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DOI: 10.7759/cureus.67524
Grapheme-Color Synesthesia and Its Connection
to Memory
Stefani Anash , Andrew Boileau
1. School of Medicine, Saba University School of Medicine, The Bottom, BES 2. Neurology, Saba University School of
Medicine, The Bottom, BES
Corresponding author: Stefani Anash, stefanianash@gmail.com
Abstract
Synesthesia is the involuntary association of different senses, where individuals experience one sensory
modality in response to the stimulation of another. For example, a synesthete may perceive colors when
reading certain numbers or associate specific tastes with particular words. Synesthesia manifests differently
for individuals grouping the condition in subcategories such as grapheme-color, sound-to-color, lexical-
gustatory, mirror-touch, and much more. This review covers grapheme-color synesthesia, described as the
involuntary perception of specific colors or color associations when seeing or thinking about certain letters,
numbers, or symbols. This review explores the performance of declarative memory tasks in individuals with
grapheme-color synesthesia. A comprehensive search of controlled trials published between 2014 to 2024
was conducted through PubMed and Google Scholar databases. In Google Scholar, the search terms
grapheme-color synesthesia, grapheme-color synaesthesia, and memory were used. In PubMed, additional
MeSH (Medical Subject Headings) terms were used which included grapheme-color synesthesia and memory.
Studies that measured declarative memory and grapheme-color synesthesia were included yielding a total of
seven controlled trials. Grapheme-color synesthetes demonstrated advanced performance in declarative
memory tasks; however, this may not have any clinical significance. Grapheme-color synesthetes
demonstrated a better performance in their ability to recall colors, but not as much recalling words.
Synesthetes were shown to outperform non-synesthetes in visual memory tasks. Synesthetes showed better
recall of paired patterns, shape-color associations, and visual grids compared to control groups, but the
influence of synesthesia on word memory remains unclear. Future research should consider adding control
for confounding factors, collaborating with other institutions, and increasing sample size.
Categories: Neurology, Psychiatry, Psychology
Keywords: sensation, synesthesia, memory, declarative memory, grapheme-color synesthesia
Introduction And Background
"Synesthesia", or “synaesthesia”, has its origin in the Greek roots, syn, meaning union, and aesthesis,
meaning sensation referring to the union of the senses. According to the most accurate study of the
prevalence of synesthesia, it affects 4.4% of the world population [1]. It is the involuntary association of
different senses, where individuals experience one sensory modality in response to the stimulation of
another. Synesthesia manifests differently for individuals, grouping the condition into about 60 different
subcategories [2]. Examples include grapheme-color, mirror-touch, lexical-gustatory, and several more.
Grapheme-color synesthesia involves the associations of colors of certain letters or numbers. Synesthetes in
this category involuntarily perceive specific colors or color associations when they see or think about certain
letters, numbers, or symbols. For example, some grapheme-color synesthete will say the number six is
orange or the letter D is brown. The assignment of color to a number or letter is subjective and can vary
between individuals. Mirror-touch synesthesia is when one watches another person being physically touched
and feels as though they are being touched. Lexical-gustatory synesthesia involves tasting certain words
when hearing or reading those words. Synesthesia uniquely blends sensory perceptions creating a diverse
number of subtypes.
Synesthesia can be traced back to the mid-1700s when famous poet and philosopher Johann Gottfried
Herder documented his confusing experience with the condition. In 1848, a French physician Charles-
Auguste-Édouard Cornaz published a medical dissertation describing synesthesia as the opposite of color
blindness. The term “hyperchromatopsie” laid a foundation for what would eventually be termed
synesthesia [3]. Famous synesthete in the 1920s, Solomon Shereshevsky, recalled lists of digits, numbers,
street names, and more in a matter of minutes. His synesthesia quickly rose to attention when he was able
to memorize anything he was told by using visual mnemonics. He was followed for 30 years by
neurophysiologist Alexander Luria who first suggested in his case study that synesthesia may be linked to
better memory [4].
