Grapheme-color Synesthesia Research Articles

Personality and cognitive profiles of a general synesthetic trait

The recent sharp increase in studies on synesthesia has taught us a lot about this fascinating condition. Still, while we define synesthesia as ‘the mixing of senses’, the great majority of synesthesia studies focus on only one synesthesia type (in particular grapheme-color synesthesia). In this study, a large group of subjects are tested on the presence or absence of different types of synesthesia. Efforts to recruit a representative sample of the Dutch population, not related to or aware of synesthesia as a research topic, helped counter a selection bias or a self-report bias in our subject group. A sharp increase in synesthesia prevalence was found, at least partially due to including many different types of synesthesia in the synesthesia ‘diagnoses’. The five synesthesia types reported in the Novich et al (2011) study were obtained; Colored Sequences, Colored Music, Colored Sensations, Spatial Sequences, Non-Visual Sequelae, as well as an additional synesthesia type, Sequence-Personality. No differences were found between synesthetes and non-synesthetes in education level, handedness, age, and sex. The synesthetes showed increased intelligence as compared with matched non-synesthetes. This was a general effect rather than bound to a specific cognitive domain or to a specific (synesthesia-type to stimulus-material) relationship. The expected effect of increased “Openness” in synesthetes was obtained, as well as two unexpected effects in personality traits (increased “Neuroticism” and decreased “Conscientiousness”). We also found increased “Emotionality” (experiencing emotions) and increased “Fantasizing”, but synesthetes did not differ in cognitive appraisal of emotions (identifying/analyzing/verbalizing of emotions). The personality and cognitive characteristics were found related to having synesthesia (in general) rather then to particular synesthesia subtypes. This supports the existence of a general synesthetic ‘trait’, over the notion of relatively independent ‘types’ of synesthesia. In further support, exploratory analyses showed that a measurement of synesthetic strength (number of subtypes of synesthesia) correlates with stronger findings (increased “Openness”, “Fantasizing”, and “Emotionality”, and decreased “Conscientiousness”). In conclusion, results are in line with the notion of a general synesthetic ‘trait’, and this synesthetic trait is associated with particular personality traits and cognitive characteristics.

Biases and regularities of grapheme-colour associations in Japanese nonsynaesthetic population.

Associations between graphemes and colours in a nonsynaesthetic Japanese population were investigated. Participants chose the most suitable colour from 11 basic colour terms for each of 40 graphemes from the four categories of graphemes used in the Japanese language (kana characters, English alphabet letters, and Arabic and kanji numerals). This test was repeated after a three-week interval. In their responses, which were not as temporally consistent as those of grapheme-colour synaesthetes, participants showed biases and regularities that were comparable to those of synaesthetes reported in past studies. Although it has been believed that only synaesthetes, and not nonsynaesthetes, tended to associate graphemes with colours based on grapheme frequency, Berlin and Kay's colour typology, and colour word frequency, participants in this study tended in part to associate graphemes with colours based on the above factors. Moreover, participants that were nonsynaesthetes tended to associate different graphemes that shared sounds and/or meanings (e.g., Arabic and kanji numerals representing the same number) with the same colours, which was analogous to the findings in Japanese synaesthetes. These results support the view that grapheme-colour synaesthesia might have its origins in cross-modal association processes that are shared with the general population.

Spatial attention to graphemes in grapheme-color synesthesia

Spatial attention to graphemes in grapheme-color synesthesia

The influence of subjective experience in grapheme-color synesthesia on synesthetic colors: by using multilevel modeling.

The influence of subjective experience in grapheme-color synesthesia on synesthetic colors: by using multilevel modeling.

Migraine in Synesthetes and Nonsynesthetes: A Prevalence Study.

Synesthesia is a neurological condition in which an inducer stimulus in one sense leads to a concurrent percept in a second sense. The immune hypothesis of synesthesia links synesthesia to immune-related conditions such as migraine. More specifically, migraine with aura may be linked to grapheme-color synesthesia as both involve cortical hyperexcitability. In this study, 161 female synesthetes, and 92 female nonsynesthetes, completed an online questionnaire about synesthesia and migraine. We found no general link between migraine and synesthesia nor between migraine with aura and grapheme-color synesthesia. Exploratory analyses, however, showed that certain types of synesthetic inducer (non-linguistic visual experiences, scent, taste, emotion and personality) were associated with visual disturbances in headache among female participants, and touch as a concurrent was associated with migraine with aura. On the basis of our exploratory analyses, we hypothesize that specific subtypes of synesthesia are related to migraine. The relationship between these two conditions is likely to become clearer as research on the underlying causes of synesthesia and migraine progresses.

