The Brain’s Illusion: The Neurobiology of Unusual Experiences

Our everyday lives follow a fairly predictable course, apart from the stumbling blocks life puts in our path. Usually, when we wake up in the morning, we already have plenty of ideas and plans about how the day will unfold, what we will do, and what we will experience, and we get on with living. This suits us, too, because we need to feel that we are in control of our lives, and doing familiar things and experiencing predictable outcomes reinforces that feeling. Until, that is, we experience something strange and unfamiliar… Although life itself can confront us with entirely unexpected oddities, this article is about what our own brains produce.

First, I would like to discuss an experience first described in Edward Titchener's book Bir Psikoloji Kitabı (A Textbook of Psychology). ‘Before the brain produces a complete perception of an experience, it creates a partial perception. It is this partial perception that creates the feeling that an event has been experienced before,’ says Titchener. What he is referring to is the phenomenon known as déjà vu. In its correct spelling, "Déjà vu" means "already seen" in French. Some of you will be familiar with déjà vu, while others may read this and think, ‘So that’s what it is?!’ It can be described as a very strong feeling that we have experienced the present moment before. We are not talking about seeing and recognizing a place or person we have seen in the past, nor about a scene we repeat every day, such as picking up our bag and leaving the house. Rather, this is the feeling of having previously experienced a specific moment—say, a cat walking past the doormat just as we slowly open the door to leave the house—a scene with nothing particularly special about its content, yet identical in every detail. What deserves emphasis here is probably not the content of the moment but the intensity of the feeling. What is distinctive about déjà vu is not so much the sense of remembering that we experience when, for example, we revisit a place we have seen before, but the strange, uncanny feeling of reliving the very same moment. So how does the human brain, in which every emotion and experience passes through so many filters, leaving little room for chance and possibility, fall into such unsettling gaps?

In The Matrix, Trinity has a very clear answer to this question: ‘A déjà vu is usually a glitch in the Matrix. It happens when they change something.’ Unfortunately, reality cannot be as clear-cut as fiction, and the answer to this question is not yet fully known. Scientists’ explanatory approaches generally involve drawing on old, familiar concepts to explain something new. Here, perhaps, I can turn to another kind of uncanniness: when you are in an unfamiliar setting for the first time, perhaps in a country you have moved to or at a crowded business dinner where you know no one, you may feel that you have seen many of the new people you meet somewhere before. This is, in fact, an example of using the old to make sense of the new—a method known as similarity-based recognition. Returning to déjà vu, it is quite common: 60–80% of people experience it at some point in their lives. It can also occur repeatedly or frequently. It is known to be more common in the second and third decades of life, that is, among young people. Could this age difference also be influenced by the gradual weakening of memory in old age, by even genuinely familiar things no longer seeming familiar, or by nothing being surprising anymore?! Considering the general methods used in neuroscience, we could say that the greatest obstacle to studying déjà vu is that no stimulus is known that can trigger it in healthy people so that it can be reproduced in the laboratory. Nevertheless, because déjà vu is related to remembering, the phenomenon is explored in memory research. In broad terms, the temporal lobes are the brain regions where long-term memory is stored; certain parts of the temporal lobes are also more closely associated with recalling specific events and detecting similarities. Moreover, in temporal lobe epilepsy, a type of epilepsy caused by uncontrolled firing in certain groups of neurons in the temporal lobes, patients sometimes report an intense feeling of déjà vu before a seizure begins. This experience, known as a seizure aura, is thought to arise when temporal lobe cells involved in recognition and detecting similarities also become involved in seizure activity. The fact that a symptom seen during an illness can also occur in healthy people suggests abnormal activity in these cells that does not develop into an epileptic seizure. Although it is not possible to conduct laboratory studies designed to elicit déjà vu in healthy people, studies involving patients with epilepsy offer clues about the brain regions responsible for it. When stimulation is delivered through electrodes placed in patients’ medial temporal regions, particularly the entorhinal and perirhinal cortices—parts of the hippocampal formation responsible for long-term memory that play a role in episodic memory and sensory processing—patients report experiencing déjà vu. Studies involving patients with damage to the entorhinal cortex also underscore this region’s role by showing that these patients experience déjà vu more frequently than other individuals.

