Synesthesia is a condition in which a sensory stimulus presented in one modality evokes a sensation in a different modality.
In the 19th century Francis Galton observed that a certain proportion of the general population who were otherwise normal had a hereditary condition he dubbed "synesthesia"; a sensory stimulus presented through one modality spontaneously evoked a sensation experienced in an unrelated modality. For example, an individual may experience a specific color for every given note (“C sharp is red”) or every grapheme – printed number or letter- is tinged with a specific hue (e.g. 5 is indigo and 7 is green; Figure 1). The specificity of evoked colors remains stable over time (Baron-Cohen, Burt, Smith-Laittan, Harrison, Bolton, 1996) within any given individual but the same grapheme doesn’t necessarily evoke the same color in different people (Cytowic, 1989). Although long regarded as a curiosity there has been a tremendous resurgence of interest in synesthesia in the last decade (Palmeri, Blake, Marois, Flanery, Whetsell, 2002; Eagleman, Kagan, Nelson, Sagaram, Sarma, 2007; Dixon, Smilek, Cudahy, Merikle, 2000; Simner & Ward, 2006; Robertson & Sagiv, 2005; Mattingley, Rich, Yelland, Bradshaw, 2001; Blakemore, Bristow, Bird, Frith, Ward, 2005; Ramachandran & Hubbard, 2003; Ramachandran & Hubbard, 2001a; Ramachandran & Hubbard, 2001b).
Synesthesia used to be regarded as a rare condition but recent estimates suggest an incidence of 4%; the most common of which appears to be grapheme-color (Simner, Sagiv, Mulvenna, Tsakanikos, Witherby, Fraser, Scott, Ward, 2006). Most individuals report having had the experience for as long back in childhood as they can remember. As Galton himself noted, the condition tends to run in families and recent work suggests a genetic basis (Barnett, Finucane, Asher, Bargary, Corvin, Newell, Mitchell, 2008). Synesthesia was previously believed 6 times more common in women than in men according to responses from newspaper ads (Baron-Cohen, Burt, Smith-Laittan, Harrison, Bolton, 1996). However, Simner and colleagues showed no difference between the sexes testing a large population for synesthesia (Simner, Sagiv, Mulvenna, Tsakanikos, Witherby, Fraser, Scott, Ward, 2006). Sometimes, sensory deprivation can lead to one sensory input evoking sensations in a different modality. For example, after early visual deprivation due to retinitis pigmentosa, touch stimuli can produce visual phosphenes (Armel & Ramachandran, 1999) or after a thalamic lesion leading to a loss of tactile sensation, sounds can elicit touch sensations (Ro, Farne, Johnson, Wedeen, Chu, Wang, Hunter, Beauchamp, 2007). This probably occurs because the tactile or auditory sensory input now begins to cross-activate the deprived cortical areas. This could be regarded as a form of acquired synesthesia caused, possibly, by pre-existing back projections becoming hyperactive, or through axonal sprouting, especially given the long period of time (on the order of years) between the loss of sensory input and the onset of the synesthetic experiences.
There are dozens of strange forms of synesthesia and descriptions of these can be found in several recent books, reviews and edited volumes (Robertson & Sagiv, 2005; Ramachandran & Hubbard, 2001b; Cytowic, 1989; Grossenbacher & Lovelace, 2001, Simner, 2007). Although these will be mentioned in passing, this short review will mainly emphasize grapheme-color synesthesia rather than attempt a comprehensive survey of the entire field. The reason for this is that grapheme–color is the most common form and also the one on which most psychophysical experiments have been done. Equally important, we have now begun to understand the anatomical and physiological basis of grapheme–color synesthesia to an extent that is not yet possible with the other more exotic variants. As such, it might provide a “model” for how we might experimentally approach the less common types.
Bearing all this in mind, we can ask several important questions regarding the phenomenon.
- Is the phenomenon authentic (“real”) and hence worthy of study? Or are the individuals simply making it up?
- Is it an early sensory process or a high level cognitive process such as a memory association (e.g. formed by having played with refrigerator magnets in early childhood)? Do the evoked colors have “quale?"
- Is it based on genes or up-bringing?
- Does it have any function?
- What is its neural basis?
- How does one account for the observation that poetry and art are more likely to be pursued by synesthetes? (Rich, Bradshaw, Mattingley, 2005; Ward, Yaro, Thompson-Lake, Sagiv, 2007; Domino, 1989).
