Why should we feel so “Blue” sometimes?
Hadjar Homaei
March 2007
“Affirm that name, whoever thus repeat
To him even death will taste soft & sweet
Whoever once that face has kissed
His bones are blessed in gravely mist.”
Mawlānā Jalāl ad-Dīn Rūmī
Introduction
Synesthesia (also spelled synæsthesia or synesthesia, plural synesthesae or
synaesthesae)1 from the Greek roots syn, meaning “together,” and aisthesis, or
“perception”, is a neurological condition in which otherwise normal people experience
the blending of two or more senses. [1] For a long time this phenomenon was often
disregarded as fakery or unconscious experiencing childhood memories and associations.
[1]
But it has recently been shown to be real. The most probable theory is that it occurs
because of cross activation between two normally separate areas of the brain when they
educe activity in each other.
Explorations in the mechanisms involved in synesthesia, not only helps to uncover mystic
ideas behind this phenomenon, scientists are also learning about how the brain in general
processes sensory information and uses it to make abstract connections between
seemingly unrelated inputs.[1]
Facts – Am I synesthetic?
Since synesthesia does not, in general, interfere with normal daily functioning many
synesthetes maybe not be aware of their condition or know it has a name. In reality most
synesthetes report their synesthetic experiences to be neutral or even pleasurable, [4] or
even in some cultures they think of it as a god given gift.
Simner et al., (in press) conducted the first random population study, arriving at a
prevalence of 1 in 23. Recent data suggests that grapheme → color, and days of the week
→ color variants are most common.2
1
2
Wikipedia – Synesthesia http://en.wikipedia.org/wiki/Synesthesia
Simner et al. in press
While cross-sensory metaphors are sometimes portrayed as "synesthetic", especially in
visual art and literature, true neurological synesthesia is involuntary. It does not require
attention and it involves a vivid perception in the secondary sense. It runs strongly in
families. A very famous synesthete was Alexander Nabakov with grapheme → color
synesthesia, whose mother and grandfather and son were also synesthetes.3 It is possibly
inherited as an X-linked dominant trait. Maybe that is why it is more common in women;
studies find a sex ratio of 1.1:1.2 Synesthesia is also sometimes reported by those under
the influence of psychedelic drugs (often LSD), after a stroke or brain injury, or as a
consequence of blindness or deafness. Interestingly there have also been reports of
synesthetes who lost their condition after a brain injury.3 This form of synesthesia that
arises from such non-genetic events is often referred to as “adventitious synesthesia” to
discriminate it from the more common inherited forms of synesthesia. Adventitious
synesthesia involving drugs or stroke (but not blindness or deafness) apparently only
involves sensory linkings; there are few if any reported cases involving culture-based,
learned sets such as graphemes, lexemes, days of the week, or months of the year.
Neurologist Richard Cytowic who was one the first modern scientists to study
synesthesia in 1980s, identifies the following diagnostic criteria of synesthesia:[3]
1. Synesthesia is involuntary and automatic.
2. Synesthetic images are spatially extended, meaning they often have a definite
'location'.
3. Synesthetic percepts are consistent and generic
4. Synesthesia is highly memorable.
5. Synesthesia is laden with affect.
6. Synesthesia is not easily forgotten.
Synesthetes often report that they were not aware their experiences were strange until
they realized other people did not have them. The automatic and ineffable nature of a
synesthetic experience means that the pairing may not seem out of the ordinary. This
involuntary and consistent nature helps us define synesthesia as a real experience which
we can scientifically tested.
Testing if someone is a synesthete is easy in most cases, and it can be very hard to fake.
The simplest reliable test involves test-retest reliability over long periods of time.
