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Synesthesia: A Normal Mode of Cognition Miri Mineh CUNY: Brooklyn College Advisor: Professor Aaron Kozbelt, Psychology Department May 2010
Synesthesia: A Normal Mode of Cognition
Miri Mineh
CUNY: Brooklyn College
Advisor: Professor Aaron Kozbelt, Psychology Department
May 2010
Abstract
This paper explores the phenomenon of synesthesia; a neurological condition that arises
when stimulation in one sensory modality evokes an additional perceptual experience in
another sensory modality. I argue that synesthesia can be viewed as a normal mode of
cognition that remains latent in most individuals. The findings that support this claim
include the fact that synesthesia can arise in altered states of consciousness, as well as the
finding that synesthesia is present in all young human infants. After close examination of
the cognitive and neurological mechanisms that give rise to a synesthetic experience, the
claim can be made that we are all born with the ability to be synesthetes. To further
consolidate this claim, this paper examines various findings that explicate how the
normal brain can be induced to elicit a synesthetic experience among non-synesthetes.
Synesthesia: A Normal Mode of Cognition
What do Vasily Kandinsky (painter, 1866-1944), Oliver Messien (composer,
1908-1992), Charles Baudelaire (poet, 1821-1867), and Richard Phillips Feynman
(physicist, 1918-1988) all have in common? Aside from the fact that they have all
attained fame and recognition, they all have been reported to have had the neurological
condition known as synesthesia (Day, 2001). This phenomenon has only recently
attracted popular and scientific attention. However, synesthesia has proven to be quite
significant. Not only has synesthesia revealed to scientists and neuroscientists about the
workings of the mind of these famous authors, artists, composers, and physicists; it has
also proven to be quite revealing in regards to the underlying mechanisms of the
“normal” human mind as well.
Synesthesia means “joined sensation” (Cytowic, 2002), and it refers to a
neurological phenomenon that arises when stimulation in one sensory modality evokes an
additional perceptual experience in another sensory modality (Sagiv & Ward, 2006). For
example, a person may see a particular color (such as, blue) when they hear a specific
tone (such as, C sharp) or they may see a particular number as always tinged by a certain
color (for example, 5 is always seen as green). It is important to note that the main feature
of this neurological phenomenon is the fact that as one sensory modality is stimulated,
another modality is simultaneously stimulated without the presence of an actual stimulus.
For example, C sharp induces stimulation in the sensory modality involved in processing
hearing. However, there is simultaneous stimulation in the modality involved in
processing color (such as, blue) even though an actual color is not present in the real
world.
People who report having experienced at least one type of synesthesia are known
as synesthetes. Over fifty types of synesthesia have been reported (Day, 2001). Some of
the more common types of synesthesia include grapheme- color synesthesia (letters or
numbers are perceived as inherently colored), number-form synesthesia (numbers,
months of the year, and/or days of the week elicit precise three-dimensional areas in
space) and sound-color synesthesia (seeing colors in response to sounds or specific
tones). It is important to note that within one category of synesthesia many individual
differences exist. For example, one grapheme-color synesthete may report seeing the
number 5 as green, while another will report seeing it as red. Furthermore, synesthesia is
generally known to be unidirectional. For example, the number 5 will induce a synesthete
to experience the color green; however, seeing the color green will not induce the
synesthete to experience the number 5. In addition, some synesthetes can experience the
synesthetic sensation without actually being presented with the stimulus. That is, they can
experience the sensation, such as color, merely as a result of thinking about a specific
person, object, or number (Cytowic, 2002).
Although this is not the commonly accepted view of synesthesia, I will make the
claim that synesthesia can be viewed as a normal mode of cognition that remains latent
in most individuals. I will discuss the evidence found in the literature that has led various
researchers to make this claim as well. I will also examine various cognitive and
neurological processes that give rise to a synesthetic experience. My goal is to show that
because synesthesia employs the same basic cognitive mechanisms that are used in
normal daily information processing, we can deduce that most individuals have the
cognitive capability to experience synesthesia. Furthermore, from a neurological
standpoint I will show that the neuronal connections within the brain have the capacity to
elicit a synesthetic response as well. To further consolidate my claim, I will present
various findings that show how the normal brain can be induced to elicit a synesthetic
experience in non-synesthetes. Understanding the mechanisms that bring about
synesthetic experiences will reveal much in terms of normal cognition, and possibly
creativity as well.
Background
Synesthesia was first clearly documented by Sir Francis Galton in 1880 (Galton,
1880). It is important to emphasize, however, that synesthesia is a subjective experience
and thus, is not necessarily “measurable” and identifiable like most neurological
phenomena. A pervasive goal of the scientific community during the twentieth century, in
medicine and particularly in psychology, was to minimize, and if possible eliminate, the
subjective aspects of scientific research. For many years, the only way to identify
someone as a synesthete was based solely on the individual‟s description of their
synesthetic experience. The technology to detect the differences between synesthetic and
non-synesthetic brains did not yet exist. Hence, synesthesia was dismissed as merely an
individual‟s overactive imagination and it was not seen as a bona fide neurological
phenomenon. It was only recently that synesthesia was more rigorously investigated and
its implications for human perception and cognition truly appreciated.
Estimates of the prevalence of synesthesia vary dramatically from 1 in 20 (Galton,
1880), to 1 in 200 (Ramachandran & Hubbard, 2001), to 1 in 2,000 (Baron- Cohen et. al,
1996) to 1 in 20,000 (Cytowic, 1997). Day (2001) explains that the percentage of the
human population which has synesthesia varies depending on the type of synesthesia
involved. For example, estimates run from 4 in 100 for basic types of synesthesia (such
as seeing colored letters), to 1 in 15,000 for people with rarer types of synesthesia (such
as experiencing the sensation of taste as a result of experiencing the sensation of touch).
Day has reported that grapheme-color synesthesia is the most common type of
synesthesia.
Developmental synesthesia arises naturally without any external agents or brain
abnormalities. It requires no medical treatment and most synesthetes find their
experiences to be quite pleasant. Developmental synesthesia is genetic and has been
reported to run in families (Hubbard & Ramachandran, 2003). It has also been reported
that synesthesia can skip generations (Hubbard &Ramachandran, 2003). Ward and
Simner (2005) have explained that the genes involved don‟t lead to a specific type of
synesthesia. Rather, factors such as gene expression and developmental traits play a role
in the type of synesthesia the individual will experience. In addition, Ramachandran and
Hubbard (2001) report that having one type of synesthesia increases the likelihood of
simultaneously having a second and a third type of synesthesia as well.
Synesthesia has also been reported to have been experienced during altered states
of consciousness by non-synesthetes, namely; after brain or nerve injury, during an
epileptic seizure, or after ingestion of hallucinogens such as mescaline or LSD (Cytowic,
2002). It can also arise in healthy individuals between sleep and wakefulness as well as
in late-blind individuals; individuals with sight at birth who lost their vision later on in
life. In addition, synesthetic experiences have also been reported in a high proportion of
people who are engaging in meditation (Sagiv & Ward, 2006). Due to the fact that
synesthesia can arise in altered states of consciousness, I postulate that it is possible that
all individuals have the cognitive predisposition and the neuronal wiring to experience
synesthesia.