There is no widely accepted explanation of the physiology behind synesthesia and theories are continuously
expanding. One theory suggests that grapheme-color synesthesia can be associated with an increase in grey
matter in the left angular gyrus [5]. Another theory suggests that grapheme-color synesthesia may develop
1 2
Open Access Review Article
How to cite this article
Anash S, Boileau A (August 22, 2024) Grapheme-Color Synesthesia and Its Connection to Memory. Cureus 16(8): e67524. DOI
10.7759/cureus.67524
during childhood while learning colors and letters. When doing so, children start to associate the two and
take those associations with them into adulthood which eventually develops into grapheme-color
synesthesia [6]. Hancock et al. support this theory by stating the association of colors and letters can be
attributed to the colors of letter magnets commonly used on refrigerators in North America [7]. Children
learn colors and letters at the same time causing them to associate the two. Another theory suggests that
individuals with synesthesia have hyperconnectivity between different sensory regions of the brain [8]. The
cross-activation theory similarly states that there is cross-activation in different processing areas of the
brain including area V4 (associated with color perception) and the inferior temporal cortex [9].
Although it is not considered to be a clinical disorder, there have been questions as to whether synesthesia
might be either beneficial or detrimental to learning and memory. Van Leeuwen et al. suggest that
synesthesia is linked to autism spectrum disorder because of hypersensitivity to senses which interferes
with daily living [10]. On the other hand, Bremer et al. argue that most individuals consider their synesthesia
as a gift that does not interfere with their daily living, but rather enriches it [11]. The type of synesthesia
should also be considered when answering this question. Although it has been reported that synesthesia can
enrich senses and creativity, in a case study with a person with reverse word synesthesia, the patient
reported her synesthesia to be debilitating rather than enriching [12]. Her form of synesthesia caused her to
impulsively flip words backward whether she was reading or talking.
This review specifically considers grapheme-color synesthesia and declarative memory. Declarative memory
covers the recall of facts through long-term memory. While the mechanism behind synesthesia has been
widely studied since the 1900s, the link between synesthesia and memory did not start being studied until
the early 2000s when case studies looked at synesthetes individually and tested their memory. A case study
by Smilek et al. tested an individual in her memory abilities to recall colored digits in which she performed
near-perfectly [13]. Studies by Yaro et al. started gathering larger subject groups and had participants
perform memory tasks further suggesting that synesthetes might have better memory than non-synesthetes
[14]. Eventually, studies started to connect the idea that individuals with grapheme-color synesthesia may
perform better than the general population in declarative memory tasks. This paper aims to delve deeper
into the intricacies of synesthesia by exploring its connection to declarative memory.
Review
Methods
A comprehensive search was conducted through two databases: PubMed and Google Scholar. In Google
Scholar, the search terms "grapheme-color synesthesia" AND/OR "grapheme-color synaesthesia" AND
"memory" were used. In PubMed, additional MeSH terms were used which included (grapheme-color
synesthesia[MeSH Terms]) AND (memory[MeSH Terms]).
Both databases were searched using filters “past 10 years” and controlled trials only, yielding 58 records
from Google Scholar and 24 from PubMed. Records from both databases were combined and 10 duplicates
were removed yielding in a total of 72 results. Of those 72 results, each article was individually screened for
eligibility. A total of 43 records were excluded for not measuring declarative memory. Four records were
excluded for not studying grapheme-color synesthesia. Additionally, 18 articles were excluded that did not
pertain to the hypothesis in other ways. In total, there were seven articles that were eligible for analysis
(Figure 1).
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 2 of 10
FIGURE 1: PRISMA-style flow diagram
PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses
Results
For the selection process in this review, seven papers fit the inclusion and exclusion criteria. All primary
papers found were controlled trials using various memory tests as their research design. Some of these
studies demonstrated an advantage to declarative memory recall in those with grapheme-color synesthesia
depending on the type of memory being tested.
Recruited from the United Kingdom Synesthesia Association website via the University of Sussex, Pfeifer et
al. compared grapheme-color synesthetes to two different control groups separated into young adults and
older adults [15]. The young adult control group (n=14) was aged 19-29 years (M=22.64) with eight females.
The older adult control group (n=14) was aged 62-83 years (M=68.79) with nine females. The synesthesia
group (n=14) had an age range of 19-31 years (M=22.50) with nine females. Participants were given three
pairs of black-and-white photos with similar patterns and five pairs of dissimilar patterns. The three groups
were instructed to memorize the eight total pairs. After learning the paired pattern associations, participants
were presented with one picture of the pairs and were required to match them to their matching photo.