Another explanation concerns the brain’s ability to derive a great deal of information from small clues. The need to process vast amounts of potentially vital information in very short periods is linked to the brain’s capacity to act on momentary data. Sometimes, for example, just a faint smell must remind us to stay away from a particular environment. However, because of certain faulty intersections in neural pathways, short circuits may sometimes occur in this process of recalling and linking information. In such a case, a momentary sensory stimulus that has not actually been encoded in long-term memory may mistakenly create the feeling that we have experienced that moment before. The concept of similarity/familiarity-based recognition is consistent with this explanation. Given that our brains try to make sense of situations using limited information, we can describe similarity-based recognition as the kind of memory that allows us to recognize that we are encountering something we have seen or experienced before. Researchers have conducted studies finding that déjà vu is a form of familiarity-based recognition. When there is a certain degree of overlap between old and new experiences, we have strong feelings about that connection, which we interpret as déjà vu. Other studies suggest that, for a consciously experienced sense of familiarity to accompany a scene perceived in the moment, a particular relationship must exist between memory-related structures and cortical structures. In déjà vu, this relationship is disrupted, so remembering remains, so to speak, a form of unconscious recognition.

Although studies of memory and epilepsy receive the most attention in the literature, one notable study concerns a patient who experienced recurrent episodes of déjà vu following hemorrhagic damage to the right thalamocallosal region, which is not directly associated with memory. The study reports that the number and frequency of episodes decreased as the patient developed awareness of these déjà vu experiences and improved the ability to reflect on the experience while it was occurring—a metacognitive skill.

The relationship between the cerebral hemispheres may be relevant to déjà vu in another way. Functions such as analytical thinking, cause-and-effect reasoning, and sequential thinking are carried out by the left hemisphere. Although sensory inputs reach both hemispheres simultaneously, integrated understanding and action depend on transmission between them. Partial disruptions in the transmission circuits of the corpus callosum, the structure composed of nerve fibers connecting the two cerebral hemispheres, may cause information to be processed at different times, and this time difference may give rise to déjà vu.

Returning to the literal meaning of déjà vu, we could also explain something being ‘already seen’ as having seen a future moment in the past—the moment in which we are now experiencing déjà vu—in other words, having foreseen a memory of something that had not yet happened. This situation, which might be described as a timing error, could arise when momentary sensory input bypasses short-term memory and, because of an error or short circuit, connects with long-term memory stores. In such a case, because there is no actual past memory in long-term storage corresponding to that moment, this connection may produce only ‘a feeling of having experienced it,’ which we cannot make sense of. The fact that we remain fully conscious during déjà vu—a common feature of these experiences—is itself evidence that the entire brain is not involved in producing it. In other words, something is going wrong, and the brain is aware of it, but it is beyond its control because the process does not involve the whole brain. Perhaps this is precisely why it can happen: because not all brain regions need to participate, this ‘small error’ also escapes oversight. Unfortunately, this seemingly logical explanation, like the others, remains no more than a hypothesis.

Some sources actually argue that déjà experiences can be divided into two categories: déjà vu, arising from a false sense of familiarity, and déjà vécu, arising from a false sense of recollection. Déjà vécu is described as a more delusional experience, triggered by newer and more unfamiliar stimuli than ‘ordinary’ déjà vu. At the risk of further complicating what I have described so far, I would like to mention the proposed explanation for déjà vécu. It is hypothesized that the signal to retrieve information from memory in order to recall or construct it can be neurally dissociated from the content being retrieved. It is proposed that, along the direction of hippocampal signal transmission, a "recollection" signal is generated by detecting the timing of firing in hippocampal output neurons relative to theta oscillations. Disruptions in this "temporal coding" mechanism result in false recollection signals that can occur without actual retrieval and, ironically, may arise particularly in situations of contextual novelty. It is in this situation that false recollection, or déjà vécu, occurs.

Like our real-life memories and the knowledge we acquire, our dreams are also stored in memory. Scientists have found that certain re-experiencing phenomena also occur in relation to dreams. Déjà rêvé, meaning ‘already dreamed,’ is the term used for these experiences. Like déjà vu, this phenomenon has been observed in patients with epilepsy and is described as re-experiencing a scene previously seen in a dream. In other words, it is the state of remembering that a moment you are experiencing while awake in the present was previously seen in a dream. Déjà rêvé is also said to have different forms: experiences associated with a specific dream; experiences known to belong to a dream that cannot be fully recalled; and, somewhat differently, experiences in which one feels as though one is currently inside a dream, which might also be understood as a dissociative state in which the connection with reality is loosened. It is noteworthy that these phenomena also occur during epileptic seizures originating in the temporal regions associated with memory.

Although explanations of déjà vu largely focus on subthreshold sensitization of neurons in particular brain regions without an underlying illness or an overt sign of disease, certain medications are also known to cause it. The combined use of amantadine and phenylpropanolamine has been reported to trigger these strange experiences. The drug-induced increase in dopaminergic stimulation has been considered responsible. Fatigue, sleep deprivation, highly stressful situations, and exposure to low-pressure conditions with reduced oxygen availability are also known to be associated with déjà vu.