Cognitive Processes Employed in Developmental Synesthesia
Neurologist Richard Cytowic (2002) outlined five clinical conditions that must be
present in order for an individual to be diagnosed as a synesthete. The synesthetic
experience must be (1) involuntary and automatic, (2) spatially extended, (3) consistent
and generic, (4) memorable and (5) affect-laden. Examination of each of these conditions
individually will enable us to understand the underlying cognitive processes that bring
about a synesthetic experience.
Involuntary and Automatic
Synesthetes report that their synesthetic response is involuntary and automatic.
For example, in grapheme-color synesthesia the synesthete may be looking at a black
printed letter A, however they will report that are seeing an inherently colored A, such as
red. The “pop-out” phenomenon demonstrates that this synesthetic response is in fact
automatic. For example, Ramachandran and Hubbard (2001) presented subjects with
displays composed of graphemes (a matrix of randomly placed, computer-generated 5‟s).
Within the display, they embedded a shape, such as a triangle, composed of other
graphemes (computer-generated 2‟s). (See figure 1.) Five‟s are mirror images of 2‟s and
are made up of identical features. Non-synesthetic subjects find it hard to detect the
triangle composed of 2‟s. Synesthetes see the 5‟s as one color (such as green) and the 2‟s
as a different color (such as red). Therefore they found that it was very easy for
synesthetes to locate the triangle. It was as if they were seeing a red triangle in a green
background. Non-synesthetes found the task to be more challenging. This “pop-out”
phenomenon seen in the faster reaction time of synesthetes indicated to researchers that
the synesthetic response is indeed automatic.
There have been other studies reported by Cytowic (2002) that indicate that
synesthesia is evoked at such an early sensory level, it has led researchers to conclude
that in certain cases, the synesthetic experience is evoked at a preconscious level. For
example, synesthetes were presented with a target grapheme in their visual periphery.
The grapheme was surrounded with other letters; rendering the target grapheme as
“invisible;” i.e. not consciously perceived. However, despite the fact that the subjects
were unable to consciously identify the target grapheme, they were still able to
experience the synesthetic color. Normally, individuals with grapheme-color synesthesia
experience the synesthetic color as a result of consciously seeing the specific grapheme.
In this case, preconscious awareness of the grapheme was enough to elicit the synesthetic
response. After seeing the synesthetic color, subjects were then able to identify the
“invisible” letter. “I see red, so it must be the letter A.” It is important to note that these
findings do not contradict researchers‟ understanding of synesthesia as being
unidirectional (seeing the letter elicits the synesthetic response). In this special case of
synesthesia, the grapheme was first “seen” on a preconscious level and then the
synesthetic color was elicited. (Conscious awareness of the grapheme then followed.)
The fact that the synesthetic experience was evoked at a preconscious level,
demonstrated to researchers that it must be that the synesthetic experience is in fact
involuntary. Synesthetes are not voluntarily making these associations across sensory
modalities. The synesthetic experience is not due to the use of overactive imagination or
metaphor.
Researchers have reported that synesthesia is more common in color-blind and
blind individuals than the rest of the population (Sagiv & Ward, 2006). For example, a
subject was reported to have had S-cone deficiency, meaning that it was hard to single
out blues and purples. This subject had grapheme-color synesthesia and stated that he saw
letters in “Martian colors” (i.e., colors that he cannot see in the real world). Again this
supports the notion that the visual systems are being stimulated without the individual‟s
control.
Researchers have also learned that synesthesia is a perceptual phenomenon. Tasks
involving random-dot stereograms have demonstrated this. When a subject engages in a
task such as a random-dot stereogram the subject‟s left eye and right eye each see a set of
black dots. During the process of visual perception, the two images from each eye merge
together in the brain, which are offset slightly with respect to each other. This enables the
individual to see a three-dimensional object pop out from the plane which they are
viewing. This phenomenon is quite common and is referred to as binocular fusion
(Cytowic 2002). The only difference between synesthetes and non-synesthetes is the fact
that synesthetes see the object in color. From this, researchers learned that synesthetes
had to have first engaged in binocular fusion and began to recognize the grapheme before
they experienced the synesthetic color. Furthermore, we see that the synesthetic mind is
engaging in a type of processing called feature binding where color is being bound to a
form as the form is recognized.
This first criterion has outlined certain cognitive processes that are employed in
synesthesia. We have learned of the “pop-out” phenomenon, the role of binocular fusion,
and the fact that the synesthetic brain is engaging in a process of feature binding. We
have also learned that synesthesia seems to employ some kind of preconscious and
involuntary processing as well.
Spatially Extended
One defining feature of synesthesia that distinguishes it from ordinary vision and
imagination is the fact that when the synesthete experiences the sensation, that
synesthetic sensation occupies a certain sense of three-dimensional space for the
individual. Whether the synesthetic experience is a color in response to a sound, or a
grapheme tinged in a specific color, that sensation is experienced as if it is seen in a
three-dimensional space. Cytowic (2002) stresses that synesthesia is not limited merely to
“sensory-sensory” pairings such as hearing a tone and therefore seeing a color. In
number-form synesthesia, for example, the individual may see the months of the year as
actually occupying a certain amount of space with certain months as “closer” or “farther”
away from the individual. In certain types of synesthesia, where days of the week,
integers, letters, and months of the year are synesthetically joined to space, color, or
shape, we have to realize that those graphemes or number forms are categories of mental
concepts.
From this criterion we have learned that the synesthetic experience is seen as if
occupying an actual amount of space. We have also established the fact that synesthesia
is not limited to just “joined sensations.” The synesthetic mind can engage in binding
mental concepts to sensory experiences as well.
Consistent and Generic
Once a synesthetic association has been established, that association remains
consistent throughout life. For example, the letter A will always be seen as red by a
particular individual. Furthermore, Cytowic (2002) explains that when we say the
synesthesia is generic we mean that it is not complex or pictorial. Specifically,
individuals with synesthesia experience sensations like seeing basic geometric shapes or
colors, feeling cold, or experiencing a sour taste.
Memorable
Experiencing the synesthetic color, when seeing a number, or seeing the days of
the week as occupying a position in three-dimensional space, often enables the synesthete
to remember information quite well. Luria (1968) described his patient Shereshevsky (S)
in The Mind of a Mneumonist as possessing an extraordinary memory. Many attribute his
flawless memory to the fact that everything he recalled was accompanied by synesthesia
in each of his senses. For example, S describes one of his synesthetic experiences as
such:
“I heard the bell ringing…A small round object rolled right before my eyes…My
fingers sensed something rough like a rope….then a taste of saltwater….and
something white.”
One stimulus was able to trigger a synesthetic experience in each of his senses, thus,
rendering that specific stimulus all the more memorable.
Results of a study conducted by Smilek, Dixon, Cudahy, and Merikle (2002),
support the notion that synesthetic experiences can have a direct influence on memory.
The subject in the study was C, a student who experienced synesthetic colors when she
sees, hears, or thinks of digits. C reported that when she is asked to remember black
digits, she simply remembers the synesthetic colors instead of the black digits
themselves. Smilek et al. explained that somehow the “synesthetic colors may increase
the distinctiveness of the individual digits or create distinctive visual patterns” (p.552)
that are easier for C to remember than the patterns of the black digits. The exact
processes are unclear, but evidence like this supports the notion that synesthetic
experiences do play a role in the cognitive processes that are employed when a synesthete
engages in recall.