Components of Attention in Grapheme-Color Synesthesia: A Modeling Approach.

Grapheme-color synesthesia is a condition where the perception of graphemes consistently and automatically evokes an experience of non-physical color. Many have studied how synesthesia affects the processing of achromatic graphemes, but less is known about the synesthetic processing of physically colored graphemes. Here, we investigated how the visual processing of colored letters is affected by the congruence or incongruence of synesthetic grapheme-color associations. We briefly presented graphemes (10–150 ms) to 9 grapheme-color synesthetes and to 9 control observers. Their task was to report as many letters (targets) as possible, while ignoring digit (distractors). Graphemes were either congruently or incongruently colored with the synesthetes’ reported grapheme-color association. A mathematical model, based on Bundesen’s (1990) Theory of Visual Attention (TVA), was fitted to each observer’s data, allowing us to estimate discrete components of visual attention. The models suggested that the synesthetes processed congruent letters faster than incongruent ones, and that they were able to retain more congruent letters in visual short-term memory, while the control group’s model parameters were not significantly affected by congruence. The increase in processing speed, when synesthetes process congruent letters, suggests that synesthesia affects the processing of letters at a perceptual level. To account for the benefit in processing speed, we propose that synesthetic associations become integrated into the categories of graphemes, and that letter colors are considered as evidence for making certain perceptual categorizations in the visual system. We also propose that enhanced visual short-term memory capacity for congruently colored graphemes can be explained by the synesthetes’ expertise regarding their specific grapheme-color associations.

Assessment of the hemispheric lateralization of grapheme-color synesthesia with Stroop-type tests.

Grapheme-color synesthesia, the idiosyncratic, arbitrary association of colors to letters or numbers, develops in childhood once reading is mastered. Because language processing is strongly left-lateralized in most individuals, we hypothesized that grapheme-color synesthesia could be left-lateralized as well. We used synesthetic versions of the Stroop test with colored letters and numbers presented either in the right or the left visual field of thirty-four synesthetes. Interference by synesthetic colors was stronger for stimuli in the right hemifield (first experiment, color naming task). Synesthetes were also faster in the right hemifield when naming the synesthetic color of graphemes (second experiment). Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield. Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere. We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.

Prevalence of learned grapheme-color pairings in a large online sample of synesthetes.

In this paper we estimate the minimum prevalence of grapheme-color synesthetes with letter-color matches learned from an external stimulus, by analyzing a large sample of English-speaking grapheme-color synesthetes. We find that at least 6% (400/6588 participants) of the total sample learned many of their matches from a widely available colored letter toy. Among those born in the decade after the toy began to be manufactured, the proportion of synesthetes with learned letter-color pairings approaches 15% for some 5-year periods. Among those born 5 years or more before it was manufactured, none have colors learned from the toy. Analysis of the letter-color matching data suggests the only difference between synesthetes with matches to the toy and those without is exposure to the stimulus. These data indicate learning of letter-color pairings from external contingencies can occur in a substantial fraction of synesthetes, and are consistent with the hypothesis that grapheme-color synesthesia is a kind of conditioned mental imagery.

Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness.

Phosphenes are illusory visual percepts produced by the application of transcranial magnetic stimulation to occipital cortex. Phosphene thresholds, the minimum stimulation intensity required to reliably produce phosphenes, are widely used as an index of cortical excitability. However, the neural basis of phosphene thresholds and their relationship to individual differences in visual cognition are poorly understood. Here, we investigated the neurochemical basis of phosphene perception by measuring basal GABA and glutamate levels in primary visual cortex using magnetic resonance spectroscopy. We further examined whether phosphene thresholds would relate to the visuospatial phenomenology of grapheme-color synesthesia, a condition characterized by atypical binding and involuntary color photisms. Phosphene thresholds negatively correlated with glutamate concentrations in visual cortex, with lower thresholds associated with elevated glutamate. This relationship was robust, present in both controls and synesthetes, and exhibited neurochemical, topographic, and threshold specificity. Projector synesthetes, who experience color photisms as spatially colocalized with inducing graphemes, displayed lower phosphene thresholds than associator synesthetes, who experience photisms as internal images, with both exhibiting lower thresholds than controls. These results suggest that phosphene perception is driven by interindividual variation in glutamatergic activity in primary visual cortex and relates to cortical processes underlying individual differences in visuospatial awareness.

Cross-modal, bidirectional priming in grapheme-color synesthesia.

