Of the five human senses, it appears that the deja vu phenomenon only occurs to the sense of sight. Whether the other senses can be affected is unclear. Since deja vu is such an undocumentable experience, even this is not a certain statement.

Going under the assumptions that it is exclusively a visual phenomenon, that it only seems to occur to certain people, and that there must be a physiological cause for the phenomenon, we consider the following possible explanation:

Hypothesis A

Any scene perceived by a normal person, is actually seen independently by each of the two eyes. That is how we accomplish three dimensional vision. Let us speculate for a moment that the signal path to the brain has a slightly different length from the two eyes. If this was the case, then the brain would get the signals from the first eye, and promptly process them and record them into memory. A moment later, the signals would arrive from the other optic nerve. The brain would then receive this signal, and immediately note that it seems very familiar, being very similar (virtually identical) to an image already in memory. But it wouldn’t be a memory from months or years earlier. It would have only been from a memory recorded a fraction of a second earlier! (The brain’s memory does not keep “time stamps” on individual memories and has no way of identifying when they first were recorded there. Under normal circumstances, other related experiences that include time-information usually give a person a cue as to when a memory was from.)

Psychological experimentation has solidly established that the human brain can only distinguish two individual visual events, with respect to time, if they occur more than about 25 milliseconds (0.025 second) apart. Since, in normal people, the signals from the eyes, through the two optic nerves, arrive at the brain and are processed well within this time interval, the brain interprets them (properly) as two slightly different views of the same scene, giving us the impression of a single event, which includes depth perception.

Let’s consider if a person had a physiological impairment in one optical signal path, such that one optic signal arrived at the brain more than the 0.025 second after the other. The brain would not necessarily know it to be the same scene and might interpret it as the Deja Vu phenomenon. An alternative way this could occur would be that the optic processing centers of part of the brain had a flaw or temporary impairment (maybe electrolyte imbalance or other biochemical deviation) that caused a delay in processing one of the signal trains.

Such an impairment could be a congenital condition, in either the optic nerve or the brain. It could also be a temporary condition due to a viral infection or an injury effect or from any other cause.

The brain’s processing of optic information is actually far more complicated than this, but the basic premise can still apply. The terminal ends of the optic nerves arrive at the optic chiasma, where half of the fibers from each eye cross over to the other hemisphere. The thalamus is then involved to get the signals to the primary visual cortex in the occipital lobe. However, the premise of a signal delay in either optic nerve, or in the brain hemisphere’s processing of the signals it has to work with, or in the corpus callosum, is still valid. Whether the ‘raw’ data in the optic nerve is delayed or the brain’s processed image is delayed, the result would be two separate images arriving at the memory, at slightly different times. If that time differential is great enough, the only noticeable result could be a Deja Vu experience.

Hypothesis B

Recently, (late 2001) I have become aware of the research being done by Vilayanur Ramachandran. In his investigations of the so-called “blind sight” condition, he has discovered something that seems to fit in well with this premise. Blind-sight is a rare condition where an individual is completely unable to see at all, and yet seems to experience awareness of some things that could only occur due to sight. Dr. Ramachandran has discovered that humans actually have TWO totally separate brain processing methods regarding visual information.

His research has indicated that the thalamus-centered pathway mentioned above is the standard processing method that our brains use today. However, he has found that there is a second, much more “primitive” processing method for visual information in the brain. He interprets this brainstem-centered as an earlier evolutionary stage of vision, which has largely been overshadowed by the newer thalamus-centered pathway.

I find his research into blind-sight quite interesting, but I see an additional possibility. IF the newer thalamus-centered pathway has taken over because of higher efficiency (a seemingly reasonable conjecture) in order to better react to more complex and changing environments that humans found themselves in, then it seems possible that this suggested higher efficiency might be associated with a quicker processing time for the visual information.

This could easily then imply that the newer thalamus-centered pathway might be more than 0.025 second faster in processing time than the older brainstem-centered pathway. Thus, in individuals where the older pathway is still relatively active, the brain could then receive two images of a scene, which it would interpret as separate experiences, as described above.