Affect-Laden
Individuals with synesthesia describe feeling strong emotions in response to the
multi-sensory stimuli they encounter. They describe an “incorrectly” colored number (a
number printed in a color that is incongruent with their synesthetic color, i.e., on the
opposite side of the color wheel (See Figure 2)) as “ugly,” and the experience is as bad as
hearing “nails scratching on a blackboard” (Ramachandran & Hubbard, 2001). A
correctly colored number (that is in congruence with one‟s original synesthetic
association) is described as feeling as one feels during an “Aha” moment when a solution
to a problem emerges. It is for this reason that many synesthetes describe their
experiences as quite pleasurable. From this criterion we learn that emotion plays a large
role in the synesthetic experience as well.
3 Types of Synesthesia: Parallels to Normal Cognitive Perception
As mentioned previously, there is a high prevalence of synesthesia during altered
states of consciousness. It is this fact that has led many in the scientific community to
wonder whether synesthesia is in fact dormant within all of us. The first step researchers
took in an attempt to answer this question was to try to figure out whether synesthesia
shares any commonalities with normal perception. Sagiv and Ward (2006) argued that to
find the commonalities between the synesthetic mind and the normal mind we must
explore situations where cross-modal interaction between various parts of the brain is
required.
Sagiv and Ward (2006) conducted their own experiments in order to assess the
processes synesthetes and non-synesthetes employ when engaging in cross-modal
interactions. They wanted to compare the associations made between auditory properties
(like pitch and timbre) with visual properties of color (like chromaticity and luminance).
The task required subjects to choose the “best” color (one sensory modality) that should
go with each tone presented (another sensory modality); thus requiring the subjects to
engage in cross-modal interactions. It was found that both sound-vision synesthetes and
non-synesthetes showed an identical trend to associate low pitches with dark colors and
high pitches with light colors. Evidence from this study suggests that there is a certain
cross-modal interaction that appears to be common across both groups of subjects,
hinting to the fact that sound-vision synesthetes may be employing the same cognitive
processes that non-synesthetes employ.
In grapheme-color synesthesia the synesthete sees the surface of the presented
grapheme (number or letter) as colored. This indicates that the individual is engaging in
feature binding, a process in which the brain binds perceived color and shape to allow for
the experience of seeing one unified object (Sagiv & Ward, 2006). However, in the case
of synesthesia the individual is only presented with the shape of the grapheme. The
synesthetic mind binds an associated synesthetic color that is not actually present, with
the perceived shape. Although differences among synesthetes and non-synesthetes exist,
this set of results indicates that the synesthetic mind is employing the same cognitive
process of feature binding that is employed in normal perception.
Sagiv and Ward (2006) further argued that attention is required in order for the
synesthete to perceive the grapheme as colored. During normal processes of feature
binding, attention is required in order to allow the individual to perceive an object as
unified. According to Sagiv and Ward, the synesthete cannot see the synesthetic color
unless attention was employed and the individual can identify the grapheme. Only then
will the synesthetic color be evoked. Researchers learned that not only is feature binding
common among synesthetes and non-synesthetes, but the process of attention that is
required for feature binding to occur is found to be a commonality as well.
In number-form synesthesia the individual experiences numbers as occupying a
particular spatial configuration (Seron , Pesenti, Noel, Deloche, & Cornet, 1992). Some
individuals see the numbers in their “mind‟s eye,” while others see them as occupying a
personal space around them. According to Walsh (2003), in normal perceptual cognition,
space is used as an organizing principle. It is used as a tool to construct concrete spatial
representations that will allow the individual to better understand and grasp abstract
concepts. This is what number-form synesthetes seem to be employing when seeing
numbers and months of the year in three-dimensional space. Numbers, months of the
year, days of the week are, as Cytowic (2002) explained, categories of mental concepts.
Seeing these concepts in a three dimensional space may reflect synesthetes‟ use of a basic
feature of human cognition: using space as an organizing principle to understand higher
order mental concepts.
Synesthesia: Neurological Evidence
Thus far, synesthesia has been presented in terms of the cognitive mechanisms
employed during a synesthetic experience. Furthermore, I have shown why researchers
believe that there exist numerous commonalities among the mechanisms used in certain
types of synesthesia and the mechanisms used in normal cognitive perception. However,
if in fact synesthesia is a truly neurological phenomenon then there must be neurological
evidence for its existence. In addition, this evidence should be able to identify differences
between synesthetes and non-synesthetes, while at the same time indicate why
commonalities among these two groups also exist.
Anatomical, physiological, and neuroimaging studies indicate that there are no
abnormalities in the brains of individuals reported to have synesthesia (Ramachandran &
Hubbard, 2001). No differences have been found, on either a macro or micro level (Sagiv
& Ward 2006). This set of findings could be taken to support many skeptics‟ beliefs that
synesthesia is nothing more than a “bogus” phenomenon.
However, neurologist Richard Cytowic (2002) argued that although no
differences have been found on a structural level of the brain, advances in medical
technology have made it possible to find neurological differences among synesthetes and
non-synesthetes. For instance, a difference has been found on a functional level of the
brain. Cytowic used a neuroimaging functional technique called regional cerebral blood
flow that is able to trace blood flow throughout various regions of the human brain. He
found that synesthetics‟ brains were much more agitated and activated by certain stimuli
than non-synesthetics‟ brains. For example, synesthetes with grapheme-color synesthesia
showed more brain activation after being presented with a letter, as opposed to non-
synesthetes. Furthermore, this study indicated that synesthesia was a phenomenon
associated with more overall activation in the left hemisphere of the brain.
Paulesu (1995) conducted a PET scan on individuals with sound-vision
synesthesia. A positron emission tomography (PET) scan is a type of neuroimaging
technique in which a radioactive element is injected into the body, and it is traced as it is
absorbed by the brain. This test reveals the area of the brain that is being activated as one
engages in a specific task or function. The synesthetic subjects in Paulesu‟s study
reported seeing colors in response to spoken words. Paulesu compared PET scans of
these synesthetes with PET scans of non-synesthetes while they were presented with
single spoken words. It was found that in both groups of subjects the auditory and
language areas of the brain were both activated. However, only in the synesthetic group
was the area of the brain associated with vision also activated.
A functional Magnetic Resonance Imaging (fMRI) study (Nunn, 2002) showed
that during a synesthetic experience, areas of the brain concerned with memory and
emotion are also activated. This type of brain scan measures the changes in blood flow
that arise throughout the brain as a result of neural activity. This study confirmed
researchers understanding of a synesthetic experience to be affect laden and to have an
influence on memory. Furthermore, the fMRI revealed that synesthesia appears to “hijack
an existing brain function” (Cytowic, 2002). In other words, when subjects reported
experiencing the synesthetic color, the area of the brain activated was the same area of
the brain that is involved in real color perception. This finding was very important. Now
that researchers understood which area of the brain is involved during experience of a
synesthetic color, they compared it to the area of the brain that is used when one engages
in imagination of a certain color. They found that when individuals engage in color
imagery they are using an entirely different functional area of the brain. Hence, this
evidence quieted skeptics that believed that synesthesia was nothing more than the use of
overactive imagination.
All these studies provide evidence indicating that synesthesia is truly a
neurological phenomenon with a strong biological basis, and differences do in fact exist
between synesthetes and non-synesthetes.
