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In the mornings, my cat often takes up a post on my chest. His presence is heralded by a chirpy meow and four quarter-sized points of pressure where his feet make contact; as he relaxes, he settles into a loud, rhythmic purr, and the pressure of his 16 pounds is more evenly distributed across my ventral torso. If I'm slow to open my eyes, he reaches out a paw and gently pricks my face with his claws ‹ enough to make an impression but not do real damage. When I do open my eyes, I see the triangles of his ears, the dense, velvety blackness of his fur and the sheen of his nose; his yellow irises are thin rings around his dilated pupils in the dim, early light.

Suppose I experienced all of those sensations up to the point of opening my eyes ‹ the pressure of my cat's weight and the pricks of his claws, his meowing and his purr ‹ and then I opened my eyes to the absence of any visual evidence of a cat. I'd be confused and disoriented, and if the tactile and auditory stimuli continued, probably panicky. A fundamental reworking of how I understand the world would be necessary to account for an invisible cat.

Now suppose that the next time I heard guitar music, I failed to perceive a soft brushing sensation around my ankles. It would not bother me a bit. But for Carol Crane, a guitar that didn't affect her ankles might provoke the same sort of confusion and anxiety an invisible cat would induce in me. To Crane, the ankle-brushing sensation has always been an integral part of guitar music, just as violins always act upon her face and trumpets on the back of her neck. Crane has a rare condition called synesthesia, in which a stimulus usually perceived in one sensory modality produces a sensation in one or more other sensory modalities. (1).

Synesthesia has many forms ‹ synesthetes may taste shapes or feel odors, for instance, or perceive alphanumeric characters in particular colors. Synesthetic perceptions are involuntary and are reliably triggered by the phenomena that induce them. They are also consistent over time for a given synesthete; that is, a true synesthete for whom the musical note E produces a percept of red triangles on a field of yellow will invariably experience that sound that way.

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There is, however, no consistency of synesthetic perceptions from one synesthete to another: while a high percentage of them experience colored letters, for instance, they do not agree with one another on what color each letter evokes. Synesthetic sensations do not replace, but add to, the perceptions of the triggering stimuli in more usual modes. In almost all cases, synesthetic correspondences are a one-way street: if sounds induce the perception of colors, for instance, colors do not induce the perception of sounds. Synesthesia appears to be a heritable trait and is more common in women than in men. (3)

Interest in synesthesia, from the Greek syn (with or together) and aisthesis (perception or sense-impression) has mushroomed in the past decade or so. Accounts in the popular news media have prompted more and more reports of the condition, and estimates of the frequency of its occurrence have risen correspondingly. In 1995, a CNN interview with Richard Cytowic, a medical doctor and synesthesia researcher who wrote the popular 1993 book The Man Who Tasted Shapes, used Cytowic's figure of 1 case of synesthesia for every 25,000 people. (4) By March 2001, a story in the San Francisco Chronicle asserted that " As many as one in a few hundred humans may be synesthetes." (5) Another recent estimate, based on the number of people who responded to a British researcher's advertisement for synesthetes relative to the circulation of the newspaper in which he advertised, is 1 in 2,000. (6)

Why the sudden interest? According to Cytowic, the phenomenon has been recognized by medicine for at least 300 years, but studying it has gone in and out of fashion. There was a wave of interest about a century ago, but after interest peaked and the phenomenon remained unexplained, synesthesia lost its appeal to researchers, a situation Cytowic attributes to the dominance of behaviorist theory that deemed the study of subjective, undocumentable experiences like synesthesia unfit objects for scientific study. (2)

Indeed, the sudden popularity of synesthesia research can be explained in part by the development of sophisticated brain-imaging techniques that give observers concrete evidence that synesthetes' brains operate differently from nonsynesthetes'. (7) Now researchers are eager to investigate the neural circuitry that results in synesthesia, and many hope that the brain functions of synesthetics can provide insight into the way sense perception is normally integrated and even the physical processes that underlie consciousness. Since the latest phase of synesthesia research began, however, there has been little progress toward a consensus about the condition's etiology. Several competing theories have emerged, but none has triumphed.