In this case, individuals who have substantial functional activity in the brainstem-centered pathway could very regularly experience the deja vu phenomenon. Individuals with lesser functionality of that pathway might only experience rare deja vu events. This would imply that most individuals probably have very well developed thalamus-centered pathways, where the brainstem-centered pathway has greatly degraded due to lack of use, and they would never experience deja vu.

A slight variation of this could involve the timing delay involved. If, in most individuals, both pathways were well functional, but where the two sets of visual information arrives at the cortex within the 0.025 second time interval, the brain would perceive it as a single event, and everything would seem normal. If such individuals had slightly longer time differentials, say 0.026 second, they could have a vague awareness of deja vu type sensations, while individuals who had longer still time differentials could conceivably almost continuously experience deja vu. For such individuals, and possibly for the rest of us as well, it seems likely that as very small children, if such confusing situations initially existed, the baby’s brain would soon establish that “time-window” of 0.025 second or whatever else was necessary in order to allow the world to appear logical. And so, there are probably no (surviving!) individuals who continuously experience deja vu, but such people with unusually long time differentials might be especially subject to having occasional deja vu experiences, especially in moments of emotion or rapid activity.

I have noticed a phenomenon that seems to support this conjecture. During late dusk, when the (clear) sky was still barely lighted, I regularly drove past a radio transmitting tower. At the top of it was mounted a white strobe light, for warning to low flying aircraft. The physics of how a strobe light works is important here. A capacitor gradually charges up electrically. Once there is enough charge in the capacitor, it is possible to suddenly discharge all of the accumulated charge to the strobe light, which then creates a SINGLE, extremely bright, extremely brief, flash of light. It is NOT possible for a normal strobe light to rapidly flash twice, since the capacitor needs time to recharge. The flash length is VERY brief, on the order of a millionth (0.000001) of a second.

One time as I approached this tower, I happened to notice an interesting looking bird flying across the sky in front of the tower. As I panned across, focused on and watching the bird fly across, the strobe light flashed. I was aware of the sensation of two distinct flashes! Since I knew that this was physically impossible for the strobe light to do, I was puzzled for quite some time. I later tried to duplicate the experiment. At other times of the day or night, there was minimal success, but at dusk, the phenomenon was often easily repeatable and noticeable.

If no bird was present, the phenomenon while panning across was also often easily obvious, but there might be a mundane explanation for that situation. It is easily reasonable that without a specific focus of attention, while panning, the two eyes might not be directed in the same direction (They might not track together). If that were the case, it could appear that a single actual flash might appear as though there were two discrete flashes (not at different times, but in different apparent locations) due to the eyes being momentarily pointed in different directions. When a bird was present, I believe this possible explanation is eliminated. The existence of the bird certainly causes both eyes to register together on it. Since both eyes are then looking in the exact same direction, the only apparently remaining explanation for the apparent appearance of two flashes where only one could have actually occurred seems to be by the process described above, where the two optic pathways to and in the brain have different response times, thereby slightly delaying the processing of the optic information received by one of the eyes (or the brainstem-centered pathway) until the head had panned to a different location. The strobe could not actually have flashed twice, and this seems to be the only remaining explanation for the sensation of two flashes.

Several possible follow-up studies seem to be available.


A possible way to prove or disprove this premise would be to survey a large number of people who have experienced the Deja Vu phenomenon and check if any of them only have sight in one eye. Whether due to accident or illness, if only one eye sends a signal to the brain to be processed, my initial premise implies that Deja Vu could not occur. If even a single one-eyed person experienced the deja vu phenomenon, that would suggest that that premise may be erroneous. If the sample is large enough, and there are NO one-eyed people that have ever experienced the Deja Vu phenomenon, statistical support might be established for this premise.

If any one-eyed people experienced deja vu, that would seem to imply that the second (thalamus vs. brainstem) premise would apply instead.