- Does the phenomenon have broader implications?
Sensory nature of synesthesia
Synesthesia is stable (i.e. has test re-test reliability) over several months (Baron-Cohen, Burt, Smith-Laittan, Harrison, Bolton, 1996), which suggests that it is authentic; not confabulatory in origin. The initial evidence that it is an actual sensory process, rather than a high level cognitive one, is suggested by six lines of evidence (Ramachandran & Hubbard, 2003; Ramachandran & Hubbard, 2001a; Ramachandran & Hubbard, 2001b).
- First as the luminance contrast of the grapheme is progressively reduced, the perceived saturation of the color decreases monotonically and disappears at less than 10% contrast even though the grapheme is still clearly visible. If the color is a simple memory association there is no reason why it be less vivid with lower contrast.
- Second, the evoked color can lead to texture segregation (Figure 2). If several 2s are scattered among a random array of 5s, synesthetes can use the evoked color difference to much more rapidly group and segregate the 2s from the 5s than “normal” non-synesthetes (Ramachandran & Hubbard, 2001a; Palmeri, Blake, Marois, Flanery, Whetsell, 2002). Such segregation strongly suggests that the colors are evoked early in sensory processing.
- Third, the synesthetically evoked color can provide an input to apparent motion perception (Ramachandran & Hubbard, 2002; Kim, Blake, Palmeri, 2003; Ramachandran & Azoulai, 2006).
- Fourth, even for a single grapheme different regions can be tinged different colors, an observation that suggests a "hardware" glitch rather than a cognitive effect (Figure 1).
- Fifth, subjects can adapt to synesthetically induced colors and experience McCollough color after effects (Blake, Palmeri, Marois, Kim, 2005).
- Sixth, direct brain imaging evidence (see below).
These experiments establish the sensory nature of synesthesia but, as we shall see later, this is only true if a subset of synesthetes ("lower synesthetes" or "projectors"). It should also be added that the view that some types of synesthesia are caused by sensory cross-activation is by no means universal. There are people who still believe - in spite of the evidence cited above - that all types of synesthesia are the result of higher level conceptual or linguistic associations. This harks us back to the dark ages of classical 'associationist' psychology.
Synesthetic color can also lead to Stroop interference (Nikolic´, Lichti, Singer, 2007) but this doesn't necessarily prove that synesthesia is sensory given that Stroop can occur at any stage in processing down all the way to motor output (MacLeod, 1991).
More recently, two group studies showed synesthetes' brain waves differ from non synesthetes with an early Event-Related Potential component being detectable 100-150 ms after seeing a grapheme or hearing a sound (Brang, Edwards, Coulson, Ramachandran, 2008; Beeli, Esslen, Jäncke, 2007). This time window argues strongly for the perceptual nature of this phenomenon, considering most language processes begin after 300 ms.
Taken collectively all these observations support the "early sensory cross-activation" theory, but what is the underlying physiological basis?
The visual grapheme area is located in the fusiform gyrus which represents visual appearance and is adjacent to color area V4 in the same gyrus. Since the condition is hereditary, it has been suggested that there is an accidental cross- activation between these areas caused by a gene mutation that causes either defective pruning of axons (Ramachandran & Hubbard, 2001a) or disinhibition (Armel & Ramachandran, 1999). This has been called the sensory cross-activation hypothesis. This is consistent with the observation that Roman numerals (e.g. V or VI) are ineffective in evoking the color; it’s the visual appearance of the numeral that is critical – not the abstract idea of number. All this appears to be characteristic of only a subset of synesthetes whom we call "lower synesthetes".
A second group called "higher synesthetes" perceive color associated with more abstract numerical concepts. Color is triggered not only by graphemes (visual shapes of numbers) but also by the abstract idea of numerical sequence as embodied in days of the week or months of the year; so called "calendar synesthetes". Additionally, phonemes associated with letters can evoke colors. It has been suggested that in these higher synesthetes, the cross-wiring occurs higher up in the vicinity of the angular gyrus where more abstract numerical ideas are represented (Ramachandran & Hubbard, 2001b). The ratio of "higher" to "lower" synesthesia is not known, but anecdotal evidence suggests that it may be as high as 6:1 (nor is it likely that the distribution is bimodal).Certain "higher" color areas which receive input from V4 are also closer to the angular gyrus than to the fusiform. Brain imaging studies (fMRI) which allow precise localization of V4 support the notion that lower synesthetes have cross activation in the fusiform gyrus (Hubbard, Arman, Ramachandran & Boynton, 2005; Figure 3).