Cytowic, the researcher who first brought techniques of modern neuroanatomy to bear on synesthesia, did his most famous research on a subject called MW, who tasted shapes. Measuring blood flow to various areas of the brain, Cytowic found that when MW was exposed to a synesthesia-inducing stimulus, blood flow to his cerebral cortex, the part of the brain generally held to be the site of higher-order mental processes, plummeted, while the metabolic activity of the limbic system, including the hippocampus, was reletively enhanced. (2) Cytowic theorized that crossmodal associations are a normal part of the limbic system's processing of information, but they are not normally available to consciousness; depressed blood flow the cortex, particularlty the left side, is what allows such associations to emerge into consciousness in synesthetes. (8)

Because the limbic system evolved earlier than the neocortex and is often assigned the more "primitive" functions of emotion, memory and attention, Cytowic says, his findings have profound implications for the relative importance of emotion and reason. He cites his evidence of the role of the hippocampus in the phenomenon as evidence that "[I]n humans, the relationship between cortex and subcortical brain is not one of dominance and hierarchy, therefore, but of multiplex reciprocity and interdependence." (2)

Cytowic's theory has not gained widespread acceptance, in part because brain-imaging technology used in later research on synesthetes did not show reduced cortical blood flow. More recent theories of causation all focus on the neocortex and its apparent segregation into sensory "modules," a disjunction that does not operate in synesthetics.

A widely accepted model of perception that has been supported and much elaborated by high-tech imaging techniques assigns each sense to its own portion of the neocortex. Using magnetic resonance imaging and positron-emission tomography scanning, researchers have discovered increasingly specific areas of subspecialization, discovering neurons in the visual part of the cortex that are activated by color, movement, horizontal or vertical orientation of a stimulus, among other things. (9)

A theory of synesthesia proposed by Simon Baron-Cohen posits that synesthetes simply have an abnormally large number of neural connections between sensory modules in the brain that are normally isolated from one another. This "cortical modularity breakdown" theory emerged from Baron-Cohen's PET scans and MRIs of the brains of six subjects who displayed lexical synesthesia, in which word evoke colors. When the trigger words were read aloud to them, areas of the visual cortex that are normally dedicated to color processing were activated. No such activity was visible in the brains of non-synesthetic control subjects. (10)

An elaboration of the cortical modulation breakdown theory has been proposed by Daphne Maurer, who hypothesizes that the abundance of neural connections among sensory modules in the brains of synesthetes was present in all of us in infancy. Infants up to four months of age, Maurer says, experience sensory stimuli in an undifferentiated way; a recording of "Lucy in the Sky with Diamonds," for instance, might trigger visual and tactile sensations as well as an auditory one. According to this theory, the neurons that allow such cross-modal sensory experience are pruned during normal development, but synesthetes retain some of the connections. (11) There is significant evidence that the sensory cortex is not as specialized in infants as in adults, Baron-Cohen notes. Cross-modal plasticity in the very young is also indicated by studies showing that the visual cortex can be recruited to process tactile information in subjects who were blind from an early age. (12)

Perhaps the most fascinating hypothesis is the "cortical feedback disinhibition theory" advanced by Peter Grossenbacher and his research team. If Grossenbacher's hunch is correct, synesthesia doesn't require an abnormal network of neurons; it uses "feedback" neurons that are present in all human brains, but are normally inhibited in certain circumstances. The abnormality in synesthetes is simply the failure of the inhibition mechanism. This, says Grossenbacher, accounts for the fact that certain hallucinogenic drugs can induce synesthesia; the drugs don't build new neural networks in each user; they simply disinhibit the flow of impulses along networks that already exist. (1)

"Feedback" connections are themselves something of a neurological mystery. Their name arises from a computational model of brain function that posits a parallel, hierarchical sensory systems in which simple representations are delivered to successively higher levels that support increasingly complex processes of differentiation and synthesis. These are "feedforward" signals. At every level of sensory hierarchy, there are "feedback" connections that reciprocate the feedforward connections. (13) Separate sensory pathways eventually converge to present an integrated experience to the perceiver. Numerous structures have been identified as possible locations of such convergence; (14) presumably, all have feedback pathways that could produce synesthesia if disinhibited.

