) and the doctor was talking to her.
The neuropsychological exams revealed that the subject had severely impaired non-verbal memory (measured using the Rey-Osterrieth Complex Figure Test.
Memory studies sometimes talk about the "two separate compartments" theory. This theory postulates that the hippocampus is involved in recollection while the parahippocampal gyrus (the area surrounding the hippocampus) is important for familiarity judgments. While the hippocampus was not directly stimulated during this study, the SPECT scan did show its activation along with other limbic (emotional system) structures. Both familiarity and recollection systems were affected during DV. This study contradicts the idea that DV is a result of a separation of these two systems.
Sadly, the article didn't cover the interesting phenomenon known as Déjà vécu (DVC). With DVC, patients experience a constant state of DV. They never lose that feeling of familiarity that accompanies DV episodes. It would be interesting to compare cases of DV and DVC to see if DVC is merely a more intense form of DV or a separate phenomenon altogether.
One thing is clear, none of the subjects with DV or DVC are clairvoyant. Although the subject of this study claimed no abilities to predict the future, past patients have claimed to know with 100% certainty that they could predict future events. When the patients are tested properly, none are able to perform better than predicted by chance. It must be an extremely frustrating disorder. I've had that sensation of "knowing" the future as well. I can't imagine what it must feel like to go through life feeling like you're repeating every event.
If you're interested in weird cases like this, I would suggest looking into the neuroscientist I mentioned or the neurologist Oliver Sacks. Case studies like this can be fascinating. They obviously don't solve all the problems (what about the dopamine hypothesis!), but they can provide good reasons to continue investigating.
-Bruce Smith
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