The difference in results was analyzed through analyses of covariance (ANCOVA) through partial eta
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 3 of 10
squared (ηp2). The accuracy of paired pattern recall of each picture was observed and the measurement of
accuracy was defined as the hit rate. The average dissimilar hit rate for synesthetes was the highest at 81.48 ±
1.54 (mean ± SD) and the lowest in the older adult group (67.22 ± 2.53). The control young adult group fell in
between the two (79.45 ± 1.90). The number of times required by each participant to learn a full set of eight
pair associates is defined as the number of runs through each round. Older adults had the greatest number
of runs (7.93 ± 1.23) while synesthetes had the lowest number of runs (3.21 ± 0.30). Young adults fell in the
middle with the number of runs (3.21 ± 0.30). The increased number of runs had a significant effect on the
contribution to the hit rate for dissimilar pairs (p<0.001, ηp2 = 0.307). In recalling the three similarly paired
pictures, the number of runs did not significantly predict the hit rate (p = 0.124, ηp2 = 0.061). Hit rates were
high in all three groups showing no difference (Young adults: 96.87 ±1.40, older adults: 91.23 ± 3.83, and
synesthetes: 98.93 ± 0.73) [15].
The design of Bankieris and Aslin's study consisted of shape-color pairings [16]. Participants included a total
of 14 self-reported synesthetes (26.7±11.6, mean±SD, four males) and a total of 15 non-synesthetes (20.1 ±
3.0, seven males). The study design comprised a three-by-three grid with nine various white snowflake
shapes along with nine randomly assigned colors with each respective snowflake shape. In the pretest phase,
participants were given the nine snowflake shapes and asked for any previously associated colors. This
method was used to solidify the control group in the study. Subjects who reported associating a specific color
with a snowflake were further analyzed to be put in the synesthetes group rather than the control group. The
learning phase and testing phases were divided into seven blocks. During the learning phase, participants
were presented with a three-by-three grid of white snowflakes. Touching each snowflake on the touchscreen
computer allowed participants to learn their respective associated colors assigned by the study. After 36
touches, the learning phase automatically ended and was quickly followed by the testing phase. In this
phase, each white snowflake shape was presented individually on a screen and participants had to choose its
assigned color via a color picker and luminescence slider. Participants were asked to return two weeks later
to go through the same testing phase to assess long-term memory.
Bankieris and Aslin analyzed the results via a “mixture model analysis” to determine if participants were
able to accurately pick color and shade based on memory. The level of significance of the hypothesis being
tested was defined as . Correctly choosing the color of the target with variability was defined as TS for
synesthetes and TC for the control group. Choosing the color of a nontarget item with variability was
defined as NS for synesthetes and NC for the control group. Randomly guessed colors were defined
as RS for synesthetes and RC for the control group. Through this mixture model analysis, it was found that
the synesthesia group was able to reproduce the hue of colors more accurately than the control group (TS =
0.77, TC = 0.54, p < .001) and fewer nontarget responses than controls ( NS = 0.06, NC = 0.36, p < .001).
Participants demonstrated similar results two weeks later with the synesthesia group reproducing the hue of
colors more accurately than the control group (TS = 0.88, TC = 0.77, p < .01) and a trend toward fewer
nontarget responses than controls (NS = 0.02, NC = 0.07, p = 0.06). The overall analysis of this study
demonstrated that synesthetes learn and recall shape-color at a higher and faster advantage in both the
short term and long term [16].
In an autobiographical memory recall study, Chin and Ward recruited 44 grapheme-color synesthetes (mean
age=40.4 years, SD=14.5; 25 females) and 40 non-synesthetes (mean age=43.6 years, SD=11.6; 39 females)
[17]. Both groups participated in an online survey where they were presented with six neutral cue words:
chocolate, train, doctor, snow, vacation, and party. Participants were then asked to describe detailed
memories from two distinct time periods: early childhood (ages 0-12 years old) and a recent timeframe
ranging from three months to three years ago, for each cue word. The description had to be specific
including all senses they could remember. Following their description, they were asked to answer 14
questions involving each sense. Senses were broken down into four subsections. The recollection section
involved five questions covering details, sound, sight, reliving, and emotion. The belief section involved
questions on spatial awareness, coherence, time, confidence, and agreement that they saw the event with
their own eyes and if the event really occurred. The rehearsal section asked questions regarding whether
they had thought of this memory before the study or if it came to them out of the blue. The last section
asked the subject how this memory impacts them.