Neurological Processes Employed in a Synesthetic Experience
Ramachandran and Hubbard (2001) investigated grapheme-color synesthesia in
an attempt to explain the various processes involved in synesthesia in general. In their
Cross-Activation hypothesis they integrated all the various evidence that researchers had
obtained, both cognitive and neurological, in order to present a unified and clear
depiction of the processes that allow a synesthetic experience to come about.
For the past 100 years many have postulated that synesthesia is the result of
neuronal cross-wiring that exists between different areas of the brain (Harrison & Baron-
Cohen, 1997). Ramachandran and Hubbard supported this notion, and they based their
hypothesis on two main findings: a) synesthesia appears to be genetic; b) all human
infants were reported to have had some form of synesthesia that is lost during
development, through a process called neuronal pruning (Sagiv & Ward, 2006). Based on
this research, as well as recently obtained neurological evidence, Hubbard and
Ramachandran hypothesized that synesthetes‟ brains have cross activation, or hyper-
connectivity, across different areas of the brain. These connections existed in infancy for
both synesthetes and non-synesthetes alike. However, synesthetes have retained these
extra projections and neuronal connections that normally would have been pruned
throughout the course of development.
In their investigation of grapheme-colored synesthesia, they found that when
subjects were presented with a visual grapheme, such as a letter, the area of the brain
known as the fusiform gyrus was activated. This area is also known to be involved in
color processing. Ramachandran and Hubbard (2001) hypothesized that in grapheme-
color synesthetes hyper-connectivity must exist within the fusiform gyrus. They
explained that when neurons representing numbers are activated in one part of the
fusiform gyrus, due to hyper-connectivity, there is corresponding activation of neurons
representing color in another part of the fusiform gyrus. It is this cross-activation that
gives rise to the synesthetic experience that allows a number to be perceived as inherently
colored.
Ramachandran and Hubbard took their Cross-Activation hypothesis a step further
and attempted to explain the finding that synesthetes who were reported to have had one
type of synesthesia were more likely to experience another type as well. They explained
that a genetic mutation in a particular gene may be responsible for the lack of pruning of
neuronal connections. They emphasized that this genetic mutation may be expressed to
different extents and in different areas of the brains in each synesthete. Therefore, the
failure of neuronal pruning can happen at more than one site, thus allowing the individual
to experience more than one type of synesthesia.
Ramachandran and Hubbard (2001) also investigated acquired synesthesia in
order to find further evidence in support of their hypothesis. They examined a blind
patient who reported seeing tactile sensations as visual phosphenes (sensations of light).
The researchers explained that as a result of not being able to process any visual input
anymore, new neuronal pathways emerged between the two areas of the patient‟s brain
that process touch and vision. These new pathways are hyperactive and therefore account
for the fact that the patient experiences touch and vision simultaneously.
Ramachandran and Hubbard (2001) stated that if new pathways are not the
answer to explain this acquired synesthesia, the other plausible explanation is back-
projections. Neuronal connections and projections may already exist between these areas
of the brain within all individuals. These projections are not active, or are pruned, in
adults. However, the researchers argued that in the case of this patient, these latent
connections became hyperactive as a result of the loss of vision and the patient was now
able to experience the synesthetic sensation.
One final piece of evidence that supports the Cross-Activation hypothesis is based
on Cytowic‟s (2002) criterion that a synesthetic experience is affect laden. Ramachandran
and Hubbard (2001) found that the aversion some grapheme-color synesthetes reported
feeling to be an unwarranted emotional aversion. (The incongruent color felt like “nails
scratching on a blackboard.”) However, neurological imaging has found activation of the
limbic system, namely the amygdala, during a synesthetic experience as well. The limbic
system is the part of the brain that is involved in emotional responses. The researchers
now understood why a synesthetic experience is accompanied by a strong emotional
response. Hyperactive neuronal connections may also exist between the fusiform gyrus
(processing of grapheme and color) and the limbic system (affect laden).
Question:
Can we view synesthesia as a normal mode of cognition that remains
latent in most individuals?
We can make an attempt, at this point, to answer our original question of whether
or not we can view synesthesia as a normal mode of cognition that remains latent in most
individuals. Based on the neurological evidence that researchers have obtained, as well as
the cross-activation hypothesis, it appears as though the answer to our question is “no.”
Neurologically speaking, the “normal” brain does not have the wiring to experience
synesthesia. It does not exhibit hyper-activation when presented with a particular
stimulus, as synesthetic brains do. Furthermore, according to the cross-activation
hypothesis, the reason as to why this occurs is due to the fact that the extra neuronal
projections that are present in synesthetic brains are pruned during the developmental
process in non-synesthetic brains (Ramachandran & Hubbard 2001).
However, we need to be extremely cautious when answering the presented
question of whether we can view synesthesia as a latent mode of normal cognition.
Researchers have clearly documented instances in which synesthesia has been
experienced by non-synesthetes during altered states of consciousness, involving sleep,
drugs, meditation, and so on (Cytowic, 2002; Sagiv & Ward, 2006). This indicates that
the normal brain does have the capacity to elicit a synesthetic response, contrary to what
appears to be the case when one examines synesthesia solely from the neurological
perspective.
Answer:
A) There exist normal cognitive processes that still cannot be explained :
found in synesthetes and non-synesthetes alike
From the cognitive perspective, we have seen that synesthesia employs basic
cognitive mechanisms that are used in everyday information processing. In addition,
there are certain functions of the normal human brain that neuroscientists are still unable
to clearly explain; for instance, feature binding and constancy operations. It is these
functions that are found to be the common ground between synesthetes and non-
synesthetes. Exploration of these functions from a cognitive perspective, in addition to
the knowledge we have gained from the neurological perspective, will enable us to view
synesthesia differently, and ultimately will provide us with a different answer to the
question previously presented.
-Feature Binding
I have previously mentioned that one of the basic cognitive mechanisms that
synesthetes and non-synesthetes share is the process of feature binding (Sagiv & Ward,
2006). For a rich understanding of the nature of synesthesia, it is important that this
mechanism be explored further. When an individual is viewing an object, he or she is
bombarded with many different perceptual attributes of the object that need to be
processed. For example, when one is presented with a red apple, he or she sees that the
apple is red, round, and edible. The computational complexity that the brain must
overcome is that the brain needs to be able to bind all these different features so that the
object can be perceived as one entity; a red apple. The problem is that these different
features are processed in different areas of the brain; moreover, they are also processed at
different times. Cytowic (2002) explained that color is processed before motion which is
processed before form, and yet the human mind overcomes this computational
complexity through feature binding. The perceptual attributes, which are processed at
different times and in different regions of the brain, are somehow bound together in such
a manner that allows the individual to experience the object as one entity.
Neuroscientists do not completely understand the mechanisms employed during
feature binding; however, we can use the concept of feature binding to better understand
synesthetic experiences. For example, in grapheme-color synesthesia, the brain engages
in feature binding to allow the synesthete to see the grapheme as inherently colored. The
brain binds the synesthetic color to the grapheme as the grapheme is recognized, so that
the synesthete experiences the colored grapheme as one entity.
-Constancy Operations
Another mode of cognition that has neuroscientists baffled is the mind‟s ability to
assign objects their constant features even though the world around us is constantly
changing. Two constant features that the mind assigns to objects are color and form. Most
people assume they are seeing the color red because it reflects red wavelengths.