Synesthetes consistently score higher on such tests than non-synesthetes. Synesthetes
may score as high as 90% consistent over test-retest intervals of up to one year, while
non-synesthetes will score 30-40% consistent over test-retest intervals of only one month,
even if warned that they are going to be retested.4
To determine whether an effect is truly perceptual, psychologists often use a simple test
called pop-out or segregation. [1] When Ramachandran and Hubbard (2001) conducted
pop-out tests with grapheme → color synesthete that is now the standard test for this
3
4
Richard E. Cytowic, M.D. lecture about synesthesia at the Hirshhorn museum, Washington D.C. 2007
e.g., Baron-Cohen et al. 1996
form of synesthesia. They presented synesthetes and non-synesthetes with displays
composed of a number of 5s, with some 2s embedded among the 5s. These 2s could make
up one of four shapes; square, diamond, rectangle or triangle. (fig 1) Unlike normal
subjects, synesthetes correctly reported the shape formed by groups of numbers up to 90
percent of the time, just as non-synesthetes would do when the numbers actually had
different colors. This result proves that the induced colors are genuinely sensory because
it is impossible for them to fake this.
Succeeding studies based on this effect show that there is considerable variability among
synesthetes of the same form5 and also while synesthesia is evoked early in perceptual
processing, it does not occur prior to attention6.
Figure 1. Pop up effect
Different Forms of Synesthesia
Synesthesia can happen between almost any two senses or perceptual modes. Because of
the substantial number of forms of synesthesia, researchers have agreed on a convention
of demonstrating the type of synesthesia by using the following notation x → y, where x
is the "inducer" or trigger experience, and y is the "concurrent" or additional experience.
[2]
For example, perceiving letters and numbers as colored would be indicated as
grapheme → color synesthesia. Also, when synesthetes perceive colors and movement as
a result of hearing musical tones, it could be indicated as tone → (color, movement)
synesthesia. Although very few but there are also some synesthetes who have bidirectional synesthesia, for example tone → color synesthetes who perceive different
pitches as colors and also hear notes when they see colors or even hear a chord or music
watching a painting or movie.7
5
Dixon, Smilek & Merikle 2004; Hubbard et al. 2005a
e.g., Edquist et al. 2006; Sagiv, Heer & Robertson 2006a
7
Richard E. Cytowic, M.D. lecture about synesthesia at the Hirshhorn museum, Washington D.C. 2007
6
In many forms of synesthesia, the inducer set and concurrent set belong to distinct sense
modalities. However, not all forms of synesthesia span two sense modalities.[2] For
example grapheme → color synesthesia involves two different modalities of the same
sense; visual. In addition, some forms of synesthesia involve inducers that are conceptual
rather than sensory such as days of the week → color.
Grossenbacher and Lovelace 2001, discriminate between two types of synesthesia, based
on their inducers types. In Synesthetic Perception, concurrents are stimulated by
perceiving particular sensory stimuli, but in Synesthetic Conception, concurrents are
stimulated by thinking about particular concepts. The most common synesthesia forms or
maybe the easiest ones to detect are synesthetic perceptions, like grapheme → color
whereas in the time period → location synesthesia which is a form of synesthetic
conception, periods of time are conceptualized in a spatial layout. (Figure 2.c) [3] Which I
believe is very close to synesthetic metaphors that most people can understand the idea
and it sounds natural to them but they do not necessarily experience the concurrent
sensation.
While almost every possible combination of experiences is logically possible, some
forms of synesthesia are more common than others.
Grapheme → color
Grapheme → color synesthesia, is one of the most common and most studied forms of
synesthesia, when individual letters of the alphabet and numbers (graphemes), are shaded
with a color. (Figure 1.a) Although each particular synesthete’s consistent graphemecolor mapping can be different from others but there are also some commonalities across
letters (e.g., A is likely to be red). [5]
A grapheme → color synesthete reports, "I often associate letters and numbers with
colors. Every digit and every letter has a color associated with it in my head. Sometimes,
when letters are written boldly on a piece of paper, they will briefly appear to be that
color if I'm not focusing on it. Some examples: 'S' is red, 'H' is orange, 'C' is yellow, 'J' is
yellow-green, 'G' is green, 'E' is blue, 'X' is purple, 'I' is pale yellow, '2' is tan, '1' is white.
