Neuroscientists Locate 'Imaginary' Colors
UCSD/Salk Team Gains Insight into Neural Basis
Perception, Finds Evidence of Cross-Activation in Brain
Regions of People Who See Letters and Numbers in Colors
By Inga Kiderra
To most people
a “red-letter day” is merely a metaphor. But it’s
everyday reality to a synesthete who sees the alphabet in colors.
Synesthesia, a condition
characterized by one sensory experience generating another –
so that shapes have tastes, for instance – is estimated
to affect between 1 in 200 to 1 in 2,000 people. The most common
form involves seeing specific letters or numbers (graphemes)
in specific colors. For these individuals, known as grapheme-color
synesthetes, an ordinary “5,” in black ink on a
white background, always appears red or a “k,” greenish-blue.
According to research
published in the March 24 issue of Neuron, not only
do these grapheme-color synesthetes really see the colors they
report, as measured in behavioral tests, but functional magnetic
resonance imaging (fMRI) of their brains also shows activation
in the color-selective regions of the cortex when they view
black-and-white letters or numbers.
The results, say researchers
from the University of California, San Diego and the Salk Institute
for Biological Studies, lend support to the hypothesis that
cross-activation of adjacent brain regions is the mechanism
designed our experiment to test the cross-activation hypothesis
we initially advanced in 2001,” said V.S. Ramachandran,
a coauthor of the study and director of the Center for Brain
and Cognition at UC San Diego. “The fMRI findings quite
clearly demonstrate cross-activation – in this case between
the number/letter region and color region of the fusiform gyrus
in grapheme-color synesthetes.”
When control subjects
viewed numbers or letters, fMRI scans showed increased activity
(increased blood-flow) only in the grapheme-selective regions
of their brains, said Edward Hubbard, former UC San Diego graduate
student and first author of the paper. Meanwhile, the hV4 area,
a part of the brain network sensitive to and specialized for
color perception, did not. In synesthetes, however, both regions
In other words, in
the synesthetic brain, the experience of a letter or number
was activating both the standard, predictable area and “cross-activating”
the color-selective area.
magnetic resonance imaging (fMRI) scans of the undersides
of a synesthete's and a control subject's brains, while
viewing a black-and-white letter or number. Each brain has
been inflated to show the entire surface, much of which
is normally hidden in the sulci, or folds. The color-selective
hVR region of the cortex is shown in pink. Brain "activation"
is shown in shades of red, orange and yellow, with brighter
colors meaning more activity. The brain of the synesthete
shows activity in the color-selective region, while the
brain of the non-synesthete does not.
At the beginning of
the project, the team first set out to determine whether synesthetes
really see their reported colors. They started with behavioral
measures. One test, for example, presented the subjects with
a pattern of graphemes embedded in a matrix of other, distracting
graphemes; 2’s that formed a triangle, say, surrounded
by 5’s. If a synesthete saw 2’s as a particular
color, the triangle shape would pop out to them from an otherwise
black-and-white field. Thanks to their synesthesia, went the
thinking behind the task, synesthetes would be able to identify
the embedded shapes more quickly than normal controls.
Most of the study’s
synesthetes (five of six) did indeed outperform control subjects
in this task. But synesthetic colors were not as “strong”
and not as effective an aid as real colors. Moreover, not all
the synesthetes performed equally well.
Even more differences
emerged among synesthetes when trying to identify letters or
numbers in a crowded display in their peripheral vision.
These differences had
been observed by scientists before, but it was difficult to
gauge whether these were due to variance in the synesthetes
or were primarily artifacts of differing research methods, Hubbard
The current study,
the first to use both behavioral measures and neuroimaging in
the same individuals, has allowed researchers to discern actual
differences among synesthetes and to discover important correlations:
The fMRI scans reveal that the stronger the activation of color-selective
hV4 in a synesthete, the stronger the color perception and,
consequently, the better the behavioral performance.
“Synesthetes are likely to be far more variable that previous
research has suspected,” Hubbard said. “Further
work in the field will need to address specific types of synesthetic
Two such types, the
researchers said, might be “higher” synesthetes,
whose colors are driven by the concept of a letter or number,
and “lower,” whose colors are driven by the appearance
of a letter or number. Ramachandran – who is beginning
to image synesthetic brains with the Diffusion Tensor Imaging
method (which captures the pathways of axons, the brain’s
connecting cells or “wires”) – plans to work
with higher synesthetes to see if they have not only cross-activation
in the angular gyrus but also more wiring.
But why trouble with
the strange, mixed-sense reality of synesthetes?
“By gaining an
understanding of how the synesthetic brain functions we may
gain an understanding of important aspects of human perception,
cognition and development,” said Hubbard. “For example,
as the infant brain grows into the adult brain, regions that
were connected to each other at birth are slowly separated or
pruned. In synesthetes, however, it seems that this pruning
process does not occur to the same degree. Understanding synesthesia
may help us to better understand how a baby brain becomes sculpted
into the adult form that we all have.”
Synesthesia may give us clues about how nurture and nature interact
to lay down neural pathways, adds Ramachandran. And it provides
a unique window into the mind.
might tell us how the brain makes metaphors, which often take
the form of cross-sensory associations – think “loud
tie” or “sharp cheddar,” Ramachandran said.
“Processes similar to synesthesia may underlie our general
capacity for metaphor and be critical to creativity.
“It is not an
accident that the condition is eight times more common among
artists than the general population,” he said. “A
quirky color/number synesthesia is not on the evolutionary agenda
– but the ability for metaphor, a flair for connection,
is. In fact, it’s one of the hallmarks that makes us human.”
The experiments were
supported by grants from the National Institutes of Health.
Geoffrey M. Boynton
and A. Cyrus Arman, both of the Salk Institute for Biological
Studies, collaborated on the project and are coauthors of the
Media Contact: Inga
Kiderra, (858) 822-0661