Most people, when not under the influence of
hallucinogenic drugs, experience the sensory world as a place of orderly
segregation.
Sight, sound, smell, taste and touch live in different
neighborhoods and commute on separate freeways: A Beethoven symphony is not
pink and azure; the name Angela does not taste like creamed spinach.
Yet there are those for whom these basic rules of the
sensorium do not seem to apply. They have a rare condition called synesthesia,
in which the customary boundaries between the senses appear to break down,
sight mingling with sound, or taste with touch.
Thus, the composer Olivier Messiaen, speaking of the
union of color and tone in his music, explained to an interviewer: "When I
hear music, I see inwardly, in the mind's eye, colors which move with the
music. This is not imagination, nor is it a psychic phenomenon. It is an inward
reality." A 21-year-old woman, participating in an continuing synesthesia
study at the National Institute of Mental Health, told researchers that when
she ate buttered toast, "it is rough, but not pointy; and if it has jelly
on it the rough texture is rounded." And Carol Steen, a New York artist
who, like most synesthetes, has had synesthetic experiences from an early age
and who uses her perceptions in her work, says she distinguishes different
types of headaches by their colors. "If it's a sinus headache, it's
green," Ms. Steen said.
Synesthesia received a flurry of attention from artists
and psychologists at the turn of the century. But until relatively recently,
modern science largely ignored it. Those who experienced synesthesia rarely
complained ("It's the most wonderful thing in the world!" exclaimed
one synesthetic woman). And the private nature of the perceptions made
investigation difficult -- there was no objective way to tell what, if
anything, unusual was taking place.
In the past 10 years, however, the arrival of imaging
techniques and other new technologies for studying the brain at work has
revived interest in synesthesia, capturing the interest of a small core of
researchers in a variety of countries and disciplines. PET scanners,
electrophysiological recording, DNA analysis and other techniques are
increasingly being used.
In the current issue of The Journal of Neuropsychiatry
and Clinical Neurosciences, for example, German researchers from the University
of Hanover Medical School report electrophysiological findings from a group of
synesthetic subjects.
An understanding of synesthesia as a perceptual anomaly,
researchers hope, may eventually help elucidate normal perception, or even shed
light on consciousness itself.
Meanwhile, much more remains unknown about the
comingling of the senses than is known. Even basic facts about synesthesia --
its prevalence, for example -- are still less than certain. One newspaper and
magazine survey, however, by Dr. Simon Baron-Cohen, a psychologist, and his
research group at Cambridge University in England, found that 1 out of 2,000
people reported synesthetic experiences.
Scientists offer differing theories of synesthesia's
cause: Some argue that it represents an innate difference in neurophysiology,
others that it is a result of associations learned at an early age. And
researchers disagree about how exactly the condition should be defined, with
some viewing it as a perceptual framework entirely distinct from normal
perception, while others envision a continuum of synesthetic experience, with
normal sensory perception at one end. Dr. Lawrence Marks, a psychologist at
Yale University, for instance, has found that normal subjects show
"implicit" associations that may be milder versions of the links
found in synesthesia: Most people, for instance, associate brighter colors with
higher pitched sounds.
Also mysterious is why the extent and form of
synesthetic perceptions differ so widely from person to person. Seeing letters
of the alphabet in different colors is a common synesthetic phenomenon. But,
Dr. Baron-Cohen said: "Even within families, people argue about the colors
of different letters. It seems to be highly individual and idiosyncratic."
And while the majority of synesthetes, who are about six
times as likely to be female as male, find their unusual sensory abilities
enjoyable, in some cases synesthesia can be disruptive. One woman, Dr.
Baron-Cohen said, not only saw colors when she heard sounds, but heard sounds
whenever she saw colors. "For her it was very unpleasant, and her reaction
was to try to control the environment and keep everything very low key,"
he said.
Amid all this uncertainty, a growing body of evidence
supports the notion that whatever synesthesia is, it represents more than the
clever use of metaphor by creative individuals. Hallucinogenic drugs like
mescaline and LSD, as well as some organic diseases, can produce synesthesia,
suggesting a physiological basis. And people who report synesthetic experiences
-- seeing particular colors evoked by particular sounds, for example --
demonstrate a remarkable consistency in their associations over time. In a 1989
study, Dr. Baron-Cohen and his colleagues found that synesthetic subjects
showed a 92 percent consistency in their color-sound associations after a full
year, in contrast to only 37 percent consistency after one week in control subjects.