(Note: October 2005. A person who has long had a permanent physical flaw in one eye has described what seem to be valid deja vu experiences. He has described having MANY of them, which has never otherwise been credibly claimed, so I am not completely sure about this. The fact that deja vu experiences can never be confirmed or proven false by anyone other than the person experiencing them, is a real complication in such cases. But if he is correct, then my theory would have to be wrong, as there are not two separate optical paths operating in him.

Experimental – Low-Expense

A volunteer is seated along one wall of a dimly lit room, which has very plain walls (no patterned wallpaper, no hanging pictures, no furniture along the opposite wall of the room. Just in front of that opposite wall, a horizontal four-foot long rod acts as a radius arm, with one end attached to a vertical rotating shaft and the other (free) end supporting a small, dim light bulb, like the small version of Christmas lights, or a grain-of-wheat light bulb. As the shaft rotates, the small, dim light bulb moves in a large horizontal circle, which has a four-foot radius. If the shaft turns once every two seconds (30 rpm), the bulb will be traveling at about 12.5 fps (about 8 mph). It will move one inch in about 0.0065 second. This moving light is meant to be the center of attention of the viewer’s eyes, so that the vision will track the moving light.

From the volunteer’s perspective (in the plane of that motion), the light will appear to move horizontally back and forth. As the radius arm crosses the point nearest the volunteer, it triggers a switch that flashes a fixed position strobe light a few inches below where the small moving light then happens to be.

In one experiment, related to the first premise, the volunteer would wear a pair of glasses similar to one type of 3-D glasses, where one lens (RIGHT) is red and the other (LEFT) is green.

This is how the experiment would be performed. The volunteer would follow the movement of the moving dim light, usually by “panning” the head back and forth. When the strobe flashes, the volunteer’s brain will receive two images of the strobe, one reddish and one greenish. If the two signal paths are identical, to within about 0.001 second, the brain will receive both images simultaneously and the light will be perceived as a single white (red + green) light. However, if either optical signal path is delayed in the optic nerve (as compared to the other), the observer would see two separated images, one red and one green. If the delay was about the 0.025 second mentioned earlier, the two images would appear to be around 4″ apart (because the viewer’s center of attention is panning at one inch during each 0.0065 second). The observer would see two images, somewhat duplicating the effect I witnessed with that strobe tower and the bird.

The explanation is as follows. The brain knows that the two eyes are registering together, because of following the dim moving light. The actual light entering the lenses of the eyes, strikes specific rods or cones, identical to how the system works every moment of every day. However, while the ‘slower’ processing is still occurring (after the ‘faster’ signal has been processed, the volunteer’s head will have rotated due to the fast panning action. Now, if there were background guides or patterns or benchmarks, the brain would recognize the similarity of the two images and laterally adjust the images to align, during the processing action. Again, this occurs all the time for us, in the instant before our eyes can adjust their tracking of moving scenes at different distances from us. But, without such necessary background clues (a plain background), the brain has no way to make such a ‘software’ adjustment. The result is that the vision’s attention remains following the moving light (bird) yet the brain knew which rods or cones were stimulated, and it knows what direction those rods or cones NOW represent (after the slight turning of the head), so the delayed image seems to appear in a location shifted horizontally from where it actually should appear. The amount of that horizontal shift is directly proportional to the difference of time or signal processing for the two eyes and also directly proportional to the rate of panning of the head.

The dim moving light in the experimental apparatus establishes and defines the panning rate, so the horizontal shift would be exactly proportional to the processing time difference we seek. The horizontal shift would be identified by the volunteer seeing two separate strobe flashes (one red and one green) which appeared to be a certain distance apart. If the red and green flashes appeared to be one inch apart, that would imply a differential time of 0.0065 second (with the apparatus as described above). They appear to seem simultaneous, but they are separated in distance due to the panning of the head. If the red and green images appeared four inches apart, that would represent 0.025 second, approximately the time distinction threshold of the brain. This might then be implied as a method of distinguishing people who could have the Deja Vu phenomenon and those who could not.

