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When I was very young, I experienced a recurring dream that I was staring, entranced, at a delicate white flower. It was like nothing I had seen or experienced in my waking life, because the pristine, thinly-veined petals were such an exquisite color that it manifested itself upon my dreaming brain as a color and a sound. The white song was a single note - like a distant choir lifting its voice in concerted wonder. I would wake from the dreams bewildered at the ease with which my brain, when asleep, could produce in me a tangle of sensations I could never enjoy while conscious.
The neurological phenomenon of cross-modal, or inter-sensory, perception stemming from a single stimulus is called "synesthesia." The word comes from the Greek syn-, meaning "union," and -aisthesis, meaning "sensation" (2). Although my childhood dreams had some synesthetic character, I have never had a comparable conscious experience. "Synesthesia proper," or neurologically-based synesthesia, is defined as the regular, involuntary experience of external, durable, and generic perceptions in senses not commonly associated with a certain stimulus (1,2) . Although there are numerous possible combinations of synesthetic events, not every permutation is observed. The estimated number of affected individuals ranges from one in five hundred, who perceive colored letters or musical notes, to one in three thousand, who experience colored sounds or colored tastes (2). It is estimated that one in fifteen thousand individuals experiences taste-touch overlap and other, rarer forms of the condition (2). Another, more conservative, estimate suggests that only one in twenty-five thousand individuals is a synesthete (3).
While certain synesthetic events are triggered by a sensory appeal to the imagination, i.e. as an artifice of literature or art, true synesthetes report only very rudimentary secondary sensory perceptions for a given input (1). That is, while many of us associate the smell of cut grass with the color green within our internal perception, a synesthete with a smell-sight perception overlap might perceive colored shapes or textures within the external environment upon smelling the grass.
Synesthesia is an additive sensory condition. Rather than replace one perception with another, a single sensory input stimulates simultaneous responses in two or more of the senses (2). It is most frequently experienced as a unidirectional condition; while one stimulus, such as sound, induces sensation within another sensory realm, such as sight, the converse does not usually occur (2).
Although rare instances of bi-directional synesthesia have been recorded, in which sight induces auditory experience and vice-versa, there is no fundamental connection between the primary and synesthetic responses for a given set of stimuli (2). For example, there is no record of a bi-directional synesthete experiencing blue visual perceptions while drinking coffee and tasting coffee when seeing the color blue.
All synesthetes have a unique and individualized set of cross-sensory perception patterns, which complicates studies of the condition. However, many theories are currently under investigation to account for the neurobiological phenomena behind synesthesia. Richard Cytowic, a neurologist and author of the book The Man Who Tasted Shapes, considers synesthetes to be "cognitive fossils:" modern holdovers of a sensory system that proved adaptive, and therefore advantageous, during the course of human evolution (3). He argues that synesthesia is caused by an abnormal redistribution of blood in the cerebrum, producing areas of cortical ischemia (reduced oxygen delivery to tissues due to a decrease in blood flow (6)) predominantly within the left hemisphere of the neocortex (2). He attributes this alteration in blood flow to the conscious surfacing of intersensory associations, which he believes occur regularly within the limbic system, especially in the hippocampus, but that, within non-synesthetic individuals, remain entrenched within the subconscious (2).
Cytowic's proposal is supported by the testimonies of epileptics who have seizures resulting from the excessive excitation of the limbic region of the brain (1), a subdivision thought to be involved in sensory awareness and emotion (7). Four percent of individuals who experience limbic seizures report simultaneously experiencing cross-modal sensations similar to those described by synesthetes (1). As the electrical perturbation during an epileptic attack spreads to the cortex of the temporal lobe, a brain region active in the integration of sensory information, the perceptions become increasingly elaborate and mimic the experience of synesthetes less accurately, resembling instead the experience produced by deliberate imagination (1). Cytowic uses this observation to root synesthesic phenomena to uncommon activity within the limbic system of the brain, a claim that is met with contradictory hypotheses (1).