Synesthetes were able to report a more recollective experience as compared to their control counterparts (p
< 0.001, ηp2 = 0.232). Synesthetes demonstrated that they were able to preserve their remote childhood
memories more than non-synesthetes (p < 0.001, ηp2= 0.544). Non-synesthetes were shown to have
forgotten more from their childhood. Because the time period between present time and childhood varies for
different age groups, age played a factor in the data. Effects of enhanced childhood memories in synesthetes
were not affected by age (p > 0.100). Whether the participant was younger or older, the sharpness of
childhood memories remained unchanged. A difference was found in rehearsal where non-synesthetes
reported better recent memory recall (p = 0.004) while synesthetes showed better childhood memory recall
(p = 0.026) [17].
Simner and Bain tested memory recall advantage in five randomly sampled 10-11-year-old children with
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 4 of 10
grapheme-color synesthesia [18]. Participants were recruited from screenings performed in previous years by
the research group. Synesthetes were compared to a control group of 40 participants with the same mean
age. The control group was broken up into 20 children categorized as having high memory and 20
categorized as having average memory. The letter-span task measured the ability to hold verbal information
for a short time period known as the phonological loop function. Participants were verbally told a series of
letters and asked to immediately recall given letters verbally at a pace of one letter per second. The letter
matrix task tested the same five synesthetes and 10 high memory and 10 low memory controls. In this test,
subjects were instructed to view a four-by-four grid filled with letters for one minute, after which the screen
went black, and they were required to fill in a paper copy with as many letters as they could remember with
no time limit.
Accuracy and speed of recall were measured in the letter-span task and performance was analyzed via
ANOVA. There were six trials and letters ranged up to nine. Participants had to correctly recall four of the six
trials to proceed. After three incorrect trials, the testing stopped, and the maximum letter-span set was
measured. The higher the letter-span number represented memory advantage. Children with grapheme-
color synesthesia showed a non-significant trend of memory advantage over the average memory control
(p=0.06) and showed no difference in comparison to the high-memory controls (p = 0.7). In the letter-matrix
task, data showed that there was no significant difference between the three groups (p = 0.7). Entering data
into a Bayes analysis gave a Bayes factor of 0.18 (a Bayes factor less than 0.33 shows strong support for the
null hypothesis) [18].
Lunke and Meier recruited 52 grapheme-color synesthetes and matched them to 52 controls according to
age, gender, and level of education [19]. Synesthetes were sourced from the Synesthesia-Check website
created by the program. Volunteers were given 24 words they had to memorize in addition to 19 filler words
such as days and months. In the study phase, they were given each word on a screen one at a time and were
required to pick one of 13 colors that they believed were associated with that word. One hour later,
volunteers were presented with old and new words and had to categorize them respectively. The accuracy of
organization tested the participants’ short-term memory.
To assess general short-term memory performance, results were analyzed via a two-by-four-by-three
ANCOVA and showed no main effect for synesthesia (p = 0.441, ηp2 = 0.01). Between synesthesia and type of
stimuli, an interaction occurred (p = 0.002, ηp2= 0.12). Age, used as a covariate, showed a main effect but did
not interact (p < 0.001, ηp2 = 0.26). Post-hoc tests showed that grapheme-color synesthetes had an
advantage for color stimuli (p = 0.012, d = 0.63) and for music (p = 0.049, d = 0.46) and a disadvantage for
words (p = 0.036, d = -0.50) [19].
Grapheme-color synesthetes showed memory advantage in correct recall of given words defined as the z-
transformed Pr scores for each type of synesthesia. Memory advantage was present in grapheme-color
synesthetes when given color stimuli (0.51±0.16, mean±SD) as opposed to their control counterparts
(0.45±0.13). Performance was consistent across different colors. However, the presence or absence of color
stimuli did affect performance. When presented with words alone without a color stimulus, synesthetes
showed little difference in memory advantage (0.47±0.26) as compared to the control group (0.58±0.16; p >
0.05) [19].
To assess recollection of words, Lunke and Meier conducted repeated measures of ANCOVA for recollection.