This simple experiment also has value regarding the second premise. If the effect should be due to a thalamus vs. brainstem time differential, our observer with the colored glasses would always see white images, but still possibly two. Now, the apparent separation of the two would represent the time differential between the two processing pathways in the brain. Again, the premise here is that, by the time the second set of information is received into cortical memory, with no obvious background identifying information, the brain would use the direction of visual attention AT THAT INSTANT (actually due to the other processing pathway!) to try to determine exactly where it actually was, and so it would appear shifted off to the side, as before.

Given the variability of life structures, it seems certain that some amount of this time differential must be present in all of us, whichever of the two premises might be valid. It is inconceivable that both images always arrive at and are then processed by the brain and then placed in cortical memory at the same nano-second. The only question that remains is just how large those differences are in various people, and whether they are ever large enough to explain the experience of the Deja Vu phenomenon.

This experiment has additional value. If two flashes are seen but they are not necessarily red and green, but nearly white, this still supports this premise of explanation for the Deja Vu phenomenon, but it might imply that much of the delay occurred in the brain processing. Since each hemisphere of the brain receives optic signals from both eyes, a delay here would delay both the red and green image processing (in that hemisphere) resulting in a basically white image that is shifted from the white image from the faster hemisphere. In other words, slightly tinted red and green images might suggest the first premise, and even suggest the proportion of delay attributable to the optic pate and the brain processing. (The second premise should always cause completely white images.)

In the first premise, if experimental results show that the two ‘whitish’ images could be distinguished somehow by the volunteer (maybe by colored fringes on the leading and trailing edges), another area of knowledge could be gained. If the image processed by the right hemisphere is always behind that processed by the left hemisphere (in right-handed people), that also supports the general concept suggested here but also implies that there might be a specific mechanism responsible for the delay, and might be associated with the corpus callosum. Such a consistent pattern would be different from that seen due to individual differences in the optic nerves (or diseases they might experience) [distinctly red and green images] or in the visual cortex [white identical images]. Such a preference would seem to only be possible if a major part of the recorded delay is due to the passage of the processed signal from the visual cortex in the non-preferred hemisphere, through the corpus callosum, to the preferred hemisphere. If such a pattern of results is noted, an analysis might establish the actual signal transfer speed between the hemispheres through the corpus callosum. (I am not familiar with any research that has examined that.) On the other hand, if the results show no such preference pattern, the implication is that the corpus callosum processing is faster than the error factor of the experiment.

Experimental – High-Expense

An extensive EEG monitoring of brain activity would be used. Only a single, fixed strobe light would be used. The EEG output would need to be either extremely high speed paper or a dual-trace oscilloscope. With the normal paper speed of EEG equipment, the few millisecond differences we are looking for would be unnoticed. Either a high-speed paper output or a dual-trace oscilloscope could display and quantify the time differentials involved. Identifying the two separate traces would allow all of the above analysis including the possible contribution by the corpus callosum activity. The time differences could be established much more accurately than by the subjective opinion of the volunteer as to how many inches apart the two images appeared to be.

Such experiments might also shed light on apparent feedback loops that seem to occur in the brain. Conditions like Epilepsy and Parkinson tremors may represent a signal processing situation where a delay is somehow included, as in the premise above regarding the Deja Vu phenomenon. Possibly, without such a delay, the brain would sense a single stimulus and react normally, but if a delay appeared in the processing action, the brain might perceive two or more separate stimuli, and then activate muscles in convulsive tremors.
Some additional comments will be added here, primarily to show how scientific research progresses. It turns out that the great majority of scientific experiments tend to prove that some hypothesis or theory is WRONG rather than right! Non-scientists do not seem to realize that! But that is progress, in a scientific sense. When Edison kept finding hundreds and hundreds of failures as he was trying to develop the light bulb, he kept eliminating wrong directions and gradually wound up narrowing it down to an approach that eventually resulted in the light bulb. If Edison had given up after the first failure, or the hundredth failure, or the eight-hundredth failure, we might not have light bulbs today!