If I write SHCJGEX it registers as a rainbow when I read over it, as does ABCPDEF."[2]
Another reports a similar experience. "When people ask me about the sensation, they
might ask, 'so when you look at a page of text, it's a rainbow of color?' It isn't exactly like
that for me. When I read words, about five words around the exact one I'm reading are in
color. It's also the only way I can spell. I remember in elementary school remembering
how to spell the word 'priority' because the color scheme, in general, was darker than
many other words. I would know that an 'e' was out of place in that word because e's
were yellow and didn't fit."
Fig 2. Some forms of Synesthesia
Music → Color
In music → color synesthesia, synesthetes experience colors in triggered by tones or
other aspects of musical stimuli (e.g., chord, key, scale). Similar to grapheme → color
synesthesia, although the mapping of inducer and concurrent is internally consistent for
each individual synesthete but there is not much agreement among different music →
color synesthetes. Synesthetes will consistently report the same experiences in response
to the same inducer if tested again.
Color changes in response to pitch may involve more than just the hue of the color.
Brightness, saturation, and hue may all be affected to varying degrees. [9] In addition,
unlike grapheme → color synesthetes, music → color synesthetes, often report that the
color patterns move with the music, or flow inside and outside their visual field.
Number form synesthesia
A number form is a mental map of numbers in space, that involuntarily comes into mind
whenever the synesthete person thinks of numbers.(figure 2.b) In particular, it has been
suggested that number-forms are a result of "cross-activation" between regions of the
parietal lobe that are involved in numerical cognition and spatial cognition. [1] This form
of synesthesia has also been explored by numerical cognition researchers for the insights
that it may give into the neural mechanisms of numerical spatial associations that is
unconsciously present in everyone’s mind.
Figure 3. 'Film for Music' interprets a piece of music from Brian Eno's album "Music for
Films." Visually by a tone → color synesthete, Fred Collopy.
Personification
Ordinal-linguistic personification – OLD sometimes referred to as a type of animism, is a
form of synesthesia in which ordered sequences, such as ordinal numbers, days of the
week, months of the year and letters are associated with personalities. [10] Although it is
harder to distinguish this type of synesthesia from non-synesthetic associations, however,
recent research has shown that this form of synesthesia co-varies with other forms of
synesthesia and cases that are automatic and maintain consistency, are required to be
considered as a form of synesthesia. [10]
Plausible Theories
From the beginning of observing synesthesia, researchers have been contemplating about
possible mechanisms underlying this phenomenon. As more research has been done,
some theories can be ruled out, while others become more plausible.
Associative learning
Calkins (1893) was the first to suggest that synesthesia could be a form of associative
learning possibly learned throughout early childhood. This hypothesis initiate several
experiments that tried to “teach” adults one form of synesthesia using Pavlov
conditioning.8 However, early experiments as well as recent works found that, although
non-synesthetic individuals could be trained to learn these associations, there is no
evidence that real conscious, involuntary synesthetic experiences accompanied these
newly learned associations. [11,12]
There are also many arguments against this theory. For example, in order to learn these
associations, synesthetes should be exposed to hundreds or thousands of occasions with
the same pairs of different stimuli. Marks and Odgaard [11] suggest that this does not seem
to be very plausible; however I think that it can be plausible for some certain types of
synesthesia tone → color and personification because there can be countless stimuli of
these types in our environment in our everyday life.
Rich and Mattingley[12] argue that for many synesthetes, a few letters consistently elicit
colors in the same range. It is impossible that children all over the world consequently see
these letters in the same colors, which is expected if these results depend on learned
association. But I think there can be explanations for this so that if children learn these
associations early in childhood by colored magnet alphabets that are used for training at
schools, maybe the manufacturers and designers of these toys are also conditioned on the
same set of similar mappings. Furthermore, behavioral experiments [12,1] showed that
synesthesia is a more perceptual experience. These and other observations argue strongly
against memory associations. So, it is assumed that synesthesia has a neural basis.