Although less widely discussed than déjà vu, several other frequently experienced unusual phenomena have been described in the literature and also have French names. For example, jamais vu means never seen; déjà entendu means already heard; and presque vu refers to something being on the tip of the tongue. Rejà vu is somewhat different: it is the belief that an event being experienced will happen again in the future. Presque vu, which we know to be very common, is regarded as a problem at the retrieval stage of memory. It is the inability to recall information you are absolutely certain you know! We might imagine the cortical structures knowing what they want but being unable to get the memory stores to retrieve the required information from their shelves. Unlike déjà vu, jamais vu can be deliberately induced. For example, if we write or say the same word repeatedly over a short period, after a while we notice that the word becomes unfamiliar, as though we no longer know its meaning. In this experience, which we can call jamais vu, the memory record of a specific image or even a taste becomes inaccessible, and that information is briefly treated as though it does not exist. In other words, the connection between a stored memory and conscious awareness of that memory is interrupted. This, too, has been observed to occur during fatigue or severe stress.

Beyond the oddities I have discussed so far, which are mostly associated with remembering and memory, there are other phenomena that the human brain experiences but cannot explain. Among these, somesthetic illusions and autoscopic phenomena deserve a closer look. Experiences such as feeling that one’s body shape has changed, that one has left one’s body and risen above it, that one is looking down at one’s own body, or that another presence is beside or immediately behind oneself are grouped under the heading of somesthetic illusions. These events, which may sound like parapsychological experiences even as we read about them, are in fact experiences generated by our own brains. Autoscopic phenomena have four main manifestations: an out-of-body experience, autoscopic hallucination, heautoscopy, and the sense of another presence. Somesthetic illusions and autoscopic phenomena are thought to result from a failure to integrate information about the body—vestibular, proprioceptive, and tactile information. The vestibular system is a sensory system concerned with balance and spatial orientation, including motor coordination and the sense of equilibrium. It is located in the labyrinth and vestibule of the inner ear, alongside the cochlea, which is part of the auditory system. Vestibular functions and the integration of sensory inputs take place in the brain’s parietal and temporal cortices and related limbic structures. Electrical stimulation of the right amygdala, inferior parietal cortex, or angular gyrus produces reproducible vestibular dysfunction.

These disturbances include imbalance, sensing the presence of someone else who is not actually there, out-of-body experiences, a feeling of floating on the surface of water, and distorted bodily perceptions. The intensity and duration of the stimulus determine the nature of the symptoms. The literature reports that the experience of sensing another presence occurs in patients with epilepsy and during electrical stimulation of the right amygdala. In Capgras syndrome, a type of thought disorder seen in patients with schizophrenia, the patient suspects that familiar people have been replaced by impostors. One study demonstrated reduced blood flow in the left hippocampus, insula, ventral striatum, and bilateral inferior frontal gyri in Capgras syndrome and in conditions involving a sense that authentic information is deceptive, such as the misidentification of familiar people. All of these brain regions are structures associated with memory and consciousness. Because highly stressful situations have also been linked to autoscopic phenomena, endorphins produced under stress have been considered a possible contributing factor, since endorphins are also known to lower the seizure threshold, creating a predisposition to epileptic seizures. The body-perception symptoms described above are thought to lie on a spectrum, with symptoms increasing as more brain regions are affected. In addition to epilepsy, electrical brain stimulation, substance effects, and severe stress, such symptoms are also experienced by high-altitude mountaineers and pilots during pressure-chamber training. Explanations have focused on acute hypoxic conditions—reduced oxygen availability—and the sensitivity of the brain’s temporoparietal junction to these conditions.

By way of a final word…

Over millions of years of evolution, the brain has developed the ability to produce an outcome, information, or action in the shortest possible time using very little data. All these stages involve processes that are far more complex than one might imagine, yet equally delicate. So much so that activation beyond the required level in a single cluster of neurons can confuse all our perceptions. Most scientific studies end their conclusions with words to the effect that ‘further research in this area is needed.’ Knowing that nature continues its cycles in an effort to maintain and regain its balance despite all interventions, I would like to take a different perspective and end this article by saying, ‘Perhaps it is also possible that the experiences I have tried to describe here need more time to evolve toward their true functions’—or perhaps this is a beginning! Who knows?...

References

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This article was first published on the web portal of the journal Bilim ve Ütopya. As the portal has been taken offline, I am republishing the text here.