The study looked for interactions due to the influence of different variables. An effect was found for
synesthesia (MSE = 0.05, p = 0.021, ηp2 = 0.03). However, no main effect was found for the type of
synesthesia (MSE = 0.05, p = 0.725, ηp2 < 0.01). There was no interaction found between the two (MSE = 0.05,
p = 0.207, ηp2 = 0.02). There was a significant interaction between the type of synesthesia and the type of
stimuli (MSE = 0.03, p = 0.012, ηp2≤ 0.05). Age, as a covariate, also showed a significant main effect (MSE =
0.05, p < 0.001, ηp2= 0.11) and interacted with type of stimuli (MSE = 0.03, p = 0.004, ηp2 = 0.03). In the
posthoc tests, grapheme-color synesthetes presented higher recollection for colors (p = 0.044, d = 0.47) and
lower recollection for words (p = 0.04, d =-0.49) [19].
In their first experiment, Mealor et al. recruited 17 older synesthetes with an average age of 64 years (SD =
6.91) and 29 control non-synesthetes with an average age of 67 years old (SD = 6.09) [20]. In the young
group, there were 22 synesthetes with an average age of 23 years (SD = 3.39) and 20 controls with an average
age of 23 (SD = 2.81). Grapheme-color synesthetes were confirmed via a standardized test on the
synesthete.org website. In a random order, participants were given memory stimuli including three-digit
numbers, images of snowflakes, and short musical passages. Each module had a training phase in which they
were instructed to memorize the stimuli and a testing phase. The testing phase presented the stimuli and
compared them to a new or old set of stimuli. Participants were instructed to click response keys if they
perceived the given stimuli as old or new.
The overall performance or accuracy was measured by d-prime in a two-by-two-by-three ANOVA.
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 5 of 10
Individually, there showed to be a significant effect of age (ηp2 = 0.096, p=0.002) and synesthesia (ηp2 =
0.150, p<0.001) on memory. Performance of both young and old grapheme-color synesthetes showed higher
d-prime scores than controls. Young synesthetes showed a d-prime score of 0.88 with a standard error of the
mean as 0.06. The young control group showed a d-prime score of 0.61 with a standard error of the mean of
0.05. The older synesthetes group showed a d-prime score of 0.69 with a standard error of the mean of 0.07.
The older control group showed a d-prime score of 0.50 with a standard error of mean of 0.05 [20].
There was no significant interaction between age and synesthesia (ηp2 = 0.007, p=0.424). Because there was
no evidence of a correlation between the two, this study showed that synesthesia does not allow for a
protective effect on age-related memory decline. There was a main effect of modality (p < 0.001, ηp2 = 0.087)
and an interaction between age and (p < 0.001, ηp2 = 0.121) with the musical stimuli showing the largest age-
related loss. No other interactions were significant (p > 0.1). Bayes factor (BF) was used to further investigate
the lack of statistical significance between age and synesthesia. The Bayes factor was found to be 0.25,
providing evidence against the theory that synesthesia has protective effects on memory decline. The
authors concluded that the memory enhancement associated with synesthesia does not influence memory
decline in aging [20].
In their second experiment, Mealor et al. recruited 18 older synesthetes aged 65±4.52 years and 21 older
controls aged 67±5.63 years [20]. They also recruited 18 young synesthetes (age 23±3.01 years) and 84 young
controls (22±2.50 years). Grapheme-color synesthetes were confirmed in the same manner as the first
experiment. In this experiment, subjects were instructed to memorize a five-by-two grid in the learning
phase. The grid contained shapes and colors along the grid and it stayed up throughout the whole learning
phase. While the grid remained within sight, 10 stimuli appeared on the computer screen for three seconds
each. Similarly to the first experiment, during the testing phase, participants were given some old and new
shapes and colors along the grid and were instructed to identify them. Participants identify old as “correct”
and new as “incorrect”. This tested shape, color, and location.
Overall performance was measured using d-prime and a two-by-nine-by-two ANOVA showed a significant
main effect of synesthetes outperforming non-synesthetes (p = 0.033, ηp2 = 0.033). There was also a
significant main effect of younger people outperforming older people (p = 0.014, ηp2 = 0.043. Synesthesia
and age did not show significant interaction (p = 0.573, ηp2 = 0.002). Bayes factor calculation shows evidence
in agreement with the null hypothesis stating there is no significance between synesthesia and its
protective effects on memory decline (BF = 0.41). The study measured color, shape, and location
performance individually by using d-prime in a three-by-two-by-two mixed ANOVA. Significance was seen
with the main effects of age (p = 0.002, ηp2 = 0.067), and synesthesia (p = 0.005, ηp2= 0.055). The interaction
between age and synesthesia was not significant (p = 0.351, ηp2= 0.006). Significance was seen with the
effect of stimulus (p < 0.001, ηp2= 0.346). Stimulus and age showed a significant interaction (p = 0.022, ηp2=
0.030). There were also significant interactions between stimulus and synesthesia (p = 0.035, ηp2= 0.024).