Regarding this premise (of the different optical path lengths for the two eyes), there are two recent additions. there have been three e-mails received that seemed especially interesting. Two of the three said that they were born with just one functioning eye, and the third is the one already mentioned above. Those three people each presented credible descriptions where they feel that they had Deja vu experiences. If so, then my theory must certainly be either wrong or incomplete. However, noting that a truly remarkably few people seem to have ever actually had Deja vu experiences, I wish there was some way to actually confirm that these three had! It just seems really remarkable that the three of them would have had the experience, while virtually all other humans never have it.

There is actually another area that is even more compelling. We noted above that the brain only senses two experiences as distinct if the sensation arrives more than 0.025 second apart. I have not found the specific speed of propagation of optic signals along the optic nerves, but in general, nerve signals travel through our nervous system at around 200 mph or 300 ft/sec. In order for a mechanically different path length to be responsible for a differential of 0.025 second, that path would have to be about 7.5 feet longer for one eye than the other! So a MECHANICALLY different path length is not realistic. However, much of the speed of the passage of a nerve impulse is involved in the chemical events that occur at, around and in the synapse where the signal is transferred from one nerve to the next. If there were still to be any value in my hypothesis, it would have to be in a flaw in the synapse structure related to one eye, where signals could be delayed long enough to account for the 0.025 second. A significantly different local chemical environment might materially speed up or slow down the transmission of such nerve impulses by that amount of time. It could also happen irregularly as the chemical environment might change, maybe even due to specific foods eaten???

For people with Possible Precognition Thoughts or Dreams

As a scientist, I see a possible value in all those people who come to this page thinking they have had Deja vu when they probably had Precognition experiences instead. As far as I know, no one has ever tried to document a statistically valuable number of such Precognition experiences. This web-page might be able to accomplish something in that regard. Until it might get to be overwhelming, I am starting a special e-mail address for a special purpose. If anyone has a special dream or otherwise thinks he or she has had premonitions of events that might later occur, I make this available so that notes, DETAILED notes, might be written down and sent in. One characteristic of e-mail is that every message gets time-stamped, so that would provide a provable way of establishing when the premonition or precognition had occurred. In general, I do NOT expect that anyone will ever read any of the e-mails sent to this address! However, if that person later actually has an experience that seems to match up to the earlier precognition, that person should immediately get any witnesses to write down their name and the time, and sign it, and also write down a description of what they experienced or witnessed. Then, a note would be sent in to me, using a standard e-mail address, informing me of the specific time and date that the predictive message had been sent in. I would then be able to find the earlier message in an archive. That would allow a level of documentation that would be useful to scientific investigation. (It REALLY relies on very accurate and complete descriptions of the Precognition, especially regarding DETAILS!) Here is the SPECIAL e-mail address for this purpose: Precognition archive

Although not a symptom, the phenomenon is especially common among persons suffering from epilepsy and brain damage.

A Scientific View

In recent years déjà vu has been subject to psychological and neurophysiological research. One explanation of déjà vu is that it is not an act of premonition or clairvoyance, but rather an anomaly of physical memory from your human brain.

Recent published material from scientific research implies that the brain produces memories from the hippocampus region. A small section of this is called the dentate gyrus and is responsible for “episodic” memories which stores information that allows us to tell similar places and situations apart. The dentate gyrus records a situation’s audio, visual, smell, sense of time, and other characteristics for the body’s future reference. When there is low performance from the dentate gyrus the supposed memory problem of Déjà vu occurs as the brain has difficulty in telling the difference between two extremely similar situations—the brain is not storing the lifetime memories well enough.

Apparently as people age déjà-vu-like confusion happens more often. It also happens in people suffering from brain diseases like Alzheimer’s.

How does deja vu happen?