However, Cytowic (2002) explained that color is actually a property of the mind, not a
property of the world, by demonstrating that we are able to assign grass its constant
feature, the color green, despite changing circumstances. For example, we still see grass
as green even if there are changes in wavelength composition (sunlight or shade). In
regards to the constant feature of form, we still see the object before us as grass even if
we view it from a different angle.
How is this relevant to synesthesia? The most common forms of synesthetic
experiences involve color and form, both of which are brought about through constancy
operations employed in normal cognition. For example, Cytowic (2002) noted that when
a grapheme-color synesthete is presented with a grapheme in their visual periphery,
despite the fact that they are seeing the grapheme from a different angle, the synesthetic
brain is still able to process the form of the grapheme and is able to assign it its
synesthetic color. This is brought about through the process of constancy operations in
regards to form.
To further understand the process of constancy operations in synesthetes, in terms
of color, the concept of thresholds should be examined. As was stated previously, when a
synesthete is presented with a grapheme in the “wrong” color, the synesthete reports
experiencing strong emotional aversion (Ramachandran & Hubbard, 2001). “Wrong”
color is usually referring to a color that is on the opposite side of the color wheel. (See
figure 2). For example, a synesthete that tends to see the letter A as red, will report strong
emotional aversion to the letter A being presented in green. What is interesting about this
finding is the fact that strong emotional aversion is reported once a certain hue threshold
is passed. Strong emotional aversion has not been reported when the letter A is presented
in crimson red, for example. This suggests that the synesthetic brain is employing
constancy operations during synesthetic experiences. These constancy operations allow
the individual not to experience aversion despite the fact that there may be certain
differences in the hue presented.
The purpose of this section is to bring attention to the fact that the normal
cognitive processes that neuroscientists are still unable to fully explain, are actually used
by synesthetes to bring about their synesthetic experiences. This is a first attempt at
showing that the differences between synesthetes and non-synesthetes may not be as
clear-cut as was once believed.
Answer:
B) Cytowic’s Distributed System: an attempt to explain both synesthetic experiences
and normal cognition
In an attempt to answer the question presented, two contradictory facts have been
established. From a neurological standpoint, it is clear synesthetes differ from non-
synesthetes on a functional level of the brain. From the cognitive perspective, however,
we have learned that both synesthetes and non-synesthetes employ the same basic modes
of cognition, thus establishing that commonalities exist and the two groups may not be so
dissimilar. Cytowic (2002) proposes a model of brain organization, referred to as the
Distributed System, in an attempt to integrate the various explanations we have obtained
from the neurological and cognitive perspectives into a unified account.
Most older models viewed the brain as consisting of specific areas that are
designated for specific functions. Each area engages in one particular task and nothing
else. This understanding led many to view processing as occurring in sequential steps;
higher level processing is not initiated until lower level processing is completed. In the
distributed system model of the brain, function is perceived as distributed across various
structures. One specific structure is not limited to performing one specific function.
Furthermore, this model employs the view of top-down processing in the brain. This view
sees cognitive processing as occurring at various levels simultaneously. The final product
of lower levels of processing is not completed, yet higher levels are obtaining partial
information from lower levels and have begun processing already.
The distributed model takes it a step further. Not only is one area of the brain
participating in more than just one cognitive function, but, this area also isn‟t isolated. It
is connected to many other non-specific areas of the brain through neuronal connections.
A given function, therefore, isn‟t localized. It is connected to various other functions
throughout the brain which are each engaging in top-down processing.
-Synesthetic Experiences
In regards to synesthesia, Cytowic (2002) states that we should not view
synesthesia as resulting from connections between areas of the brain that have specific
functions. Rather, he proposes that we view synesthesia as employing trans-modal
modules. He explains that this term refers to certain areas of the brain that are not
function specific. These areas are also connected to other non-specific areas of the brain
through neuronal connections. The main point he stresses is the fact that when the brain is
triggered to perform a specific function, the delocalization of the function and the
connectivity to other areas and functions of the brain results in the individual having a
synesthetic experience.
These trans-modal modules allow for the construction of multi-sensory
representations of the world in synesthetes. As one type of sensory processing is
occurring (such as sound) another is simultaneously triggered (such as vision) due to
delocalization of function as well as neuronal connections. Furthermore, these trans-
modal modules help explain why synesthetic experiences are memorable and affect-
laden. These cognitive functions are also triggered through the same processes.
-Metaphorical Thinking
Cytowic (2002) argued that this model can also explain why the normal brain is
able to engage in higher order cognitive functions such as metaphorical thinking. In
synesthesia, neuronal connections between areas of the brain involved in perception
(color, form, and space), emotion, and memory allow for a synesthetic experience to
arise. Cytowic stated that the process of metaphorical thinking is quite similar. The
difference lies in the fact that these neuronal connections are between areas of the brain
related to abstract concepts and not sensory processing, as is the case with synesthesia
(Ramachandran & Hubbard, 2001). Metaphorical thinking arises when connections and
delocalization of function allow the individual to perceive the similar in the dissimilar
“an imaginative understanding of one thing in terms of another”(Cytowic 2002, p.24).
Furthermore, both synesthesia and metaphorical thinking engage in the mode of cognition
of constancy where the individual can see the same concept/object even though it can be
perceived or processed from various perspectives.
In sum, the distributed model of the brain (Cytowic, 2002) integrates the
information obtained from the neurological and cognitive perspectives. While I concur
that differences exist among synesthetes and non-synesthetes, this model is very
significant in that it clearly shows that metaphorical thinking is the link between
synesthetes and non-synesthetes. This model illustrates that the synesthetic brain and the
normal brain are structurally and functionally similar, due to the fact that neuronal
projections connecting various parts of the brain result in synesthesia (in synesthetes) and
metaphorical thinking (in non-synesthetes). Furthermore, both synesthesia and
metaphorical thinking employ the same cognitive mechanism of constancy, among other
mechanisms. Synesthesia is brought about due to neuronal connections between areas of
the brain that process sensory stimuli. The ability to engage in metaphorical thinking is
brought about in a similar way, through neuronal connections between areas of the brain
that process abstract concepts.
At this point we are better able to understand why synesthetic experiences have
been reported by non-synesthetes during altered states of consciousness, such as under
the influence of drugs, during an epileptic seizure, during meditation, and so forth
(Cytowic, 2002). Based on all the research thus presented, it is reasonable to argue that
the normal brain has the cognitive capacity and neurological wiring to elicit a synesthetic
response. The claim that synesthesia can be viewed as a normal mode of cognition that
remains latent in most individuals can now be made, and it has also been supported. The
distributed model has illustrated how the normal brain employs the same mechanisms
when engaging in metaphorical thinking as does the synesthetic brain during a
synesthetic experience. Furthermore, the fact that synesthesia is experienced during
altered stated of consciousness clearly attests to that claim.
The question that arises however, is why is it latent? If the normal brain has the
neurological wiring and cognitive capability to experience synesthesia why isn‟t
synesthesia experienced consciously by the non-synesthete? Why is it experienced only
during altered states of consciousness?
Savant Skills Latent in Us All
Through close examination of Snyder‟s (2009) hypothesis on savant skills we will
be better equipped with the knowledge to answer the questions previously stated. Snyder
explains that the savant syndrome is a condition in which people who are autistic or have
other mental disorders exhibit extraordinary skills. These skills are typically exhibited in
one of five areas: art, music, calendar calculating, mathematics, and mechanical/spatial
skills (Treffert, 2005). These skills are also usually associated with an exceptional
memory and a high incidence of synesthesia as well (Sacks, 2007).