There was no significance in interactions between stimulus, age, and synesthesia together (p = 0.198, ηp2=
0.012) [20].
Lunke and Meier studied grapheme-color synesthesia and its role in recognition memory through retaining
words and colors individually [21]. The study design consisted of 19 grapheme-color synesthetes (17 female
and two male) and 79 controls recruited via the Synesthesia-Check website from the University of Bern. The
mean age for synesthetes is 46.79 years (SD=18.19) and 48.21 years (SD=18.03) for the control group. The
study was broken down into two phases: the study phase and the recognition phase. In the word study phase,
participants were instructed to read a list of words on the screen, one at a time. They were told to select a
color that was best associated with each word. In the color study phase, participants were given different
color patterns and had to study them for three seconds each. After that, they rated how much they liked the
patterns on a seven-point scale. During the word recognition phase, participants were given a mix of new
words and words used in the study phase, and they had to recognize whether the words were old or new. In
the color recognition phase, participants were presented with old and new color patterns and had to identify
which one was old and new. The test was conducted through two sessions which were one year apart.
The results of their study showed grapheme-color synesthetes had a longer memory advantage because they
had a smaller forgetting score as compared to the control group. The forgetting score was used in both the
word and color recognition phases. Using a two-by-three ANOVA showed a significant effect of synesthesia
(p = 0.048, MSE = 0.06, ηp2 = 0.11) and a significant effect of type of stimuli (p < 0.001, MSE = 0.03, ηp2 = 0.36)
but no interaction between the two (p = 0.492, MSE = 0.03, ηp2 = 0.02). Between the first and second
sessions, grapheme-color synesthetes had a forgetting score significantly smaller for color recall (p = 0.051,
Cohen’s d = −0.55), and words (p = 0.045, Cohen’s d = −0.57), whereas the decay for music was not significant
(p = 0.477, d = −0.24). Overall, the study showed a smaller forgetting score for colors rather than words
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 6 of 10
supporting grapheme-color synesthetes’ long-lasting memory with colors specifically [21].
Table 1 shows a summary of the findings of the included studies.
TpFirst
Author Objective Study
Design
Level of
Evidence Study Population Therapy or
Exposure
Results
Summary
Bankieris
and Aslin
(2016)
[16]
To assess whether
synesthetes demonstrate
superior short-term
memory tasks in recalling
snowflake-color pairings.
Control
Trial 2
14 synesthetes and 15 non-
synesthetes from the Rochester
area
Snowflake color
pairings
Synesthetes
learned
snowflake-color
pairings more
quickly than
controls.
Synesthetes
demonstrated
better color recall
than their peers.
Chin and
Ward
(2017)
[17]
To study childhood
memory recall in
synesthetes and non-
synesthetes.
Control
Trial 2
44 grapheme-color synesthetes and
40 non-synesthetes in the
University of Sussex
Autobiographical
memory tests.
Synesthetes
reported sharper
childhood
memories as
compared to non-
synesthetes
reported more
vivid memories
from adulthood
Lunke
and Meier
(2018)
[19]
To assess memory recall
in grapheme-color
synethetes when given
color stimuli.
Control
Trial 2
52 grapheme-color synesthetes and
matched them to 52 controls
according to age, gender, and level
of education
Color and word
recognition in
random order.
The results
showed a memory
benefit with color
stimuli in those
with grapheme-
color synesthesia.
Lunke
and Meier
(2020)
[21]
To evaluate long-term
recall in grapheme-color
synesthetes compared to
their control counterpart.
Control
Trial 2
19 grapheme- color synesthetes
recruited via the synesthesia-check
on the website of the University of
Bern and 76 healthy control-
participants matched for age,
gender and education participated.
Recognition
memory tests.
The advantage for
memorizing color
shown by
grapheme-color
synesthetes was
persistent after
one
year compared to
their matched
controls.