Grapheme-color synesthetes perceive achromatic graphemes to be inherently colored. In this study grapheme-color synesthetes and non-synesthetes discriminated (1) the color of visual targets presented along with aurally presented digit primes, and (2) the identity of aurally presented digit targets presented with visual color primes. Reaction times to visual color targets were longer when the color of the target was incongruent with the synesthetic percept reported for the prime. Likewise, discriminating aurally presented digit targets took longer when the color of the prime was incongruent with the synesthetic percept for the target. These priming effects were absent in non-synesthetes. We conclude that binding between digits and colors in grapheme-color synesthetes can occur bidirectionally across senses. The results are in line with the idea that synesthesia is the result of linking inducing stimuli (e.g. digits) to synesthetic percepts (colors) at an abstract - supra-modal - conceptual level of processing.

Synesthesia and release phenomena in sensory and motor grounding. Cases of disinhibited embodiment?

Synesthesia is an unusual condition occurring in at least 4% of the population (Simner et al., 2006) in which certain stimuli trigger unusual perceptions which the physical properties of the stimulus alone are not sufficient to account for. For instance, in grapheme-color synesthesia, the sight of black-and-white printed letters or numbers triggers the experience of consistent and specific colors (e.g., A might be red, 7 yellow: Smilek et al., 2001). The associations reported by any given synesthete tend to be remarkably consistent over time (e.g., Simner et al., 2005). Hence if A is red for a certain synesthete, it will tend to always be red. There is evidence for a genetic component to the condition (Ward and Simner, 2005; Asher et al., 2009; Tomson et al., 2011), and evidence too that the brains of synesthetes are structurally and functionally different to those of non-synesthetes. For example, in a sound-color and sound-taste synesthete, Hanggi et al. (2008) found altered patters of brain volume and functional anisotrophy (FA) values (indicative of greater white matter coherence). These included increased FA values in primary auditory cortex, as well as increased gray and white matter volumes in the same region, and also structural differences in gray and white matter in visual and gustatory regions, i.e., increases in occipital regions, and increases and decreases in insular cortex (for review see Hubbard, 2012; Rouw, 2012).

Red, green, blue equals 1, 2, 3: Investigating the bidirectionality of digit-colour synaesthesia

In grapheme-colour synaesthesia, letters and digits elicit vivid and highly consistent experiences of colour. Typically, the conscious experience is unidirectional: digits elicit colours but colours do not elicit digits. If synaesthesia reflects an involuntary connection between representations, however, a link between a digit and colour should have some bidirectional effects. Recent evidence shows subtle bidirectional interference effects reflecting this implicit link. Here, we examined whether the implicit activation of a digit by a colour for grapheme-colour synaesthetes could aid their performance, and whether this reflected slow strategic or rapid effortless processing. We presented grapheme-colour synaesthetes with a stream of coloured squares, and asked them to report the colours in order at the end of each trial. Within a trial, all the colours were individually presented for 200ms (fast condition) or 500ms (slow condition). The colours were matched to each synaesthete's colours for the digits one to nine. In half the trials, the order of the colours corresponded to part or all of a sequence of numbers (e.g., red, green, blue if 1=red, 2=green, 3=blue). For the other trials, there was no sequence in the order of colours. If synaesthetes activate, either involuntarily or strategically, the digit that typically elicits the colour when viewing colours, they should be able to use the sequential information to aid the recall of colours in order. Early results suggest that the synaesthetes were able to recall sequential colour sequences more accurately than pseudo-randomized colours. This effect appears greatest for the slow condition, where there is time for strategic transformation, suggesting that synaesthetes are able to use their unusual experiences to effectively cue recall.

¿Colores, sabores, números?: la sinestesia en una muestra española

Synesthesia is a neural phenomenon in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream. These activations occur involuntarily, automatically and consistently over time. To estimate the relative frequency of the different modalities of the phenomenon in a Spanish sample. Study performed in educational (55.04%), labor (20.54%) and digital contexts (24.4%) using the Synesthesia Questionnaire created by Artecitta Foundation. The analysis of the responses given by 803 participants suggests that 13.95% of the sample experience any synesthesia. The analysis of the relative frequencies shows that the most frequent modality is spatial sequence synesthesia (44.6%). 33.9% see colors when listening to sounds and/or music, 25.9% associate colors to temporal concepts, 20.5% assign gender and personality to letters and numbers, 10.7% experience grapheme-color synesthesia and 5.4% feel a specific flavor when hearing words. These data suggest that the presence of synesthesia in the Spanish sample under study is high, and that the investigation of the phenomenon and its different modalities needs to be approached on the basis of the current knowledge about its phenomenological variability and its genetic and neurophysiologic characteristics. Likewise, the results are useful to adjust the questionnaire items and increase their discriminative power.