The scientific explanation is that it has to do with memory processes. I’ll make it as simple as I can here. The basic idea is that there are portions of the brain that are specialized for the past, the present and the future. In general, the frontal lobes are concerned with the future, the temporal lobes are concerned with the past, and the underlying, intermediate portions (the limbic system) are concerned with the present. When these are all doing their normal thing, in normal states of consciousness, the feeling that ‘something is going to happen will only come up when we are thinking about the future, worrying about it, anticipating it or making plans for it. The sense of the past will only come up when our memories have been triggered in some way.

The structure that overwhelms our consciousness when we are ‘in the present are ‘being here now’ is the amygdala. It assigns an emotional ‘tone’ to our perceptions. When you step into the street and see a car speeding towards you, and you instantly freeze in terror and jump out of the way, that terror is the amygdala at work. Present. Here and now. The amygdala also recognizes expressions the expressions on people’s faces. When we are talking to someone, we can recognize their expressions and change the way we are talking to them just as quickly as we recognize danger. Words can often seem dangerous to the one hearing them. “we’re thinking of letting you go.” “I’ve been thinking that our relationship is holding me back.” “You are under arrest.”

Phrases like these need instant, appropriate responses, and the amygdala is specialized to provide them. For example, one function it participates in, the maintenance of the sense of self, is repeated 40 times per second. Each instance of the self is able to manifest a new emotional response, but only if circumstances have changed. Every 25 milliseconds. In fact, the duration of the ‘present’ in neurological terms is so brief that we don’t experience it so much as remember it.

The next level could be called ‘being around here-just about now.’

Short term-memory deals in periods of a few minutes. Its mostly based in the hippocampus. We know this because problems with the hippocampus. often lead to severe short-term memory problems. It helps us to stay oriented in time. There have been a few people who have lost all hippocampal functions, and they are unable to remember anything that happened after their brain problems began. Humans are a linguistic species, and an intensely social one. We relate to each other through words. We have conversations. In order to do this, we have to be able to remember what people say to us. We also have to be to think about it long enough to be able to respond to it. We have to remember what we have just finished doing in order not to have to do it again.

There is a joke I heard while working in a nursing home: Happiness is finding your glasses before you forget what you need them for.

Then there is long-term memory. Its ‘seated’ in the surface of the brain, along the bottom of the temporal lobes. The area has been called the parahippocampal cortex, and its very closely connected to the hippocampus.

Ordinarily, there is a fairly seamless integration of the past, present and the future. In simple terms, we experience something in the present, compare it to similar experiences in the past, and decide how we will respond. The time frame can be very brief; even a few seconds. Once in a while, though, there can be too much communication between short-term and long-term memories. When this happens, then the present can feel like the past.

If perceptions from the present are shunted through the parts of the brain that process memories from the past, those perceptions will feel like they are memories, and the person will feel that they are re-living a moment stored in long-term memory.

There is another experience worth mentioning; Jamais Vu. Its the opposite of deja vu. Instead of feeling extra familiar, thing seem totally unfamiliar. In this case there is too little connection between long-term memory and perceptions from the present. When a person is in this state, nothing they experience seems to have anything to do with the past. They might be talking to a person they know well and suddenly they person seems totally unfamiliar. Their sense of knowing the person, and knowing how to relate to them simply vanishes. A room in which they spend a lot of time suddenly becomes totally novel; everything seems new. Details they will have seen a thousand times suddenly become engaging.

Jamais Vu is not so common as deja vu, but it can be just as compelling.

How do I respond to Deja Vu?

That depends on whether you enjoy it or not. Some people are just terrified when it happens. Others find it mildly euphoric.

As with all other altered state experiences, most people who enjoy it think of the experience in spiritual terms, and those who don’t, think about it in psychological terms. I have talked to people who had it often, and found the experience to be terrifying. There is nothing frightening about deja vu in itself, but it can happen that activity from the hippocampus. can spill over into the neighboring structure, the amygdala, which is a highly emotional structure. If it gets into the one on the right, the emotion is going to be unpleasant, most likely fearful.