Limbic activation, mediated by cortical inhibition
Cytowic [3] suggested that synesthesia is actually a normal brain function generated in
subcortical areas, mainly in the limbic system but its activations only reach consciousness
level in little number of people, mediated by cortical inhibition. His theory was based on
a fMRI experiment with xenon-133 inhalation in which a Shape → Taste synesthete
showed extensive decrease in cerebral blood flow which indicates a decrease of activity
in neocortex. This result sounds to be in line with the results of an ERP study of
grapheme → Color synesthetes. Schiltz and his colleagues observed positivity, which
indicates an inhibition, over frontal and prefrontal structures. Nevertheless, no further
evidence is found for this theory, and no other imaging study has confirmed these results.
On the contrary, activation of frontal areas is present when there is a mismatch between
the grapheme surface color and the induced synesthetic color.9
Cross-activation
Baron-Cohen suggests another theory based on the assumption that synesthesia represents
a breakdown in modularity of sensory areas. The hyperconnectivity theory indicates that
brains of synesthetes differ from normal brains in the way that they have extensive crosswiring between two or more different sensory modalities. The reason for these extra
connections could either be due to additional unnecessary growth or a failure of natural
pruning throughout development. Grossenbacher and Lovelace [2] proposed that dis8
9
eg. Kelly, 1934 and Howells, 1944
eg. Weiss et al. 2001 and 2005
inhibited feedback can be the mechanism that causes cross-activation. The dis-inhibited
feedback theory assumes no abnormal neural connections, but proposes that synesthesia
is entirely mediated by neural connections that exist in normal human brains. This theory
says that in normal individuals, the inhibitory mechanism prevents the activation between
different modules, while in synesthetes this inhibitory mechanism is not functioning
properly and this leads to a cross-activation of concurrents in a subordinate modality. [11]
The main support of this theory comes from primate studies which show that conscious
sensory perceptions are characterized by feedforward and feedback information flow,
mainly in the visual system.
These theories have led to an updating of the cross-wiring theory of Ramachandran and
Hubbard, and the developing of possible models. [11] For example, Grossenbacher and
Lovelace [2] proposed a mechanism characterized by feedforward and feedback
connections, in which three different routes can lead to an activation of the concurrent
representation, either through hyperconnectivity, or through dis-inhibition. [11] Smilek et
al. [13] suggested a re-entrant model based on dis-inhibited feedback. In his model, feedforward connections from V1, through V2, to V4 take color and form information of a
grapheme, which is processed and sent to Anterior fusiform and Posterior Inferior
Temporal (PIT) cortex. From here the synesthetic processing departs from normal
processing. Feedback connections from these areas back to V4 carry information about
the synesthetic meaning of the grapheme which influences the perception of an externally
presented figure. Although producing slightly different results, neuroimaging studies
support this idea that synesthesia results from cross-activation between different
modalities, for example between grapheme/auditory and color selective brain areas.
Studies that studied the response to hearing words in phoneme → color synesthetes,
reported an activation of areas ranging from visual association areas such as parietooccipital junctions and the posterior inferior temporal (PIT) cortex [15], to early visual
areas V4 and V8 [14], and even V1 10 that were not present in control experiments. Other
studies that examined grapheme → color synesthesia found activation in extra striate
cortex [18], intra-parietal cortex, fusiform gyros [16] and V4 [17], after perceiving words.
These activation patterns appear to be related to the synesthetic experience. V4 is
associated with color perception, while the PIT cortex is concerned with the integration
of color with shape.
It is known that both areas show strong attentional modulation which is consistent with
cognitive studies showing that graphemes can not induce synesthetic colors when
presented peripherally, and that synesthetic experience highly depends on attention. [1]
Also, it seems that the color percepts of synesthetes can be generated without primary
visual area activation. This finding matches with the report of a synesthete who
experienced ‘uncanny’ colors that he had not experienced in real world since he has scone weakness, and also the description of blind people that still perceive synesthetic ally
evoked colors. [20] This implies that if synesthesia works because of dis-inhibited
feedback, the feedback connection may not go as far back as primary visual areas.