Mealor et
al. (2019)
[20]
To evaluate protective
effects of memory in
grapheme-color
synesthetes compared to
non-synesthetes.
Control
Trial 2
17 older synesthetes, 29 older
control, 22 young synesthetes, 20
control
Recognition
memory tests.
Grapheme-color
synesthesia does
not have a
protective effect
against memory
decline in aging
Pfeifer et
al. (2014)
[15]
To compare memory
advantage in grapheme-
color synesthetes
compared to non-
synesthetes in young and
old age groups.
Control
Trial 214 young synesthetes, 14 young,
and 14 older adults.
Visual
associative
learning
computer
program.
The results show
a subtle
associative
memory
advantage in
synesthetes for
non-synesthesia
inducing stimuli,
which can be
detected against
older adults.
Synesthetes
demonstrated
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 7 of 10
Simner
and Bain
(2018)
[18]
To evaluate enhanced
abilities in performing
tasks in synesthetes
compared to their control
sounterpart.
Control
Trial 2Group of randomly sampled child
synesthetes age 10 and 11 years.
Cognitive
memory tests.
above-average
performance in a
processing-speed
task and a near-
significant
advantage in a
letter-span task.
TABLE 1: Summary of study findings
Discussion
Every study focused on the short-term memory of participants and one study additionally included a follow-
up test to assess long-term memory one year later. All the studies involved experiments focused on testing
memory. Examples included tasks such as memorizing and recalling words, pictures, grids, or a combination
of different stimuli.
The studies that used pictures for the memory studies were by Pfeifer et al. [15] and Bankieres and Aslin [16].
Pfeifer et al. gave participants eight pairs of different black-and-white photos [15]. Three of those pairs
posed pictures that were similar but not similar enough to be obvious. They were tested on their visual
memory and observed based on age and the presence or absence of grapheme-color synesthesia. Overall, the
study found a memory advantage in grapheme-color synesthetes over their control counterparts. Although
they presented with an advantage, it was not enough to be seen with the young controls. Advanced memory
performance was more apparent when synesthetes were compared to older control participants suggesting
the performance was not enough to consider synesthesia as an advantage to memory. Synesthetes may use
their synesthesia as a mnemonic tool, but it would not be enough to set them apart from non-synesthetes.
Bankieries and Aslin conducted a similar study but with black-and-white snowflake shape pairs on a three-
by-three grid [16]. The study concluded that grapheme-color synesthetes learn and recall visual images
better and faster both short and long term. Although synesthetes performed better at memory tasks, age was
not considered a confounding factor in their study. This suggests that these memory tasks may appear
impressive, but it is unclear how they may translate to everyday situations.
Similar to how Bankieries and Aslin [16] used a three-by-three grid to conduct their studies, this type of
methodology was popular among other experiments such as those by Simner and Bain [18] and Mealor et al.
[20]. The second experiment done by Mealor et al. involved memorizing a five-by-two grid filled with shapes
and colors [20]. When instructed to recall the grids, participants had to recognize whether they were
presented with those specific shapes and colors before categorizing them as “new” or “old”. The study
showed that there was no significance between synesthesia and its protective effects on memory decline.
This can suggest that there is no relation between memory and synesthesia, or that synesthesia’s impact on
memory is more of a slight enhancement than a significant advantage. Simner and Bain had a study design
containing random letters on a four-by-four grid they called their letter matrix test [18]. They required
participants to memorize the grids for one minute and rewrite them on a blank sheet of paper. Their study
found that grapheme-color synesthetes performed better and faster in memory tasks than their control
counterparts. Because of the small sample size, results were trending but statistically insignificant.
Additionally, the study design could be questioned as being too simple: memorizing a four-by-four grid is
relatively straightforward. Developing more advanced memory tests and increasing sample size would be
required to get more accurate results.
In addition to grid tests, Simner et al. conducted letter span tasks [18]. After being verbally presented with a
sequence of letters, participants were required to orally recall as many of those letters as they could.
