LUCID PAGES		PHYSIOLOGY OF SYNESTHESIA
HOME HUMAN MIND		This page is based on Martin Dak's eBook THE HUMAN MIND and other sources. THE PHENOMENON OF SYNESTHESIA Synesthesia is a fascinating condition that is attractive not only for what it does to human cognition, but also because of the way how synesthesia does it. This mental disorder mimics various brain mechanisms and obscures its true physiology. To fully understand synesthesia, one would have to know the cognitive architecture of the human brain. The following discussion only outlines general principles of synesthesia, but does not explain their deeper neuropsychological meaning. In the 1990's, an unusual book, The Man Who Tasted Shapes by Richard E. Cytowic, appeared in American bookstores. The described sensory experiences were weird, and no one seemed able to understand them. The phenomenon of crossed sensory perceptions got the name synesthesia, and the subjects affected by such a condition have been called synesthetes. Synesthesia is essentially a condition of activated sensory perceptions that do not occur in the majority of people. Normal people have five basic senses that are isolated from each other. Sounds are processed by hearing; images are perceived by vision; physical objects are sensed by our fingers, and so on. In synesthetes, the usual sensory stimuli of one modality also evoke perceptions in additional unrelated senses. For example, sounds can produce perceptions of touch or vision; letters can have colors, and words can have flavors [5]. Although most people do not have combined sensory perceptions and keep their senses largely isolated from each other, there are significant advantages when other sensory modalities are coactivated. Some people have difficulty hearing or understanding language, but their difficulty seems to fade when they see the speaker's face. The subjects are unable to do lip reading, and yet they benefit from the visual input. In general, stimuli introduced in multiple sensory modalities lead to faster processing. These traits hint that everyone has the ability to process different senses as one sensory packet, regardless of the modalities involved. Such an ability suggests that some integration of senses must occur in the brain. If not consciously, then at least on the subliminal level. In addition to purely sensory perceptions, humans have mental perceptions. A steel pin can be imagined by any person without actually seeing the pin. And any person can mentally imagine being pricked by the pin. Concurrently, the subject can have an emotional reaction to the imagined stimulus. Because of these abilities, emotion and mental visualization should be considered "internal senses" of the human organism. We intuitively know that senses for processing external environmental stimuli are different from those that internally react to our bodies and minds. Negative emotions, such as grief, pain, regret, or fear are typically associated with sensory faculties of vision, hearing, and touch, and sometimes even with smell and taste. The emotional states are possible because the human organism senses the cognitive meaning of the sensory stimuli. This ability is lost in people with damage to the amygdala, or in people whose temporopolar connections to the limbic system are severed. A frequent trigger of mental perceptions is language. The emotional meaning of words is usually acquired through cultural associations. For example, the English word "hell" means a place where the devil lives. The word has a negative emotional valence. The German word "hell" means light-colored. Also this word has an emotional meaning that is often associated with light-colored beer. The Scandinavian word "hell" means luck and has a positive connotation. Interestingly, just by itself, the sound "hell" or its written equivalent is neutral and evokes no emotional responses. By contrast, some words or sounds have universal emotional valence, whether we understand them or not. A chalk making screeching sounds on the blackboard would be perceived as a negative emotional stimulus in any culture. This is also a case of "synesthesia." We hear the sound, and on top of that we have an emotional experience. By contrast, the sounds of Mozart's music usually evoke pleasant emotions in everyone. The examples suggest that some degree of "synesthesia" is inseparable from the functioning of the brain. The mentioned functional associations do not exist between dissimilar senses, but between our senses and our mental concepts. Our mental concepts and our external senses map into each other correctly most of the time. The relationship is bidirectional. We can see with our eyes, and we can mentally understand what we are looking at. In addition, we can activate thoughts to visualize previously experienced sensory input. Subjects with temporopolar damage do not have this ability. They can see real images arriving from the retinas of the eyes, but are unable to form/recall the previously seen images in their minds. The subjects have lost links between their sensory functions and mental concepts. Mental concepts represent categories of one's knowledge about the world. They allow us to distinguish an object, plant, or animal among other objects, plants, or animals. Likewise, we can easily tell the difference