Biology 202
1998 First Web Reports
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A normally unobserved brain process brought to the foreground

Meredith Ralston

Those who have watched Disney's Fantasia or followed an art teacher's injunction to listen to music and let it suggest images have roughly approximated synesthesia. Synesthesia is a condition in which automatic cross-modal associations are formed; or in other words, a stimulus perceived with one sense will be evaluated by a second sense as well. The word "synesthesia" actually means "joined sensation". This means that a synesthete may experience colored hearing (this is the most common combination), tactile tastes, colored words or sounds, names or sounds producing tastes, or many other possible combinations of the five senses. Even stranger cross-modal perceptions have been recorded as well, such as a youth who generated a specific body posture for different words or nonsense sounds, or a man who tasted in shapes.

There are distinguishing factors between synesthesia and the average person's associations between words or colors. The most important one is that the synesthete's associations are automatic. They do not make conscious efforts to produce the joined sensations; these come unbidden. Synesthetic associations are stable for life, as well. In the case of audiomotor synesthesia above, the physician who recorded the boy's movements tested the boy without warning ten years later by reading the same word list. The boy assumed the exact same positions as before.

Synesthetic associations are generic. Listening to Chopin's Funeral March, a non-synesthete might have a very specific image come to mind, such as plodding cadavers in a dreary landscape, or a detailed funeral procession. But a synesthete who "sees" music does not conjure up something like that-- he or she might see balls, lines, or other generic shapes instead. Perhaps most interesting to me is the fact that synesthetic perceptions are not projected onto the mind's eye, but are perceived as being outside the body, perhaps right in front of the face. They are often in peri-personal space, the limb-axis space immediately surrounding the body.

Though synesthetics have been mistaken for schizophrenics or madmen before, they are not abnormal. They test normal on standard neurological exams. In addition, it's not uncommon for synesthesia to come hand in hand with superior memory skills. The Wechsler Memory Scale tests both short and long term memory, as well as verbal and non-verbal memory. Synesthetes perform very well on this exam. They are often of high intelligence, though most have some mathematical deficiencies, and family history of dyslexia, autism, and attention deficit may be found in 15%. In addition to the poor math skills, most are not right-handed indicating that synesthesia depends on the left brain hemisphere. Synesthetes have been thought to be inherently artistic, but Richard E. Cytowic, author of The Man Who Tasted Shapes, considers this to be a sampling bias.

Women synesthetes are more common, with synesthesia running in families according to either autosomal or x-linked dominant transmission. Approximately one person in 25,000 is synesthetic.

Though there are definite specifications which separate synesthetes from average people, cross-modal associations are readily apparent in day to day life. For example, anyone in the bi-college community sees many hand-lettered signs, with random or planned arrangements of colors among words. Societal values probably play a large part in which colors are used; for example, an American might use the color black for the word "death," and the color red for "love," but in China the color white would be associated with death and the color red with fortune. Synesthesia is different because the associations are personal, not external. But an interesting question is, who first made the color associations we now use?

Currently there seems to be confusion as to what happens in the brain to cause synesthesia. Certain experiments or abnormal brain functions have approximated the condition. For example, seizure discharges in the hippocampus of the limbic system may produce synesthesia in non-synesthetics in the form of flashing lights, tastes, a high-pitched whine, or etc. Seizures which stay in the hippocampus produce generic reactions, whereas those that progress to the cortex of the temporal lobe are more specific.

For this reason Cytowic feels that synesthesia is dependent on the limbic system, and most especially on the hippocampus. Indeed, one subject's cortical metabolism dropped so low during synesthesia that he should have been blind or paralyzed, yet he functioned normally and completed a neurological exam unimpaired. Cytowic goes on to state that the limbic system has been overlooked in science, and that our emotions are regarded as primitive. He hypothesizes that the limbic system is just as evolved as the neocortex, and that "the relationship between cortex and subcortical brain is not one of dominance and heirarchy,...but of multiplex reciprocity and interdependence" (From his web-article "Synesthesia: Phenomonology and Neuropsychology"). He goes on to assign a much greater importance to the limbic system than is usually accorded it, reasoning that non-rational thought easily overwhelms rational thought, and using the fact that upon emergence from a coma rational thinking is the last function to be recovered, and non-rational thought must be recovered first.

Luciano da F. Costa of the Cybernetic Vision Research Group, however, feels that Cytowic describes synesthesia as an abnormality, when in reality it is "a much more generalized effect that may take place throughout a range of scales in the brain." He cites examples in which normal mammalian brains have produced synesthetic-like results under abnormal circumstances. In cats, in response to visual deprivation portions of the anterior ectosylvian cortex, which processes vision, received auditive and somatosensory input. This is called a cortical plasticity response. However, I feel that Costa has misinterpreted Cytowic, as Cytowic makes it clear that he considers cross-modal associations to be a tool that average brains use to different degrees depending on the age of the individual and the activity the brain is undergoing. His emphasis seems to be simply that synesthesia is merely a different and more noticeable expression of this ability in certain people, which is what Costa is suggesting.

In an experiment using Positron Emission Tomography, or PET scanning, synesthetes and a control group of non-synesthetes were blindfolded and asked to listen to words played through headphones. Activity in different regions of the brain was measured during this time, and it was observed that the classical language areas of the perisylvian regions were activated in both groups. In the synesthetes, visual associative areas such as the posterior inferior temporal cortex and the parieto-occipital junction were activated. Thus higher visual regions were activated when there was no visual input.

Especially interesting to me is the role that hormones play in synesthesia. Synesthetic experiences are much more common in children, but they often cease with puberty. My mother has suffered from intense migraines which last for several days and occur twice monthly for all of her adult life. They are sparked by the menstrual cycle's change in hormones. My mother, as well as many other migraine sufferers, reports synesthetic-like phenomena during this time, such as smells producing a clangor, and strange effects on her vision and taste.

Thus it seems clear that synesthesia is a capability we all have, that regions of our brains can interpret stimulus coming from any of the five senses given the need. Or possibly "we are all synesthetic, and that only a handful of people are consciously aware of the holistic nature of the perception" as Cytowic believes. In other words, in synesthetics our natural tendency to make non-rational associations is simply pronounced.










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