Pinning down the operation of feedback connections in synesthetes might help researchers understand how they normally function. Feedback connections have been proposed as the physiological mechanism of an emerging "top-down" model of sensory perception in which high-level judgments about a sensory stimulus are relayed back down the chain of processing to instruct lower levels in how to perceive it. Raw visual data, for instance, might be compared to a stored mental representation of an object; if there's a match, that information returns to a lower level and adjusts the perception, filling in information where the stimulus is weak. A study reported in 1998 supports the hypothesis that feedback connections have this kind of role. (15)

In this model, the brain is an active participant in the creation not only of meaning and emotion, but of the contours of physical reality. And there is evidence cross-modal associations can "feed back" and significantly alter the perceptions of nonsynesthetes. Researchers at Caltech recently discovered a visual illusion that is consistently induced in normal subjects by a sound. Subjects who watched a white disk flash a variable number of times as they simultaneously heard a variable number of beeps. They were asked to judge how many flashes were presented in each trial, and they consistently reported seeing multiple flashes whenever a single flash was accompanied by more than one beep. (16)

The failure of researchers to reach a consensus about the causes of synesthesia is hardly surprising: there are, after all, so many varieties of it. New studies are addressing subtleties that were ignored in earlier research. The fact that some (indeed, most) forms of synesthesia are triggered not by low-level sense perceptions but by symbolic representations like alphanumeric characters has been addressed recently by several researchers. An Australian team's findings suggested that an alphanumeric trigger could not be bound to a color-perception response until the letter or digit had been recognized and understood at a semantic level ‹ a very high level of processing. (17)

The Australian researchers' findings don't conclusively support any of the theories about what causes synesthesia. But they may provide insight into a major problem of consciousness research: how and where in the perception process are certain features bound to, or associated with, features in a different sensory mode to produce an integrated experience. If the Australian findings are a guide, it may be that before I've fully registered his washboard purr or his triangular ears, my brain brings into play my whole memory file about my heavy black cat who always wants me up exactly half an hour before the alarm goes off. I wish I could measure the time it takes me to throw him off my chest.

WWW Sources

1) ,Everyday fantasia: The world of synesthesia," in Monitor on Psychology, a publication of the American Psychological Association.

2), Richard E. Cytowic, "Synesthesia: Phenomenology And Neuropsychology," from Psyche, an interdisciplinary journal of research on conconsciousness, on a Butler University Web page ,

3) ,Synesthesia page maintained by Sean Day

4),"Ever taste a shape, or smell a color?" CNN Food & Health

(5),"They See Where Others Hear," San Francisco Chronicle

6), Burt Woodruff, "Research Group Readings in Synesthesia," Butler University

7), Brad Lemley, "Do You See What They See?" Discover, December 1999

8), Christopher T. Lovelace1, Peter G. Grossenbacher1, and Carol A. Crane, "Functional connectivity underlying synesthetic perception: Theories and data." Poster presented at the 1999 meeting of the Cognitive Neuroscience Society, Washington, DC.

9), Ethan Montag, "Brain Processing," a University of California, San Diego Web page

10) Charles Downey, "Senses Working Overtime," Praxis Post

11), Simon Baron-Cohen, "Is There a Normal Phase of Synaesthesia During Development?" Psyche

12), Leonardo G. Cohen et al., "Functional relevance of cross-modal plasticity in blind humans," Nature

13), Peter G. Grossenbacher and Christopher T. Lovelace, "Mechanisms of synesthesia: cognitive and physiological constraints," Trends in Cognitive Sciences ,

14), John Foxe, "Multisensory Integration: Candidate Brain Areas"

15), J. M. HUPÉ, et al., "Cortical feedback improves discrimination between figure and background by V1, V2 and V3 neurons," Nature

16) Ladan Shams, et al., "What You See is What You Hear," Nature

17), Mattingly, et al., " Unconscious priming eliminates automatic binding of colour and alphanumeric form in synaesthesia," Nature
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