between cars of various manufacturers. The information is stored in our categorized memories, and we compare the concepts with the arriving sensory stimuli. A seen car that matches our memories is associated with the appropriate category (a specific car make). By contrast, loss of access to categories leaves a subject unable to make a recognition. The subject can tell that the "thing" is a car, but is unfit to make any better classification. Anyhow, when the mapping between senses and mental concepts becomes faulty, synesthesia-like perceptions may be produced. NOT JUST SYNESTHESIA Synesthetes are best known for their ability to perceive nonexistent attributes of real sensory stimuli, but these subjects differ from most other people in additional ways. Cytowic has recognized that synesthetes are likely to be left-handed and to confuse right with left, and north with south [6], and the synesthetic experience is perceived in the peripersonal space [3]. Synesthesia has also been shown to have a strong hereditary component and may be related to autism and the existence of savants [2]. By now, people who have read Dak's work already recognize some tantalizing patterns and possibilities, but it is too early to tell what they really mean. The sure thing is that synesthesia is a condition that involves much more than simple misconfiguration of senses. Synesthesia is produced by neuropsychological defects in major brain systems, which warrants the label disorder. The constellation of symptoms suggests that synesthetes suffer from a dissociative disorder that Dak labeled GCD. This hunch would have to be confirmed by clinical examinations. THE APPARENT NEURAL MECHANISM Ramachandran points out that the fusiform gyrus is a place where neurons of various sensory modalities lie physically close to each other, and cross-connections of neural fibers might result in synesthetic experience [4]. But if synesthesia is explained by focusing on the fusiform gyrus, which is activated by stimuli in various sensory modalities, the outcome is baffling at best. To find the mechanism behind synesthesia, one has to approach the problem with the knowledge of the physiology of the whole brain. The key aspect in such an exploration is Dak's discovery of the communication in the brain. The fusiform gyrus does not function in an isolated manner, but is part of larger neural networks. As the various neural structures communicate, they need to understand each other. This is achieved by correctly mapping vision to vision, taste to taste, hearing to hearing, smell to smell, and touch to touch. The neurons of different sensory faculties are often physically close and share a common sensory purpose. Because of these functional similarities, the neurons of all five senses have similar neurobiological properties. Some properties are the same in each sense, and some properties differentiate one sense from another. When the dissimilar properties are suppressed, the common properties of sensory neurons make it possible to use the involved neurons for other senses than was originally intended. If this change in sensory purpose happens in one neural structure, there may be no issue. The extra neurons are recruited for the new function and make it usually better. For example, neurons dedicated to vision can be used to perceive touch or to process sounds in a person who becomes blind at a young age. A problem only arises when another neural structure innervates the incorrectly mapped neurons. One structure signals: "This is your visual input", but the other structure experiences the input as taste or sound or shape. Hence, cross-connections between neural structures appear to be responsible for the phenomenon of synesthesia. In reality, the neurobiology of senses is more complex than outlined above. Senses frequently arrive to neural structures via multiple pathways. Shorter pathways usually produce very vivid, sharp, and palpable sensations, while longer neural pathways are associated with less prominent, but still significant sensory stimuli. The longer pathways are the ones that evoke the sensation of synesthesia; that is a perception in an additional sensory modality. Synesthesia is not the only phenomenon that is produced by the purported neural cross-connections. Phantom pain and sensation of phantom limbs also depend on multiple pathways. Some paths are short and some are long. Unlike in synesthesia, phantom limbs only lead to incorrect somatosensory perception of body parts. Amputation of a limb, for example, may produce a sensation of the limb in the face. There is no additional sensory modality, such as vision, hearing, or taste that would be coactivated during the sensation of the perceived movements of the now nonexistent limb. THE ROLE OF DNA The development and function of the human brain are controlled by the DNA. The DNA code determines what needs to be done, and the "junk DNA", nowadays called the epigenetic material, determines when, how, and to what degree will the master plan of the genetic code be fulfilled. The activity of the epigenetic material is normally modulated by environmental stimuli originating both inside and outside the human organism. The epigenetic DNA initiates processes during neurocognitive development, and the created neural structures then signal