10
Aleman et al. 2001
Finally, there appears to be cross-activation in synesthetic perception, but unfortunately,
we’re not sure if this is because of inhibition or excessive cross-wiring, as areas of brain
activity do not point to a certain mechanism.
Some important questions
Despite the similarities that let us define the general phenomenon of synesthesia, it
should also be noted that experiences of individual synesthetes vary in many ways. Some
grapheme → color synesthetes report that the colors seem to be "projected" out into the
world, while most report that the colors are experienced in their "mind's eye".11 Maybe
that is why no research has demonstrated a consistent association between synesthetic
experience and other neurological or psychiatric conditions yet, although this is an active
area of research.
It might be difficult to study on synesthetes for whom this experience is involuntary, so
we can not turn the phenomenon on or off and study the change in the mechanism that
controls their perception; however there are other situations that normal people can
experience synesthetic perception. Many people that have used hallucinogens claim to
have "seen sounds" or "heard colors" and this is clearly synesthetic experience which has
been “turned on” by a drug. Using hallucinating drugs is not the best way to experience
such marvel, but there also have been many reports on people experiencing the same
phenomena during meditation. Although in meditation one makes an attempt to minimize
perceptions through senses by detaching mind from sense organs in the brain to help to
silent the mind but in fact it increases consciousness. Also studies show that it causes
clearer perception. [8] My personal experience for several years of meditation gives me
the idea that not only the consciousness is increased but it’s quite different level because
during meditation and a little while after you can have a total consciousness of every part
of your body and every stimulus from environment somehow that it feels like a whole
new body or even your feet on the floor although you shouldn’t be conscious of this
perception because of adaptation.
So are the forms of synesthetic experiences based on altered consciousness, similar when
caused by drugs such as LSD and when caused by long meditation? Are there different
types of mechanisms that can cause synesthesia? If there is only one mechanism it
should be found in the mechanisms that Meditation and Hallucination share.
We know that meditation can cause some rewiring in the brain where hallucination in a
short term does not. [8] But they both cause changes in the level of neurotransmitters in
the brain. It is determined that hallucinogenic drugs structurally resemble serotonin and
cause an increase in the level of brain serotonin, but they inhibit the rapid firing of
neurons containing serotonin. This is a negative feedback system in which as the
serotonin level rises, the activity of serotonergic neurons decreases. On the other hand
meditation has no proven effect on serotonin level but long meditations dramatically
11
Dixon, Smilek & Merikle 2004
increase the level of melatonin in the brain which is a hormone secreted by the pineal
gland in proportion to the body's exposure to light. Considering the fact that serotonin is a
precursor of the hormone melatonin, hallucinogenic drugs must also increase the
melatonin level. I believe that study of this similarity, the melatonin level, might be a
worthwhile idea. One can compare the melatonin level of synesthetes and normal people
or to test the effect of harmless melatonin supplements that in low dose can help regulate
sleep but in higher dose can bring vivid dreams and nightmares – if taken at night before
sleep. Time release melatonin pills can help people with depression but can it cause
normal people to experience synesthesia?
As described earlier Cytowic suggests that cross-modal associations are a normal part of
our thinking that happen at an unconscious level. On the other hand synesthetes are aware
of these associations and discern them as “experiences”. All people experience what they
hear and see as distinct events - but we integrate these diverse experiences in forming our
thoughts based on the sensations they bring to our brains and we call them sensations and
feelings because we are not exactly aware of how they integrate. That is where synesthete
metaphors come from. Language metaphors are part of our everyday life and they are
trivial to every normal person but why a sound is sour when it’s unpleasant? How can
person be sweet if she’s nice?
Cytowic goes on to state that though we discriminate what we hear and see as distinct
events, experience shows that we can integrate them in forming thoughts about what
these sensations bring to our brains. The integration occurs at a level that we are not
accessible by our consciousness while the synesthetes’ experiences imply that there is a
conscious mixing of these sensory channels.
This is the theory that sounds the most interesting to me. I think I can design some
experiments to compare synesthetes and normal individuals based on their characteristics
of their conscious experiences in their concurrent modality. For example on the minimum
duration of stimulus presentation that evokes the conscious experience. My hypothesis is
that grapheme → color synesthetes’ time threshold for perceiving color might be lower
than normal people.
