Baylor: A set of multi-colored magnetic letters that children used in the
1970s and 1980s to learn the alphabet have provided neuroscience
researchers at Baylor College of Medicine, Stanford University and New
York University a clue to the role of environment in synesthesia – a
condition in which the senses are blended. About three percent of the population has colored-letter synesthesia,
a perceptual condition in which letters of the alphabet trigger color
experiences. For example, J might trigger the experience of green, or M
may trigger red.
The standard theory is that synesthesia results from random
over-wiring between brain areas—in this case, between regions that
process letters and those that process color. This model of random
connectivity predicts that each synesthete’s color associations will be
different: Sue might have a yellow “B” while Tom’s “B” is red.
However, a suspicion about the standard model arose in 2006, when two
Stanford scientists, Drs. Nathan Witthoft and Jonathan Winawer, found a
synesthete whose colors had a distinctive pattern, cycling through red,
orange, yellow, green and blue. When asked about the pattern, the
synesthete pointed out that her colors matched those on the popular
Fisher Price magnet set she had as a child—in which A is red, B is
orange, C is yellow, D is green, and so on. By 2013, Winawer and
Witthoft had found 10 more synesthetes whose colors echoed the Fisher
Price magnet set. (Winawer is now with New York University department of
psychology.)
This finding opened the possibility that synesthetic associations can
be shaped by associations in the environment, rather than simply
resulting from random cross talk between brain areas. However, it was
difficult to test the prevalence of such imprinting, given the challenge
of finding and testing a sufficiently large population of synesthetes.
To address this challenge, Dr. David Eagleman collected data from
6,588 synesthetes. Eagleman is an assistant professor of neuroscience at
Baylor College of Medicine, and a pioneer in researching the subtle
differences in the synesthetic brain, from genetics to brain imaging.
Eagleman’s laboratory developed the Synesthesia Battery,
the gold standard test for measuring the condition. Through this online
testing, Eagleman has rigorously verified the world’s largest
collection of color-letter synesthetes.
“Often, if you want to understand a single human brain,” said Eagleman, “you have to study many thousands.”
Witthoft, Winawer and Eagleman collaborated to analyze this large
data set, and their findings are forcing a re-evaluation of theories of
synesthesia. Their results are published in the journal PLOS One this week.
They found that 6 percent of the synesthetes had 10 or more letters
that matched the magnet set, much higher than could be expected by
chance.
To see if their hypothesis about the magnet set was on track, the
researchers then analyzed the birthdates of all the synesthetes. They
found that no one born before 1967 matched the Fisher Price colors; all
the “magnet synesthetes” were born 1967 or later. This was a crucial
clue, because the Fisher Price magnet set went into production in 1971.
Strikingly, for synesthetes born in certain time frames (for example,
1975-1980), almost 15 percent of them seem to have imprinted on the
colors of the toy. For those synesthetes born after the magnet set went
out of production in 1990, the percentage of magnet imprinting goes
down, in parallel with the shrinking prevalence of the toy.
“It was entirely unexpected that we would find so many synesthetes
with letter-color pairs that are traceable to the same external source,”
Eagleman said. “Fifteen percent during the peak production of the
toy—that’s more than anyone could have guessed.”
He is quick to point out that 15 percent needs to be considered a
lower bound. It is possible that most or all of the synesthetes learned
associations from their environment, but it would be infeasible for the
researchers to identify the idiosyncratic displays in their homes and
schools, such as posters, murals, quilts, and other letter toys. As
surprising as 15 percent is, Eagleman noted, it presumably just
scratches the surface.
“When combined with genetics and brain imaging, these findings open a
powerful inroad into understanding the brain mechanisms of learning,
memory and perception,” Eagleman said.
Eagleman is a co-author of a book on synesthesia, Wednesday is Indigo Blue: Discovering the Brain of Synesthesia
(MIT Press). He is also the writer and host of a six-hour PBS series on
the brain, airing in October 2015. Funding for the synesthesia research
was provided by the Mind Science Foundation.