back to the epigenetic DNA that they are functional. In most cases, the epigenetic DNA stops controlling the biological functions of the brain when the feedback pathways report that the brain has acquired the correct form and function, and brain plasticity dramatically reduces. The termination of brain plasticity creates a potential problem, however. An injury to the head can damage the functions and structures of the brain. The damage does not reactivate the now dormant epigenetic DNA; it stays out of the picture. Under the circumstances, the affected neurons and their connections can only activate the common neurosensory properties that remain active even at this time. Unfortunately, the properties do not distinguish between the individual senses. The neural damage can be repaired by growing new connections, but they are only restored under the guidance of general biological forces. The individual senses are not correctly recognized and may become cross-connected within the same neural structure, as well as between different neural structures. Cross-connections can happen in the somatosensory cortex after a loss of a body part. The new connections are only restored by biological forces. The repaired connections are biologically functional, but may cause that a lost limb becomes remapped into some other part of the body, often the face. The same concept applies to some types of false memories. After a brain injury, the memory circuits can be restored in the biological sense, but they are connected to cognitively mismatched neurons. The person is now absolutely sure that his or her memories are correct, even though they may be false, irrational, or impossible to be true. THE EFFECTS OF NEUROTRANSMITTERS The effects of DNA on the development of synesthesia suggest that faulty DNA causes neural fibers to land improperly at the receiving neural structure. Interestingly, synesthesia can also be evoked by drugs, such as LSD or mescaline [1, 6]. This cause of synesthesia appears irreconcilable with cross-connected neural pathways. It is hard to imagine that drugs would create faulty synapses, and the synapses would vanish after recovery from the influence. There must be some other mechanism behind the physiology of synesthesia, a mechanism that makes the various senses appear to be cross-connected, even though they are not interconnected in the physical sense. Brain research has determined that various neurotransmitters activate or inhibit neuronal populations. The same properties are exhibited by some drugs. They affect the functions of synapses. If genetic defects cause sensory malfunction by making neurons respond to the wrong neurotransmitters, the wrong senses may become activated. This scenario hints that all senses are connected to a common cognitive pathway, and only the desirable senses are engaged by the correct neurotransmitters acting as passwords. Presence of the passwords engages the desirable sensory neurons and results in perception. Problems with coding or decoding of the passwords may coactivate additional senses and may result in synesthesia. Similarly, disruption of the password receptors by drugs could produce the same type of malfunction. The proposed physiology is surprisingly consistent with normal human perceptions that combine senses with mental processes. When we hear the word orange, we can perceive a complex category that specifies the object. The orange evokes visual imagery, a taste, a sense of shape, and perception of its surface qualities, along with the size and weight. This normal human perception is essentially synesthetic. Several correct sensory modalities are coactivated to give the person a broad understanding of the object. In the case of true synesthesia, a nonexistent sensory perception is produced. The sensory experience not only appears to reflect crossing from one sensory modality into another, but may also dramatically differ from normal perceptions produced by healthy senses in the conscious mind. The difference is largely qualitative. THE TRUE MECHANISM OF SYNESTHESIA The above overview of various possible causes of synesthesia was necessary to eliminate some promising but wrong approaches. Explanation of synesthesia by means of neural cross-connections or chemical passwords sounded good, but both possibilities have been discredited by a recent discovery. Kadosh et al. found that synesthesia can be produced by hypnosis in normal subjects [7]. The discovery changes the focus of this analysis and brings forth the importance of clinical work. Without the experiment, the proposed mechanisms could have been considered to be correct. The acquisition of synesthesia in hypnotized subjects does not completely invalidate cross-wiring as a possible cause of synesthesia, but seriously limits the likelihood and frequency of such an occurrence. Cross-connections and activations of multiple sensory pathways do exist in amputees, but these mechanisms have nothing to do with synesthesia or hypnosis. With the knowledge that hypnosis can produce synesthesia, further exploration of this phenomenon is easy because Dak has extensively analyzed the physiology of hypnosis, dreams, false memories, and other related topics. Now it is obvious that even sensory perceptions can be fabricated and have the nature of false