Colour fluctuations in grapheme-colour synaesthesia: The effect of clinical and non-clinical mood changes.

Synaesthesia is a condition that gives rise to unusual secondary sensations (e.g., colours are perceived when listening to music). These unusual sensations tend to be reported as being stable throughout adulthood (e.g., Simner & Logie, 2007, Neurocase, 13, 358) and the consistency of these experiences over time is taken as the behavioural hallmark of genuineness. Our study looked at the influence of mood states on synaesthetic colours. In Experiment 1, we recruited grapheme-colour synaesthetes (who experience colours from letters/digits) and elicited their synaesthetic colours, as well as their mood and depression states, in two different testing sessions. In each session, participants completed the PANAS-X (Watson & Clark, 1999) and the BDI-II (Beck, Steer, & Brown, 1996, Manual for Beck Depression Inventory-II), and chose their synaesthetic colours for letters A-Z from an interactive colour palette. We found that negative mood significantly decreased the luminance of synaesthetic colours. In Experiment 2, we showed that synaesthetic colours were also less luminant for synaesthetes with anxiety disorder, versus those without. Additional evidence suggests that colour saturation, too, may inversely correlate with depressive symptoms. These results show that fluctuations in mood within both a normal and clinical range influence synaesthetic colours over time. This has implications for our understanding about the longitudinal stability of synaesthetic experiences, and of how mood may interact with the visual (imagery) systems.

Implicit interactions between number and space in digit-color synesthesia

In digit-color synesthesia, a variant of grapheme-color synesthesia, digits trigger an additional color percept. Recent work on number processing in synesthesia suggests that colors can implicitly elicit numerical representations in digit-color synesthetes implying that synesthesia is bidirectional. Furthermore, morphometric investigations revealed structural differences in the parietal cortex of grapheme-color synesthetes, i.e., in the brain region where interactions between number and space occur in non-synesthetic subjects. Based upon these previous findings, we here examined whether implicitly evoked numerical representations interact with spatial representations in synesthesia in such a way that even a non-numerical, visuo-spatial task (here: line bisection) is modulated, i.e., whether synesthetes exhibit a systematic bisection bias for colored lines.Thirteen digit-color synesthetes were asked to bisect two sets of lines which were colored in their individual synesthetic colors associated with a small or a large digit, respectively. For all colored line stimuli combined, digit-color synesthetes showed – like control subjects (n = 13, matched for age, gender, IQ and handedness) – a pseudo-neglect when bisecting colored lines. Measuring the color-induced change of the bisection bias (i.e., comparing the biases when bisecting lines colored according to a small number vs those lines corresponding to a large number) revealed that only digit-color synesthetes were significantly influenced by line color.The results provide further evidence for the bidirectional nature of synesthesia and support the concept of a mental number line. In addition, they extend previous reports on bidirectionality in synesthesia by showing that even non-numerical, visuo-spatial performance can be modulated by implicit bidirectional processes.

Adults can be trained to acquire synesthetic experiences.

Synesthesia is a condition where presentation of one perceptual class consistently evokes additional experiences in different perceptual categories. Synesthesia is widely considered a congenital condition, although an alternative view is that it is underpinned by repeated exposure to combined perceptual features at key developmental stages. Here we explore the potential for repeated associative learning to shape and engender synesthetic experiences. Non-synesthetic adult participants engaged in an extensive training regime that involved adaptive memory and reading tasks, designed to reinforce 13 specific letter-color associations. Following training, subjects exhibited a range of standard behavioral and physiological markers for grapheme-color synesthesia; crucially, most also described perceiving color experiences for achromatic letters, inside and outside the lab, where such experiences are usually considered the hallmark of genuine synesthetes. Collectively our results are consistent with developmental accounts of synesthesia and illuminate a previously unsuspected potential for new learning to shape perceptual experience, even in adulthood.

Both "나" and "な" are yellow: cross-linguistic investigation in search of the determinants of synesthetic color.