Snyder (2009) explains that savants tend to have some kind of atypical brain
function that always allows them access to “raw” less processed information before it is
packaged into a holistic label. He terms this as failure in top-down inhibition (Snyder,
2004). Most normal brains are inhibited from accessing this raw information, and only
the end product (a label or concept) is what is usually brought to conscious awareness.
What differentiates savants from the rest of the population is the fact that they have
conscious awareness, or special access to this temporary information, even before it is
consolidated into a concept or label. This allows the savant to concentrate more on the
parts rather than the whole. It is this ability that is ultimately responsible for the savant
skills that are exhibited by the autistic brilliant mind.
Snyder (2009) hypothesizes that all savant skills, including synesthesia, are latent
within all members of the population. Snyder‟s argument is based on the finding that
savant skills can be induced through inhibition of the left anterior temporal lobe (LATL)
in the normal brain. He explains that most normal brains are concept-driven. This means
that processing is usually only concerned with meaning and labels. Therefore, networks
in the brain involved in processing and formulating concepts, found in the LATL, tend to
inhibit access to other networks that are concerned with detail. This is what is referred to
as top-down inhibition (Snyder, 2004). For example, when we are confronted with a
picture of a face, it is a lot harder to draw the face and see the details when we are aware
of the meaning of the picture. Being aware of the meaning suppresses our ability to focus
on the details. However, Edwards (1989) explains that we can learn to draw better by
turning the picture of the face upside down. Once we do that, we obscure the overall
meaning of the picture and therefore, we are better able to focus on the details of the
picture and ultimately we can draw better.
Low frequency repetitive Transcranial Magnetic Stimulation (rTMS) temporarily
inhibits neural activity in the LATL, where meaning and conceptual networks exist. Once
these networks are inhibited, they can no longer hinder our access to networks where
temporary information is processed. Therefore, we can now have conscious awareness of
the details that the brain is processing, and ultimately, artificially induced savant skills
can result.
It only follows then to ask, why are these savant skills, including synesthesia, not
consciously accessible? Why does the brain engage in top-down inhibition? Snyder
(2009) explains that deliberate top-down inhibition is used by the brain as a “principle of
economy.” Once a concept of an object is formulated, the actual temporary details used
by the brain to formulate the label or concept don‟t need to be brought to the individual‟s
conscious awareness (Snyder & Mitchell, 1999). To the brain, the object‟s concept is of
ultimate importance when processing the ever changing world around us. The details
used to arrive at that concept are not necessary to be brought to the individual‟s conscious
awareness.
This is a very useful strategy for the brain because it allows the brain to make
quicker decisions and it also allows for faster processing. For example, this is seen when
the brain is presented with partial or ambiguous information. (See Figure 3.) The fact that
the brain is concept-driven and focused on labels, means that the brain can see the holistic
“bigger picture” of a familiar situation, THE CAT, even when not all the information has
been clearly presented. Furthermore, this top-down inhibition also helps accelerate the
process of learning, because without consolidating information into meaningful groups
and labels the brain may become overwhelmed with the details (Seidenberg, MacDonald,
& Saffran, 2002).
In regards to synesthesia, the same argument can be applied. We are aware of the
fact that synesthesia arises during altered states of consciousness (Cytowic, 2002). This
means that synesthetic experiences (such as synesthetic color) are activated in the non-
synesthete when processing information like graphemes or sounds. The reason why the
synesthetic experience does not penetrate consciousness is due to top-down inhibition.
When a non-synesthete is presented with a letter, a synesthetic color arises as a result of
the neuronal connectivity (Ramachandran & Hubbard, 2001) and distributed function
(Cytowic, 2002) throughout the brain. However, this synesthetic color isn't necessary to
process the letter being presented. Based on Snyder‟s (2009) reasoning, the brain is
concept-driven and does not want to be overwhelmed with information. If every letter
that we encountered in our daily lives triggered a different synesthetic color that we were
consciously aware of, the brain would be overwhelmed with information. Hence, it is
plausible to assume that top-down inhibition prevents the non-synesthete from conscious
awareness of synesthetic experiences that are activated, due to the fact that the normal
brain is concept-driven and avoids information overload.
Is synesthesia inherited or learned?
-A Close Examination of Odor-Taste Synesthesia
Thus far, I have shown that synesthesia can be viewed as a normal mode of
cognition that remains latent in non-synesthetes. Furthermore, I have explained why the
normal brain engages in top-down inhibition to prevent synesthetic experiences from
penetrating consciousness. However, there are other aspects of synesthesia that remain to
be explored. Why is it that the synesthetic brain does not engage in top-down inhibition,
and yet the non-synesthetic brain does?
Based on the research reported by Hubbard and Ramachandran (2003), as well as
Ward and Simner (2005) there appears to be strong evidence for a genetic factor that
gives rise to synesthesia. These researchers have reported that synesthesia runs in
families. Furthermore, they have determined that specific genes that exist on the X-
chromosome may be responsible for the type of synesthesia that is expressed in
synesthetes. Findings such as these, have led many to accept the common view that
synesthesia arises as a result of gene expression.
However, further research has led me to believe that the neurological
phenomenon of synesthesia may not necessarily be fully explained by genetic factors
alone. Ward and Simner (2005) may have presented findings that suggest that
synesthesia is inherited. However, they have also found that it is actually quite common
for family members to experience different types of synesthesia. This suggests that the
gene(s) involved do not lead to a specific type of synesthesia. Furthermore, I have
previously stated that within one category of synesthesia many individual differences can
exist. One synesthete may report seeing the number 5 as green, while another may report
as seeing it as red. There is very little research found in the literature addressing and
explaining why different colors are experienced by two synesthetes when it is the same
stimulus that is presented. Both facts regarding synesthesia suggest that the genetic
explanation is inadequate.
Heer (2000) explained that with grapheme-color synesthesia (among other types),
the synesthetic experience arises as a result of seeing a number or letter, emphasizing that
number and letter systems must be learned. They are not inherited. Therefore, he
suggests that there must be some other factor that is contributing to the development of a
specific type of synesthesia during the learning process of letters and numbers.
Stevenson, Boakes, and Prescott (1998) have investigated odor-taste synesthesia,
that is, when an odor alone can induce the sensation of taste. Data regarding the
prevalence of odor-taste synesthesia has not been widely collected or investigated.
Ironically, the reason is due to the fact that this form of synesthesia is very common. It is
experienced by all members of the population, and therefore, it has escaped popular and
scientific attention (Stevenson & Boakes, 2004). For example, the majority of people
tend to experience odor-taste synesthesia when smelling certain odors such as vanilla.
These odors are almost always reported as smelling sweet. The reason these experiences
are considered a form of synesthesia is because there are no sweetness receptors in the
nose. Sweetness is associated with the sensation of taste. Therefore, this experience can
be identified as synesthetic, because as one sensory modality is stimulated (odor) another
is simultaneously stimulated (taste).
Stevenson and Boakes (2004) have investigated the mechanism by which odor-
taste synesthesia arises. They point out that describing an odor as sweet does not
necessarily indicate that the odor is actually producing the sensation of taste-sweetness.