memories. In retrospect, synesthesia is nothing else but a benign form of hallucination produced by the unconscious mind. Unlike in classical hallucination, which is only occasional, infrequent, and involves complex mental processes, the hallucination in synesthesia is rudimentary and persists over years and decades. The effect is achieved by false associations that are never challenged by the conscious mind. The same phenomenon exists in repeated dreams. They are unchanged and can replay the same imagery faithfully. The dreams are never dealt with by the conscious mind, and that is why they keep intruding into the circuits of consciousness when such an opportunity arises during sleep. Some repeated dreams can occur decades later and can be exactly the same as the previous versions. Experience shows that uncanny human abilities can be acquired in dreams. People can be jumping over cars, horses, and even buildings while running. The dreamer can jump from a high cliff and land gently without an injury. Some dreamers can fly by moving their arms like birds. These unreal physical abilities are seamlessly combined with true human physiology in dreams. The combined characteristics are stored as conceptual memories. It is assumed by the unconscious mind that this is how the real world is and that the dreamer truly has such abilities. The dreamer accepts the fantastic imagery as real. Similarly, the unconscious mind often combines real and mentally produced "sensory perceptions" into concocted cognitive schemes. This is what causes that true sensory perceptions are coactivated with the fabricated ones when a sensory stimulus is received by the brain. The fabricated sensory construct acquires the nature of a concept and is used to interpret the real world in all future times. The factually absent sensory attributes of the stimulus are not identified as unreal and are not corrected by the unconscious mind. When a real sensory stimulus arrives, it activates the whole memorized category of a synesthete, including the fabricated attributes. For example, the view of the number "5" may add yellow color to the number; the sound of a barking dog may evoke the taste of tangerines, and the shape of a cube may evoke the sound of wind. These examples mean that the unconscious mind is divorced from reality and creates its own reality. This is why hypnotized subjects frequently produce bizarre or severely distorted accounts of true events. The censoring role of the conscious mind is suppressed, and the unconscious mind rules. Dak's work reveals that dissociation and disrupted brain hierarchy is the usual reason for loss of conscious censorship. The affected subjects firmly believe that their false sensory perceptions or memories are real. Such blending of true and fabricated information is understandable, but no one had suspected that the unconscious mind could do the same with senses. The physiology means that synesthesia is typically not attributable to biological forces, but is produced by cognitive processes of the unconscious mind. The creations are purely cognitive and psychological, while the inability to censor the fantastic creations is caused by neural damage to the conscious mind. Although the unconscious mind can generate various distorted constructs of reality, healthy neural circuits of consciousness normally prevent the unconscious ideas from entering one's awareness. Only when the censoring abilities of the conscious mind subside, can the unconscious mentation reach a person's awareness. The usual time this happens is during dreams or hypnosis. Similarly, subjects who suffer from dissociative disorders generally have poor censorship of the unconscious mind. They can perceive true sensory imagery of the conscious mind or they perceive the fabrications of the subconscious mind. For example, a person suffering from hallucinations may see her body as it truly is while she is fully conscious. But during moments of dissociation, when her unconscious mind dominates her awareness, she may see bugs crawling on her legs. Dak's research suggests that false sensory associations are likely produced during sensory integration. This process occurs early in the cognitive process. Clinical work shows that an overwhelming number (more than 99%) of false sensory perceptions is created within 250 ms post-stimulus. In fact, the false constructs of synesthesia probably occur much sooner, because sensory integration of true senses already happens within 50 ms post-stimulus. In case of fabricated sensory attributes, the process may usually take up to 150 ms post-stimulus. Because of this tendency, highly hypnotizable subjects should have stronger expression of synesthesia than poorly hypnotizable subjects have. In support of this assumption, Dak offers numerous examples and explanations of cognitive distortions produced by the unconscious mind. Most distortions are created early in the cognitive process, and there is little distortion of reality between 250 and 500 ms post-stimulus. Some false memories and false sensory perceptions can be created even during this later stage, but the majority of preexisting cognitive attributes remains unaffected. One of the most fascinating aspects of synesthesia is the fact that the false sensory perceptions are not produced by senses, but