01/06/2003
When Carol Steen injured her knee while hiking, she didn't
just feel the pain – she saw a bright orange hue. And the
prickling needles of her acupuncture treatment evoked colorful
geometric shapes. Patricia Duffy sees letters and numbers in brilliant color,
and words take on the cast of their first letter. When
envisioning the new year before her, she sees the months
written in their individual shades, linked together like
a multicolored ribbon. Ms. Steen and Ms. Duffy share an extraordinary condition
called synesthesia – a "blending of the senses." In
synesthesia, a sight, sound, taste, touch or smell prompts a
response from that sense plus another one. Synesthetes may
taste shapes, see numbers or letters in color, or perceive
colors when hearing particular sounds. "For as long as I could remember, each letter of the
alphabet had a different and distinct color," says Ms. Duffy,
author of Blue Cats and Chartreuse Kittens, a book
on synesthesia published in November. "Until I was 16,
I took it for granted that everyone shared those perceptions
with me." Although doctors first described synesthesia in 1710,
scientists have only recently begun to understand the biology
behind it. The latest studies of synesthesia, using brain
scans and genetic screening, have exploited synesthesia's
commingling of the senses to learn how the brain perceives
and categorizes sensory information. Research is even
providing new clues to how language evolved. Studies suggest that at least one in 2,000 people
experience a blending of the senses while smelling, seeing,
tasting or hearing. For some, more than two senses are
involved. Synesthesia runs in families and occurs up to six
times more frequently among women than men. The Russian
writer Vladimir Nabokov was a synesthete, as was physicist
Richard Feynman. Synesthesia comes from the Greek words syn
(together) and aisthesi, (sensation). In its most
common form, people see letters or numbers in color. Most
synesthetes see such colors internally, in "the mind's eye."
Some, such as Ms. Duffy, see their visions projected outside
the body, as if on a movie screen. "I would compare it somewhat to watching a black-and-white
movie," she says. "Even though in the movie everything is in
shades of gray, as you see the young actress your mind
says, 'Oh, she has blond hair and blue eyes.' Your mind
is not saying she has gray hair, because somehow it is
making that correction for what you know the colors ought
to be." Unlearned, enduring "Once these associations are made in childhood, they remain
fixed for life," says neurologist Richard Cytowic, whose
investigations with a synesthetic friend and 1993 book
The Man Who Tasted Shapes helped renew scientific
interest in the topic. Confirmation of such long-term associations was first
established in the late 1980s by Simon Cohen-Baron and
colleagues at the University of London. The researchers read a
list of 100 words to a synesthete and asked her to describe
a color associated with each word. Retested without warning
a year later, the synesthete was more than 90 percent
consistent in her initial responses. Comparison subjects
given the same list and told they would be retested in
two weeks were consistent only 20 percent of the time.
In the mid-1990s, the same research group found that in
synesthetes who perceive colors when hearing words, a part of
the brain used in vision is activated in response to sound.
No such activity occurs in nonsynesthetes. "These studies showed that the synesthetic experiences were
real, that synesthetes are doing something different from
the rest of us," says Edward Hubbard, a researcher at
the University of California, San Diego. Some researchers have argued that synesthesia is a
"conceptual" event, based on learned associations between
colors and letters or numbers. Such associations might be made
in early childhood from playing with objects such as colored
blocks or refrigerator magnets. Convincing evidence For example, the colors seen by synesthetes can help in
distinguishing objects from a cluttered background. Dr. Blake's group studied a man who saw letters and numbers
in vivid hues. A visual search task involving motion and
stereo-vision required considerable effort for nonsynesthetic
subjects. The synesthete, however, completed the task
with relative ease because the target of the search appeared
to be a different color from the distractors. The group's findings, published last year in the
Proceedings of the National Academy of Sciences,
provide strong evidence that synesthetic experiences originate
from a binding of color and form in the central stages of
visual processing. Another synesthesia study published last year identified
activity in the central processing region. The London research group used MRI scanning to determine
precisely what regions of the brain were activated in
synesthetes with colored-hearing. For them, the researchers
found, a word evoked neural activity in the brain region
V4, the visual area associated with color perception and
color processing. Comparison subjects showed no such activity,
despite being trained to associate colors with certain
words, the researchers reported in Nature Neuroscience
. An MRI study under way at the UC, San Diego, is testing
what areas of the brain are activated in four synesthetes who
see letters and numbers in color. Mr. Hubbard and colleagues
Vilayanur Ramachandran and Geoffrey Boynton at the Salk
Institute plan to present their results at a synesthesia
conference in May. Various explanations Perhaps synesthesia reflects "cross-wiring" between areas
of the brain that process different sensations. Advocates of
this theory point out that babies are born with many neural
connections between adjacent brain regions. "We propose that some type of genetic factor prevents these
connections from being eliminated or pruned the way they
normally would be, leaving the adjacent areas of the brain
connected," Mr. Hubbard says. He notes that a primary color area lies next to an area
that processes numbers and letters. Another color region in
the brain borders a primary auditory area. Mr. Hubbard says
that such natal neural connections, if not pruned, might
explain why some people see colors when they read or hear
sounds. He argues that the same sort of cross-wiring that might
give rise to synesthesia may also be present in a weaker form
in all of us. Thus, brain wiring lies along a continuum
and synesthetes just happen to be at one end of the spectrum.
Another theory suggests that sensory information from
separate brain areas is somehow merged during processing. Phil
Merikle, of the University of Waterloo in Ontario, notes
that perception does not occur at once, but forms as low-level
sensory areas send and receive information from higher-level
sensory areas. A link between these processing regions
may somehow allow the concepts associated with letters
and numbers to become associated with colors, he says.
Dr. Merikle currently is collaborating with neurogeneticist
Maria Karayiorgou of Rockefeller University to pursue studies
on the genetics of synesthesia. Last year, his research
group published a study on identical twin girls. One twin
has synesthesia and the other doesn't. "Just because they have the same gene structure doesn't
necessarily mean all of the genes will function the same –
especially if it's an X-linked gene," Dr. Merikle says. Because synesthesia is often seen in families, yet never
passed from father to son, it is widely believed to be a
genetic, X-chromosome linked trait, he says. Although each
of the twin girls possesses two X chromosomes – one from
each parent – only one X chromosome is activated in each.
Activation is randomly selected and can be influenced
by differences in the uterine environment during development.
Language theories For example, a concept referred to as the "arbitrariness of
the sign" argues that there's no connection between the sound
of a word and the object or idea to which it refers. People
in linguistics are now viewing this bit of wisdom with
fresh eyes, he says, and arguing that the correspondence
between sounds and specific types of meaning is not arbitrary.
"For example, if you look at words like fudge, trudge and
sludge, all these 'dge' words seem to involve some sort of
arrest of motion. And there seems to be similar stickiness
in the meaning," Mr. Hubbard says. The similarity in these words might be explained by common
features of the brain. "We might choose certain words like
fudge and trudge and sludge because of a similar structure
in the brain for the physical sense of this action or
the motion that you would make with your mouth," he says.
Studies with synesthetes may help scientists understand how
such neural connections are made. "In synesthesia we see a very clear example of how the
senses are being combined," says Mr. Hubbard, "how information
from vision and audition, for example, combine to create
colored-hearing. By exploring these multisensory interactions,
we can see what sort of biases are present in the brain."
Susan Gaidos is a free-lance writer in Maine.