Synesthesia also appears to run in families, leading
some researchers to believe it has a genetic basis. Certainly, many synesthetes
report that a family member shared their ability. The writer Vladimir Nabokov,
for example, wrote that as a young child, he informed his mother that the
painted colors on his wooden alphabet blocks were "all wrong." She
understood immediately, Nabokov recalled, because she, too, saw each letter in
a distinctive hue.
For her part, Ms. Steen remembers a family visit 30
years ago, when she was in college: Sitting at the dinner table, she mentioned
casually that the numeral "5" was yellow, but her father corrected
her. "No, it's yellow-ocher," he said. Her mother and brother, Ms.
Steen said, sat in silent perplexity.
Perhaps most intriguing as arguments for the
"realness" of synesthetic phenomena, however, are the handful of
brain studies that show differences in brain functioning of subjects who have
synesthesia. In 1982, Dr. Richard Cytowic, a Washington neurologist and the
author of "The Man Who Tasted Shapes" (Putnam, 1993), led the way by
measuring brain metabolism in a single synesthetic subject -- a man who at a
dinner party announced that the meal would be late because "there are not
enough points on the chicken." Dr. Cytowic found that during synesthetic
experiences, the man showed decreased blood flow in brain areas of the cortex
responsible for language and abstract thought -- findings the neurologist
argued were indications that synesthetes were not simply using their
imaginations or playing with language.
A more sophisticated PET scanning study, published in
1996 in the journal Brain by Dr. Eraldo Paulesu, Dr. Baron-Cohen and
colleagues, compared brain functioning in six synesthetes to that in six
members of a control group. The subjects, all women, were blindfolded and
listened to sound cues delivered through headphones. Synesthetes, the
researchers found, showed increased activation in some areas of the visual
cortex when responding to sounds; control subjects did not.
The areas showing increased activity are not the same as
those activated when someone is imagining images, said Dr. Baron-Cohen. He said
an M.R.I. study, as yet unpublished, by researchers at the Institute of
Psychiatry in London had replicated these results.
Most recently, the team of German researchers compared
recordings of electrophysiological activity in the brains of 17 synesthetic
subjects, who experienced color images upon reading letters or numbers, with
recordings from 17 control subjects.
The results, said Dr. Thomas Muente, a neurology
professor at Hanover who took part in the study, suggest that during
synesthetic experiences, regions of the frontal cortex of the brain that
control attention and also play a role in processing sensory information are
inhibited. "But this paper is just the starting point," said Dr.
Muente.
Murkiness in science is not without its rewards, and one
benefit of the absence of conclusive data about synesthesia is that it has left
ample room for the construction of titillating theories. Dr. Cytowic, for
example, argues on the basis of his own research and the subsequent brain study
findings that synesthesia is a kind of "cognitive fossil," left over
from a time before the separation of sensory pathways evolved. In this sense,
synesthesia may be "closer to the essence of what it is to perceive,"
Dr. Cytowic has said, and may be tied to phylogenetically older, subcortical
brain structures involved in emotion and other primal functions.
Dr. Peter Grossenbacher, a senior staff fellow at the
National Institute of Mental Health, who with Christopher T. Lovelace and Carol
Crane has interviewed more than two dozen synesthetic subjects as part of a
current study, has a different theory. He suggests that in synesthetic
experiences, neural pathways that normally act to suppress irrelevant sensory
input and allow focused perception in a single sensory mode, may be selectively
disinhibited, resulting in multimodal perception.
For Patricia Duffy, a 46-year-old instructor in the United Nations' language and communication training program, the cause of her perceptions is less important than the richness they have brought to her life. She sees the words she speaks fly by in a rainbow of colors. She sees a year as an oblong circle, a week as a sidewalk with seven colored squares of pavement. The month of January is garnet red; December is dark brown. "I don't really know where it comes from," she said. "I just know it's always been that way."