by memories. The memories are not true memories of some specific sensory experience. The false memories are derived and manufactured from the overall wealth of remembered sensory stimuli. An equivalent phenomenon is a repeated dream that occurs several times and is always the same. Also fabricated sensory attributes are recalled repeatedly and without any change. All that is needed to access the memories is to provide the appropriate true sensory stimuli. For example, the view of the digit eight acts as an address to the concocted memorized attributes, and the digit eight is always associated with the red color. Thus, the modality of the memory address (the sensory input) gives the impression that synesthesia is a phenomenon of crossed sensory modalities. In reality, synesthesia combines a sensory stimulus with unconsciously fabricated memories and presents the overall creation to the conscious mind as a composite "sensory signal."      TREATMENT POSSIBILITIES Treatment for synesthesia is a tricky problem. Treatment can mean doing away with the false sensory associations. This process is amenable to deep hypnosis and is well known from treatment of specific phobias, which are created by the same mechanism as synesthesia is. Phobic associations are typically formed in response to traumatic dissociation and reduced participation of consciousness. The usual trigger that leads to such a condition is a traumatic experience of sexual abuse or satanic ritual abuse. The events are real, but are misinterpreted by the unconscious mind. The mental state results in a false association of the abusive episode with some innocent stimulus that was present during the abuse. This stimulus can later trigger an aversive reaction or a panic attack. The true reason behind the panic (the memory of the abuse) is not accessible to consciousness. In synesthesia, the false sensory associations are typically not formed in response to traumas, but in response to normal everyday experiences. Either way, the unconscious mind creates a distorted interpretation of the world. The false sensory association could be abolished within the unconscious mind, or the unconscious mind might be banned from reporting such information to consciousness. Both methods might be effective. The bigger problem is restoration of the censoring functions of the conscious mind. Hypnosis usually works well with specific topics, but is difficult to apply in general. In fact, a total ban on the unconscious activities should not be the goal of the treatment. The unconscious supplies consciousness with useful functions and is needed for normal operation of the brain. In most cases, therapy for synesthesia will mean treatment for dissociative disorders. The results of such treatment vary widely and usually do not produce complete recovery. Similarly, synesthesia that (at least in theory) has genetic cause is unlikely to lead to restoration of successful conscious censorship. The genetic damage may cause that the conscious mind cannot perform its censoring functions. The conscious mind itself often has considerable neuropsychological damage that hampers treatment. Naturally, this conclusion challenges the beliefs of scientists who have conducted their research with "otherwise healthy" synesthetes. Dak explains the discrepancy between the pathology and the apparent normalcy of dissociating subjects in great detail. In all fairness, treatment is not something most synesthetes want. Similarly as people who were reportedly kidnapped by aliens, synesthetes are fond of their experiences and would not like to lose them. Likewise, believers in the supernatural do not want to abandon their concocted reality. They cherish it. They want God, Allah, or Buddha to be in charge of their lives. Also this attitude is a consequence of dissociation and defective conscious control. REFERENCES [1] Ker Than, LiveScience Staff Writer (22 February, 2005). Rare but Real: People Who Feel, Taste and Hear Color. Retrieved June 1, 2009 from http://livescience.com/humanbiology/050222_synesthesia.html. [2] Elizabeth Landau (February 9, 2009). Seeing color in sounds has genetic link http://www.cnn.com/2009/HEALTH/02/09/synesthesia.genes/index.html [3] Synesthesia: Phenomenology And Neuropsychology. A Review of Current Knowledge Richard E. Cytowic 1995, PSYCHE, 2(10), July 1995. Retrieved July 1, 2009 from http://psyche.cs.monash.edu.au/v2/psyche-2-10-cytowic.html [4] Neurology Reviews.com. Clinical Trends and news in Neurology. Vol. 10, No. 7.July 2002. The Mind's Eye—Neuroscience, Synesthesia, and Art. Retrieved June 5, 2009 from http://www.neurologyreviews.com/jul02/nr_jul02_mindseye.html [5] Vicki Mabrey (August 14, 2002). A Sixth Sense. One Feels Music, Another Tastes Words. Retrieved June 1, 2009 from http://www.cbsnews.com/stories/2002/01/08/60II/main323596.shtml [6] The Heath Report. Synesthesia. Radio National Transcripts. Monday, 8th July, 1996. Retrieved July 1, 2009 from http://www.abc.net.au/rn/talks/8.30/helthrpt/hstories/hr080796.htm. [7] Hypnosis can trigger synaesthesia, study shows (October 27, 2008). Retrieved July 1, 2009 from http://cordis.europa.eu/fetch?CALLER=EN_NEWS&ACTION=D&SESSION=&RCN=30027 Note: References to undocumented ideas are listed in Dak's work.
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