Although grapheme-color synesthetes performed better than average memory controls, there was no
difference between synesthetes and high memory controls. Grapheme-color synesthetes showed an
advantage to visual learning. It could be less advantageous for them to complete tasks involved in auditory
learning. Participants in Lunke and Meier's 2018 study were also asked to memorize words but were allowed
to associate words with a color of their choice giving them a grapheme-color stimulus [19]. When they were
presented with various words, they were asked to identify them as “old” or “new” providing grapheme-color
synesthetes with the opportunity to stimulate their synesthesia through choosing color associations with
given words. They demonstrated a significant advantage in their memory performance. Further research is
needed to explore this phenomenon, specifically focusing on how stimuli can trigger and enhance the
memory advantage in synesthetes.
Autobiographical memory was tested by Chin and Ward who gave participants different cue words to
determine if it triggered memories differently in grapheme-color synesthetes compared to non-synesthetes
[17]. Memories from adulthood to childhood were analyzed in subjects. Synesthetes reported sharper
childhood memories as compared to non-synesthetes who reported more vivid memories from adulthood.
There was no definitive explanation as to why childhood memories seemed to be clearer. However, one
2024 Anash et al. Cureus 16(8): e67524. DOI 10.7759/cureus.67524 8 of 10
explanation is that the vibrant colors of toys and cartoons during childhood may trigger synesthesia. On the
other hand, it can be argued that synesthesia may be learned through associations based on exposure in
childhood. It raises the question of whether synesthesia is a learned phenomenon. Synesthetes may have
learned to associate senses but is unclear why some people experience it and some do not.
Some studies used mixed stimuli tests to see how it affects memory in grapheme-color synesthesia. Mealor
et al. used a mixed stimulus of three-digit numbers, images of snowflakes, and musical passages in their
first experiment [20]. Subjects were instructed to memorize the three stimuli that were presented in a
consistent order. After studying various quantities of stimuli, they were presented with new and previously
seen stimuli and were required to identify them as old or new. The study found that synesthesia does not
have a protective effect against memory decline in aging again suggesting there may not be a connection
between synesthesia and memory.
Common limitations
Several limitations can arise in studying grapheme-color synesthesia irrespective of the design of the study.
Self-reported synesthesia is a common limitation across all studies. A significant proportion of the
population may not be aware of their synesthesia. Participants in the studies who are aware of their
synesthesia may introduce bias to the study. They may consciously create their own synesthetic mnemonics
during studies potentially leading to skewed results. Memory performance can be affected by other factors
outside of synesthesia such as education level, general intelligence levels, social factors, etc. The outcome of
results can also be attributed to the Hawthorne effect in which participants may have increased or decreased
performance in memorizing since they all knew they were being studied. Blinded studies should be
considered in the future. Additionally, synesthesia is not a linear experience. Although all subjects here
reportedly had grapheme-color synesthesia, they may not experience it in the same way. Limitations arise in
this review structure as well. There were only seven studies that met the criteria. Further reviews should
expand the criteria. The variability in types of studies also poses a limitation in the consistency of
results. Finally, a majority of the primary articles included in this review were conducted by the same or
overlapping research groups. Seven authors from these primary articles are based at the University of
Sussex, which could possibly introduce bias into their experiments. It is crucial to diversify research on
grapheme-color synesthesia to reduce potential bias. Collaborations should be done with other institutions
to ensure wider viewpoints to extend research.
Conclusions
This review examined how grapheme-color synesthesia can affect declarative memory performance.
Synesthetes showed to outperform non-synesthetes in visual memory tasks. Synesthetes showed better
recall of paired patterns, shape-color associations, and visual grids compared to control groups. The
influence of synesthesia, either grapheme-color or other subtypes, on memorizing words remains unclear.
Future research should address limitations identified in this review such as controlling for confounding
factors, sample size and selection, and standardized testing.
Additional Information
Author Contributions
All authors have reviewed the final version to be published and agreed to be accountable for all aspects of the
work.
Concept and design: Stefani Anash, Andrew Boileau
Acquisition, analysis, or interpretation of data: Stefani Anash, Andrew Boileau
Drafting of the manuscript: Stefani Anash
Critical review of the manuscript for important intellectual content: Stefani Anash, Andrew Boileau
Supervision: Andrew Boileau
Disclosures
Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the
following: Payment/services info: All authors have declared that no financial support was received from
any organization for the submitted work. Financial relationships: All authors have declared that they have
no financial relationships at present or within the previous three years with any organizations that might
have an interest in the submitted work. Other relationships: All authors have declared that there are no
other relationships or activities that could appear to have influenced the submitted work.
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