The gene mutation- based cross-wiring hypothesis also receives support from the fact that if you have one type of synesthesia you are also more likely than chance to have one or two other types (again, this would depend the gene being expressed more widely but in a patchy manner; Ramachandran and Hubbard, 2001b).
Reality of number forms and their relevance to cognitionGalton also described another strange variant of synesthesia. When asked to visualize numbers, some people will say that every number always occupies a specific location in space and the integers are represented sequentially along a virtual line called a number line or number form. Remarkably, these lines are often elaborately convoluted - even doubling back on themselves (so 5 might be spatially nearer to 19 on the number line than it is to 7; Figure 4).
The importance of number forms in studying cognitive processes in normal people was largely overlooked until recently (Seron, Pesenti, Noel, Deloche, Cornet, 1992; Ramachandran and Hubbard, 2001b). To establish the “reality" of these convoluted number lines, synesthetes were tested using a "stroop memory" task (Azoulai, Hubbard, Ramachandran, 2005). The subject was asked to memorize a subset of numbers that were randomly selected from 1 to 50. There were three conditions a) the selected subset were placed randomly on the line b) A new subset of numbers was selected and they were presented congruently; i.e. on the "correct" location on the line or c) They fell randomly throughout the screen. The accuracy of subsequent recall was highest in (b) and lowest in (a) - perhaps a result of stroop-like interference in memory. Similar, but more conclusive evidence has been more recently reported by Sagiv, Simner, Collins, Butterworth, and Ward (2006).
What causes number lines? Given hardware constraints and the way it evolved, the brain may lack the ability to represent ideas such as numerical sequence in abstract form, and may therefore map them on to pre-existing spatial maps which are phylogenetically ancient. A genetic glitch in this “remapping” of numbers on to space may result in a convoluted number line (Ramachandran & Hubbard, 2003). The observation implies a “hardware“ rather than “software” glitch and cannot be accounted for by any straightforward associationist model of synesthesia.
In another curious variant of the phenomenon, months of the year are represented sequentially in a hula-hoop like circle - a "calendar line"- around the subject's head (Smilek, Callejas, Dixon, Merikle, 2007). We have observed that the head often occupies the rim of the circle with the current month centered on it; e.g. if the current month is march, the head occupies March and February is to the left and April to the right, but when it is May, April will be to the left of the head (we have observed this in 3 out of 15 subjects). Once again this demonstrates the brain's tendency to represent time in spatial and (in this case) bodily representations.
Genetics of synesthesia
While a proven genetic basis for synesthesia remains elusive, the phenomenon tends to run in families, as nearly 50% of synesthetes report a first degree relative with the phenomenon (Barnett, Finucane, Asher, Bargary, Corvin, Newell, Mitchell, 2008; Baron-Cohen, Burt, Smith-Laittan, Harrison, Bolton, 1996). Importantly, the ‘’type’’ of synesthesia can vary within families, and the qualitative experience usually differs even between individuals of the same family. Preliminary work by Brang and Ramachandran (2007a) has suggested, based on pharmacological models of synesthesia, that HTR2A on chromosome 13q may be one of the genes involved in this phenomenon.
Potential benefits in synesthesia
Is synesthesia a gift or a curse? Most people who have it claim it enriches their lives. For example, if the many letters of a word (but especially the first letter) evoke the same color (say blue) and the word happens to be sea, they say a pleasing harmony between word and letter is set up that has great aesthetic appeal. This link between aesthetics and synesthesia is intriguing and may well repay further study.
A clever experiment by Smilek, Dixon, Cudahy, and Merikle suggests that synesthesia can enhance memory (2002). They asked a grapheme-color synesthete to memorize a random selection of letters which were either randomly colored or colored in a manner consistent with their synesthesia. On subsequent testing they found that the letters with concordant colors were more accurately remembered. Luria (1968) described an individual ("S") whose prodigious memory was based largely on using synesthetic associations evoked by the things to be memorized. Enhancements of memory based on synesthesia have also been reported more recently by other groups (Yaro & Ward, 2007).
