One theory on deja vu is that hyperactive neurotransmitters (e.g., dopamine) that help shuttle information into both short-term memory and long-term memory centers change lanes. The visual memory you are seeing for the first time ends up getting sent directly into long-term memory before it's processed into short-term memory. The result? Your brain "recalls" the image from the long-term memory storage area, which tells you it's a memory from the past, not the present. Deja vu!
Another deja vu theory is that these been-there-done-that feelings are a result of one eye sending information to the brain faster than the other. Almost everyone has a dominant eye. If the stronger eye sends information to the subconscious before both eyes focus and register the input as a conscious experience, your brain will tell you, "I've seen that before." You have, but only a nanosecond ago. Or, maybe you've just "seen" it on the computer or TV.
More Answers from Michael Roizen, MD