Odors are usually difficult to describe. The term “sweet” may just reflect the lack of a
better term to describe what sweet odors have in common perceptually. Therefore, they
have tried to find evidence that shows that the sensation of sweetness experienced as a
result of smelling an odor, is the same sensation of sweetness that is experienced as a
result of tasting something sweet.
They conducted experiments that investigated a certain phenomenon known as
sweetness enhancement. When participants were presented with two sweet-tasting
solutions, one having a sweet odor and one without an odor, the solution that had the odor
was rated as sweeter than the solution without the odor. The participants experienced
what the researchers termed as sweetness enhancement when tasting the solution that had
the odor (Stevenson, Boakes, & Prescott, 1998). Stevenson and Boakes (2004) claimed
that this is a significant phenomenon. On the one hand, it has shown that odor-taste
synesthesia does in fact exist. The fact that both solutions contained the same amount of
sucrose, and yet the one that had the odor was rated as sweeter, indicated that the odor
component must be producing an added taste sensation. This experiment suggested that
the sensation of sweetness (taste) that is produced as a result of smelling an odor, is the
same sensation that is produced when actually tasting something sweet.
These experiments were also significant because they showed that odor-taste
synesthesia is the result of a certain type of learning that is related to classical
conditioning (Heer, 2000). Stevenson and Boakes (2004) refer to concepts found in
animal-learning theory in order to explain how this type of synesthesia can be learned.
They refer to a concept known as “unitization” (Hall, 2001). Unitization within the brain
occurs when certain types of events are experienced simultaneously; such as experiencing
a smell and taste when eating something containing vanilla. The chemicals within the
food or drink can stimulate both the taste and the olfactory system (smell)
simultaneously. Once these sensations are stimulated, they are then represented within
the brain as one event (smell-taste of vanilla), as opposed to two events (smell of vanilla
and taste of vanilla). Therefore, these two sensations are experienced as one perceptual
event, and are stored in odor memory as an odor-taste “unit”. The individual is not aware
of the fact that two events, smell of vanilla and taste of vanilla, have actually taken place.
For them, eating a food containing vanilla produced only one perceptual experience.
However, the individual learns to link the two events of smell of vanilla and taste
of vanilla even though they are unaware of the fact that these two events happened. How
does this occur? Later on, when the smell of vanilla is presented alone, this stimulus
activates the odor-taste “unit” of vanilla that has been previously formulated and is now
in odor-memory. When the memory is activated, the memory is actually experienced, and
the taste of vanilla is therefore simultaneously stimulated. The smell of vanilla alone
resulted in the sensation of taste; i.e. odor-taste synesthesia. This process where the part
(smell) is able to retrieve the whole (smell-taste) is what Stevenson and Boakes (2004)
believe is responsible for odor-taste synesthesia.
They explain that this synesthesia arises through a process known as odor-taste
learning that is associated with classical conditioning. Classical conditioning refers to the
process where we learn to make associations between various elements within our
environment that tend to occur simultaneously or in succession. These associations help
us predict various outcomes of certain events that may arise in the near future because we
have learned that these events tend to occur together. The interesting fact about odor-taste
synesthesia is that this learning can actually take place without a person being
consciously aware of it. Stevenson and Boakes (2004) have explained that we learn to
associate the taste of sweetness with the smell of vanilla without even realizing that these
two events had occurred together. We were only aware of the resulting unitary perceptual
experience, however, we nonetheless learned to associate the two events.
Stevenson and Boakes‟ (2004) research has revealed much in terms of synesthesia
in general, and their theory can be applied to other types of synesthesia as well. In regards
to grapheme-color synesthesia, we can postulate that throughout the learning process of
letters and numbers, grapheme-color synesthetes formulated some type of “unit” between
a grapheme and a certain color. For example, they may have seen the number 5 as red on
a refrigerator magnet (Campen, 2007). This results in one perceptual experience (seeing
a red 5) that really consists of two separate events (the color red and the number 5) that
occur simultaneously. This would surely explain why differences among synesthetes
occur. One synesthete sees the number 5 as green while another reports seeing it as red,
because while learning the grapheme, each individual formulated their own grapheme-
color “unit.” The fact that that this type of learning occurs without the individual‟s
awareness, explains why synesthesia occurs involuntarily and without the individual‟s
control. Furthermore, the concept of unitization can also explain how other factors related
to synesthesia arise. The association between grapheme and color remains consistent
throughout life because it is the same grapheme-color “unit” that is triggered every time
the grapheme is presented. In addition, the fact that synesthetic experiences tend to be
affect laden can also be explained by this reasoning. During the learning process, there
may have been a certain emotion that was experienced that also became integrated into
the grapheme-color “unit.” Therefore, when the grapheme is presented, not only is it
unconsciously associated with a specific color, but is it also associated with a particular
emotion.
There have been researchers who have been so bold as to claim that we all can
learn to become synesthetes. The fact that differences in associations among synesthetes
of the same type exist indicates that synesthesia is just a matter of each individual
learning to make their own subjective associations (Campen, 2007). Stevenson and
Boakes‟ (2004) theories may support their claims. However, the final answer to the
question of whether synesthesia is inherited or learned is not so apparent. Heer (2000)
states that the evidence presented by Stevenson and Boakes (2004) is not sufficient to
overturn the commonly accepted theories that view synesthesia as an inherited
neurological phenomenon. He explains that taste and smell are known to be highly
integrated senses; therefore, evidence for a learned synesthesia across smell and taste is
far from convincing to many in the field. Learned synesthesia would have to be exhibited
among more disparate senses in order for this theory to become the commonly accepted
view.
Conclusion
Throughout this paper I have attempted to explain the neurological phenomenon
known as synesthesia. The five clinical conditions that must exist in order for an
individual to be diagnosed as a synesthete were closely examined (Cytowic, 2002). In
addition, I discussed the evidence in the literature indicating that synesthesia can arise in
altered states of consciousness, as well as the research that has found that synesthesia is
present in all young infants (Sagiv & Ward, 2006). Studies that revealed that synesthetes
and non-synesthetes behaved quite similarly across various tasks that required cross
modal interactions, such as choosing which colors go “best” with a specific pitch, were
also presented and closely examined. All this evidence supported the claim that
synesthesia may in fact be a mode of cognition that is latent in all of us.
In order to assess this claim the various cognitive mechanisms that give rise to a
synesthetic experience were examined. Researchers have found that synesthesia employs
the cognitive process of binocular fusion, an aspect of normal cognition. Furthermore, we
saw that synesthesia is similar to normal cognition in the sense that both require attention
for feature binding to occur. We also found that synesthesia employs two very important
and fundamental aspects of normal cognition: feature binding and constancy (Cytowic,
2002).
The distributed model of the brain (Cytowic, 2002) integrated the information
obtained from the neurological and cognitive perspectives. This model clearly illustrated
that it is possible that we all have the predisposition to be synesthetes. Metaphorical
thinking is one link between synesthetes and non-synesthetes. Both synesthesia and
metaphorical thinking employ the same mechanism of constancy, and furthermore both
can be explained by the distributed model of the brain quite similarly.