Individuals with grapheme-color synesthesia experience "colors" when viewing achromatic letters and digits. Despite the large individual difference in synesthetic association between inducing graphemes and induced colors, the search for the determinants of synesthetic experience has begun. So far, however, research has drawn an inconsistent picture; some studies have shown that graphemes of similar visual shape tend to induce similar synesthetic colors, while others suggested sound as an important factor. Moreover, meaning seems to affect synesthetic color. In the present work, we sought to investigate the determinants of synesthetic color by testing four multilingual grapheme-color synesthetes who experience "colors" upon viewing Korean (hangul), Japanese (katakana and hiragana), and English (Latin alphabet) characters on a standardized color-matching procedure. Results showed that pairs of characters of matched sound tended to induce similar synesthetic colors. This was the case not only between two scripts within the same language (Japanese hiragana and katakana) but also between two different languages (Japanese and Korean). In addition, pairs of characters with similar initial phonemes tended to induce similar colors; this was general across multiple languages. Results also showed that pairs of sequential words in Korean, Japanese, English, and Chinese that have the same meaning tended to elicit similar synesthetic colors. When those pairs of words shared not only meaning but also sound, the similarity of the induced synesthetic colors was even greater. Our work is one of the few initial attempts to examine the influence of visual shape, sound, meaning, and their interaction on synesthetic color induced by characters across multiple languages.

Two plus blue equals green: Grapheme-color synesthesia allows cognitive access to numerical information via color

In grapheme-color synesthesia, graphemes (e.g., numbers or letters) evoke color experiences. It is generally reported that the opposite is not true: colors will not generate experiences of graphemes or their associated information. However, recent research has provided evidence that colors can implicitly elicit symbolic representations of associated graphemes. Here, we examine if these representations can be cognitively accessed. Using a mathematical verification task replacing graphemes with color patches, we find that synesthetes can verify such problems with colors as accurately as with graphemes. Doing so, however, takes time: ~250 ms per color. Moreover, we find minimal reaction time switch-costs for switching between computing with graphemes and colors. This demonstrates that given specific task demands, synesthetes can cognitively access numerical information elicited by physical colors, and they do so as accurately as with graphemes. We discuss these results in the context of possible cognitive strategies used to access the information.

Associative memory advantage in grapheme-color synesthetes compared to older, but not young adults.

People with grapheme-color synesthesia perceive enriched experiences of colors in response to graphemes (letters, digits). In this study, we examined whether these synesthetes show a generic associative memory advantage for stimuli that do not elicit a synesthetic color. We used a novel between group design (14 young synesthetes, 14 young, and 14 older adults) with a self-paced visual associative learning paradigm and subsequent retrieval (immediate and delayed). Non-synesthesia inducing, achromatic fractal pair-associates were manipulated in visual similarity (high and low) and corresponded to high and low memory load conditions. The main finding was a learning and retrieval advantage of synesthetes relative to older, but not to younger, adults. Furthermore, the significance testing was supported with effect size measures and power calculations. Differences between synesthetes and older adults were found during dissimilar pair (high memory load) learning and retrieval at immediate and delayed stages. Moreover, we found a medium size difference between synesthetes and young adults for similar pair (low memory load) learning. Differences between young and older adults were also observed during associative learning and retrieval, but were of medium effect size coupled with low power. The results show a subtle associative memory advantage in synesthetes for non-synesthesia inducing stimuli, which can be detected against older adults. They also indicate that perceptual mechanisms (enhanced in synesthesia, declining as part of the aging process) can translate into a generic associative memory advantage, and may contribute to associative deficits accompanying healthy aging.

Back to the future: synaesthesia could be due to associative learning.

Over the last century, researchers and laypeople alike have been captivated by synaesthesia—a developmental condition present in approximately 5% of the adult population, where the presentation of an “inducer” stimulus gives rise to additional “concurrent” sensation absent from the veridical sensory world (Ward, 2013). For example, the most common variant is grapheme-color synaesthesia, where letters and digits induce concurrent sensations of color (Day, 2005). Early theories proposed that synaesthesia resulted from associative learning between the inducer and concurrent sensory representations (Calkins, 1893; Claparede, 1903). Specifically, the inducer and concurrent stimuli have been present in a correlated, or “contingent,” fashion in the synaesthete's environment; hence presentation of the inducer alone now activates the representation of the concurrent. However, these theories have been widely rejected. A range of structural and functional mechanisms are proposed by contemporary theorists, including hyperconnectivity between brain regions (Hubbard and Ramachandran, 2005) and feedback disinhibition (Grossenbacher and Lovelace, 2001), but all are united in the assertion that “associative learning in and of itself cannot explain synaesthesia” (Marks and Odgaard, 2005). Given recent evidence consistent with learning explanations of synaesthesia, we consider here whether the associative account may have been dismissed prematurely. We suggest that common objections to this account may be rooted in misunderstandings about what it would predict. We consider these objections in turn, highlighting how they are easily countered by contemporary learning theory. The objections are that synaesthetic experiences (1) do not reflect environmental statistics, (2) remain consistent over time, (3) cannot be trained into neurotypicals, and (4) are domain-specific. We close by describing the types of experiment that would provide the associative account of synaesthesia with the rigorous empirical assessment it deserves.