Another potential explanation of synesthesia is the Cortical Modularity Breakdown Theory, espoused by Simon Baron-Cohen, et. al. (2). Baron-Cohen and his colleagues assert that the portions of the brain responsible for the processing of external sensory stimuli are separate "modules" (2). According to their hypothesis, synesthesia results when so many neural connections exist between the separate sensory units that their autonomy suffers, shunting stimuli to multiple "modules" of the brain simultaneously (2). They propose a Neonatal Synesthesia Hypothesis as a necessary corollary explaining the origins of the inter-modular connections (5). They aver that in human neonates up to approximately four months of age, sensory input is experienced without conscious differentiation of stimuli, which indicates that the modular autonomy they observe in non-synesthetic adults is the culmination of a multi-step developmental pathway that contains a synesthetic phase in early postnatal development (5). The persistence of synesthesia into adulthood, according to the hypothesis, results from the improper postnatal brain growth, in which the sensory connections between the modules that exist during early neonatality are not adequately reduced (2).
Other research supports the idea that human neonate brains are organized in a way that allows for minimal sensory differentiation (4). One experiment documented instances in which both auditory and visual stimuli provoked eye movement in infants younger than four months old (4). These findings buttress the validity of the Neonatal Synesthesia Hypothesis in their correlation of observable behavior with the proposed neonatal organization of the brain. Because the infants responded in analogous ways to two distinct stimuli, the researchers concluded that the infants had not yet developed the ability to connect a certain type of stimulus to an appropriate behavioral response, potentially due to incomplete separation of the sensory pathways. This phenomenon is observable in the neonates of other animals, as well, and indicates that the brains of many animal newborns, including humans, are more multi-sensory than the brains of adults of the same species (4). While this is a necessary condition for the Neonatal Synesthesia Hypothesis, it is not a sufficient one; the conclusions beg further investigation.
A final hypothesis for the synesthetic experience has been proposed by Peter Grossenbacher (2). He does not suggest that synesthetes have abnormal neural connections, just as Cytowic believes, but rather that multi-sensory areas of the cerebral cortex, which integrate sensory input, transfer the combined information as feedback into the areas of the brain that are responsible for processing a single external set of sensations, thus triggering a cross-modal experience (2). Grossenbacher states that synesthetes merely lack an inhibitory function that prevents this reverse flow of information in non-synesthetes (2).
While synesthesia poses many neurobiological and evolutionary riddles, it has also made a dramatic impact on human society through synesthetic expression in the fine arts. Many synesthetic composers, writers, and visual artists consider their condition a boon rather than a burden, allowing them to experience firsthand the sensual unification that is the goal of many artistic endeavors. Vassilly Kandinsky (1866-1944), a painter, composer, and synesthete, commented on synesthesia, emphasizing that the synesthetic experience itself is of value, even removed from its biological implications (1). He said, "Lend your ears to music, open your eyes to painting, and . . . stop thinking! Just ask yourself whether the work has enabled you to 'walk about' in a hitherto unknown world. If the answer is yes, then what more do you want?" (1). Although synesthesia research has exposed no definitive explanations or justifications for its existence, the very elusiveness of its origins allows researchers and curious lay people alike to "walk about" in its "hitherto unknown world," simultaneously challenging the current ideas of brain organization, development, and evolution, and incorporating those ideas into our understanding of ourselves as sensory and thinking beings (1).
1)Psyche Website Psyche, Volume 2, Issue 10. July, 1995, This paper contains Cytowic's hypothesis of the limbically-centered synesthetic experience, as well as supporting evidence for his theories.
2)Sean Day's Personal Synesthesia Website , This page is a very interesting organization of facts, current hypotheses, and personal experiences regarding synesthesia.
3)CNN Website , This article is a brief summary of Cytowic's research.
4) Paper: Synesthesia - The Mixing of the Senses. , This paper highlights research relating observable behavior to the type of brain organization proposed in Baron-Cohen's Neonatal Synesthesia Hypothesis.
5)Psyche Website Psyche Volume 2, Issue 27. June, 1996., This article outlines Baron-Cohen's Neonatal Synesthesia Hypothesis.
6) Ischemia Technologies Website. , This site provides information about cerebral ischemia, as well as current treatments and technologies relating to ischemia.
7) Information About Acupuncture Website , This site provides information about the function of the limbic system and neocortex, as well as acupuncture treatments and observations of behavior induced by acupuncture.