All this knowledge regarding synesthesia served as evidence for the original
claim. We are all born with the ability to be synesthetes. Many can experience a
synesthetic sensation when meditating. Many get synesthesia as a result of becoming
blind. The question is why? The research all point to some kind of cross-activation of
neuronal connections among various areas of the brain, thus giving rise to a synesthetic
experience. The reason as to why the synesthetic experience is perceived on a conscious
level by only a few individuals, may be due to the fact that synesthetic brains don‟t
engage in top-down inhibition as non-synesthetic brains do (Snyder, 2004). Some have
claimed that this is controlled by the expression of a particular gene associated with the
X-chromosome (Ward & Simner, 2005 and Ramachandran & Hubbard 2001). Others
have been so bold as to claim that we all can learn to become synesthetes. (Campen,
2007). They reason that ultimately synesthesia requires an individual to make certain
associations among different senses. Their reasoning may not be so far fetched, as is
indicated by the existence of learned odor-taste synesthesia that is present in all
individuals (Stevenson & Boakes, 2004).
One of the reasons as to why this phenomenon has started to receive so much
attention by the scientific community is because it is so closely related, and may in fact
be a part of normal cognition. In fact, many have come to view synesthesia as an extreme
mode of normal cognition. This understanding of synesthesia has enabled scientists and
researchers to gain a clearer understanding of human perception and cognition in general.
It has revealed much in terms of organization of function throughout the brain.
Synesthesia has also enabled a clearer understanding of how the brain performs higher
order cognitive functions such as learning and metaphorical thinking. Most importantly,
however, synesthesia has revealed that there remains much more to be investigated and
clarified. Researchers have only begun to understand the mechanisms employed during a
synesthetic experience. They are still unclear as to whether synesthesia is inherited or
learned. We have only begun to answer the question of whether or not we can all learn to
become synesthetes. Experiments and research must continue to be conducted and
obtained. We may one day discover how to access the mechanisms, that synesthetes like
Messien and Feynman have accessed, that ultimately contributed to their famed success.
References
Baron-Cohen, S., Burt, L., Smith-Laittan, F., Harrison, J., & Bolton, P. (1996).
Synaesthesia: Prevalence and familiarity. Perception, 25, 1073–80.
Campen, C. van. (2007). The hidden sense: Synesthesia in art and science. Cambridge,
MA: MIT Press.
Cytowic, R. E. (2002). Touching tastes, seeing smells - and shaking up brain science.
Cerebrum, 4, 7-26.
Day, S. (2001). Types of synaesthesia. Retrieved May 5, 2010 from Sean Day‟s website:
http://www.users.muohio.edu/daysa/types.htm.
Day S.(2001). Famous synesthetes. Retrieved May 5, 2010 from Sean Day‟s website:
http://home.comcast.net/~sean.day/html/famous_synesthetes.html
Edwards, B. (1989). Drawing on the right side of the brain. New York, NY:
Tarcher/Putnam Press.
Harrison, J.E., & Baron-Cohen, S. (1997). Synaesthesia: A review of psychological
theories, in Baron-Cohen & Harrison (1997).
Hall, G. (2001). Perceptual learning: Association and differentiation. In R.R. Mower &
S.B. Klien (Eds.), Handbook of contemporary learning theories, (pp.367-407).
Hillsdale, N.J.: Erlbaum.
Heer, J. (2000). A review of synesthesia. In Psychology 127: Cognitive Neuroscience.
Psychology Department, University of California, Berkeley.
Hubbard, E.M., & Ramachandran, V.S. (2003). Refining the experimental
lever: a reply to Shannnon and Pribram. J. Consciousness Stud. 9, 77–84.
Galton, F. (1880). Visualised numerals. Nature, 21, 252-256.
Luria, AR.(1968) The Mind of a Mnemonist. New York: Basic Books.
Nunn, J.A., Gregory, L.J., Brammer, M., Williams, S.C., Parslow, D.M., Morgan, M.J.,
Morris, R.G., Bullmore, E.T., Baron-Cohen, S., & Gray, J.A. (2002). Functional
magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken
words. Nature Neuroscience, 5(4), 571-575.
Paulesu, E., Harrison, J., Baron-Cohen, S., & Watson, J.D.G. and others (1995). “The
physiology of coloured-hearing: a PET activation study of colour-word
synaesthesia.” Brain, 118, 661-676.
Ramachandran, V.S., & Hubbard E.M. (2001). Synesthesia: a window into perception,
thought, and language. Journal of Consciousness Studies, 8(12), 3-34.
References Cont.
Sacks O. (2007) Musicophilia: tales of music and the brain. New York, NY: Knopf
Publishing Group.
Sagiv, N., & Ward, J. (2006). Cross-modal interactions: lessons from synesthesia. In
Martinez-Conde, S., Macknik, S.L., Martinez, L.M., Alonso, J-M., & Tse, P.U.
(Eds). Progress in Brain Research, 155, 263-275. London: Elsevier
Science.
Seidenberg, M. S., MacDonald, M. C. & Saffran, J. C. (2002). Does grammar start where
statistics stop? Science, 298, 553–554. doi:10.1126/science.1078094
Seron, X., Pesenti, M., Noel, M.P., Deloche, G., & Cornet, J.A. (1992). Images of
numbers, or "when 98 is upper left and 6 sky blue". Cognition, 44, 159-196.
Smilek, D., & Dixon, M.J. (2002). Towards a synergistic understanding of synaesthesia:
combining current experimental findings with synaesthetes‟ subjective
descriptions. http://psyche.cs.monash.edu.au/v8/psyche-8-01-smilek.html.
Smilek, D., Dixon, M.J., Cudahy, C., and Merikle, P.M. (2002). Synesthetic
color experiences influence memory. Psychol. Sci. 13, 548–552.
Smilek, D., Moffatt, B.A., Pasternak, J., White, B.N., Dixon, M.J., Ward, J., & Simner, J.
(2005). Is synaesthesia an X-linked dominant trait with lethality in males?
Perception 34, 611–623.
Snyder A.W., Bossomaier T., & Mitchell D.J. (2004). Concept formation: „object‟
attributes dynamically inhibited from conscious awareness. J. Integr. Neurosci. 3,
31–46. doi:10.1142/S0219635204000361.
Snyder, A.W., & Mitchell, D. J. (1999). Is integer arithmetic fundamental to mental
processing? The mind‟s secret arithmetic. Proc. R. Soc. Lond. B, 266, 587–592.
Stevenson, R.J., & Boakes, R.A,.(2004). Sweet and sour smells: learned synesthesia
between the senses of taste and smell. Cambridge, MA: MIT Press.
Stevenson, R.J., Boakes, R.A., & Prescott, J. (1998). Changes in odor sweetness resulting
from implicit learning of a simultaneous odor-sweetness association: an example
of learned synesthesia. Learning and Motivation, 29, 113-132
Treffert D.A. (2005) The savant syndrome in autistic disorder. In M.F. Casanova (Eds.),
Recent developments in autism research (pp. 27–55). New York, NY: Nova
Science Publishers, Inc.. ch.2.
Walsh, V. (2003). A theory of magnitude: common cortical metrics of time, space and
quantity. Trends Cogn. Sci., 7, 483-488.
Ward, J., & Simner, J. (2005) Is synaesthesia an X-linked dominant trait with lethality in
males? Perception, 34(5), 611 – 623.
Figure 1:
(Ramachandran & Hubbard, 2001)
Figure 2:
( http://www.tigercolor.com/color-lab/color-theory/images/color-wheel-300.gif)
Figure 3:
(http://cliff.uconn.edu/TheCat.png)