Also another very simple experiment can involve measuring the effect of unconscious
synesthete process - if it exists by forcing nonsynesthetes to pick colors for graphemes
although they do not have a conscious perception and check if there is any commonality
among them and if that matches of synesthetes.
References
[1] Vilayanur S. Ramachandran, Edward M. Hubbard,” Hearing Colors, Tasting Shapes”,
Scientific American, 2003, p 53-59
[2] Peter G.Grossenbacher, Christopher T. Lovelace, “Mechanisms of synesthesia:
cognitive and physiological constraints”, Trends in Cognitive Sciences Vol.5 No.1 p 3641, 2001
[3] Richard Cytowic, “The Man Who Tasted Shapes”, New York: Tarcher/Putman, 2003
[4] Sean Day, "Some Demographic and Socio-cultural Aspects of Synesthesia", in L.
Robertson & N. Sagiv, Synesthesia: Perspectives from Cognitive Neuroscience, Oxford:
Oxford University Pres, 2005
[5] Julia Simner, Jamie Ward et al., "Non-random associations of graphemes to colors in
synesthetic and non-synesthetic populations", Cognitive Neuropsychology 22(8): 10691085
[6] B.L. Jacobs. “How Hallucinogenic Drugs Work.”, American Scientist 75:3 p 85-92,
1987
[7] Alicia Ebbitt, “The Effects of Hallucinogenic Drugs on the Brain”
[8] M.P.Bhattathiry, “Neurophysiology of Meditation”,
[9] Sean Day, “Synesthesia and Synesthetic Metaphors“, in Simon Baron-Cohen,
Synesthesia: Classic and Contemporary, 1996
[10] Simner, J.; C. Mulvenna & N. Sagiv et al. , "Synaesthesia: The prevalence of
atypical cross-modal experiences", Perception 8(35): p 1024-1033, 2006
[11] Marks, L.E., & Odgaard, E.C. “Developmental constraints on theories of
synaesthesia. “ In L. Robertson and N. Sagiv (Eds.), Synesthesia: Perspectives from
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[12] Rich, A.N. & Mattingley, J.B.“Anomalous perception in synaesthesia: a cognitive
neuroscience perspective.” Nature Reviews Neuroscience, 3, p 43-52. 2002
[13] Smilek D., Dixon M.J., Cudahy C., Merikle P.M. “Synesthethic colour Experiences
Influence Memory” Psychological Science 13(6), p 548-552, 2002
[14] Nunn J.A., Gregory L.J., et al, “Functional Magnetic Resonance Imaging of
Synesthesia: Activation of V4/V8 by Spoken Words.” Nature Neuroscience 5(4), p 371375, 2002
[15] Paulesu E., Harrison J., Baron-Cohen S., et al. “The Physiology of coloured Hearing:
A PET Activation Study of colour-Word Synaesthesia.” Brain 118, p 661-676, 1995
[16] Weiss P.H., Zilles K., Flink G.R. “When Visual Perception Causes Feeling:
Enhanced Cross-Modal Processing in Grapheme-colour Synesthesia.” Neuroimage 28, p
859-868, 2005
[17] Hubbard, E. M.; A. C. Arman & V. S. Ramachandran et al. "Individual differences
among grapheme-color synesthetes: Brain-behavior correlations", 45(6) Neuron, p 975985, PMID 15797557, 2005
[18] Weiss P.H., Shah J., Toni I., Zilles K., Fink G.R. “Association colours With People:
A Case of Chromatic-Lexical Synesthesia.” Cortex 37, 750-753, 2001
[19 ] Mondloch C.J., Maurer D. “Do Small White Balls Squeak? Pitch-object
Correspondences in Young Children.” Cognitive, Affective, and Behavioral Neuroscience
4(2), p 133-136, 2004
[20] Steven M.S., Blakemore C. “Visual Synesthesia in the Blind.” Perception 33, p 855868, 2004