Less studied variants and aspects of synesthesia
Ordinal linguistic personification
The cross activation (of brain maps) model explains many aspects of synesthesia. But some synesthetes report that each grapheme is a specific sex, or good or evil (Ramachandran & Hubbard, 2005b), and it strains credulity to assume that there are brain maps for classifying sexes. Our brains tend to binarize the world (black/white, male/female, good/evil, ugly/beautiful, yin/yang, etc.) to simplify cognitive processing and it's not inconceivable that there are dedicated brain regions involved in such functions, but why they should get hooked up to graphemes is unclear. Perhaps certain shapes are more feminine (or masculine) through association even in ‘normals’ and these are made explicit by the enhanced connectivity caused by synesthesia genes. The boundary between such phenomena and higher cognitive functions remains elusive.
The fact that the "sex" effect is genuine was shown in a clever manner by Simner and Holenstein (2007). Assume S is "feminine" for a particular synesthete. For this individual it takes longer to identify the male name "Sam" than the female name Sue, reflecting the inherent incongruity. The manner in which high level phonemic representations can interact with and modulate synesthesia have also been explored by Simner and Ward in a series of intriguing experiments (2006).
The simpler and more common forms of synesthesia -such as graphemes evoking colors- can be explained readily in terms of cross-activation and the psychophysical and physiological evidence for this is overwhelming. But the existence of certain "weird" idiosyncratic forms of synesthesia is harder to explain - e.g. "8 is female and has a demanding personality; wants to be a larger number than she is and is difficult for the other numbers to be around." Many of us also make such seemingly random associations from time to time but we don't get "stuck" with them. In synesthesia such Hebbian associations may have a "self-enhancing" tendency - akin to kindling - once they are set in motion.
Mirror neurons and synesthesia
A curious form of acquired synesthesia can be observed in patients with phantom limbs. If a system of mirrors is used to optically resurrect the phantom and it is made to appear visually that the phantom is being touched with a pencil, the subjects feels her phantom being touched even though no part of her body is being touched. This implies that the visual sensations must be feeding back to activate somatosensory maps in the brain (Ramachandran, Rogers-Ramachandran, Cobb, 1995). Even more remarkably it was noticed that patients with phantom arms will experience their phantom being touched in a precisely localized manner even if they merely observe another persons intact arm being touched (Ramachandran, Rogers-Ramachandran, 2007). Similarly they experienced their phantom moving if the experimenter put his own hand in the vicinity of the phantom and moved it. Such effects may be mediated by a class of neurons in the premotor cortex and parietal lobes called “mirror neurons” ((Rizzolatti, Fadiga, Gallese, Fogassi, 1996). These neurons are activated when a subject moves his hand - as expected. But surprisingly they also fire when the subject watches another person making similar movements. Such activation does not lead a normal observer to experience sensations presumably because the regular somatosensory neurons (i.e. those that are not mirror neurons) signal the absence of real proprioceptive and tactile inputs. When the arm is amputated, however, this normal sensory input is removed, leading the patient to quite literally experience the touch (or proprioceptive) sensations in the phantom (Ramachandran and Rogers–Ramachandran, 2007). The patient also noted that watching another persons intact hand being rubbed caused relief from his phantom pain.
There is anecdotal clinical evidence that synesthesia is more common in TLE (temporal lobe epilepsy). This can be explained by assuming that the repeated seizure volleys might indiscriminately strengthen certain brain connections through a process known as kindling. This would lead to pathological cross activation (Ramachandran & Hubbard, 2001b).
Certain rarer forms of “congenital” synesthesia can also be partially explained by the cross activation model. For example, some people “taste shapes”. For these synesthetes, every taste has a shape which they perceive alongside their gustatory experience (i.e. chicken tastes “pointy”, Cytowic, 1989). It has been suggested that this is caused by cross activation between taste neurons in the insula and S2 somatosensory cortex involved in discerning tactile texture and shape (Ramachandran and Hubbard, 2001b). Similarly, in a newly discovered form of synesthesia - tactile textures evoking highly specific emotions (e.g. velvet = guilt). We postulate that there may be enhancement of connections that already exist between tactile textures (S2 cortex) and adjacent insula (emotion) as well as between the insula and the orbito frontal cortex (Brang & Ramachandran, 2007b; Ramachandran & Brang, in press). Not coincidentally, perhaps similar "touch to emotion" activations are also common in cross-sensory metaphors ("sharp taste," "flat taste," and "touching remark") suggesting that metaphors may, speaking statistically, respect the same cross sensory anatomical constraints as “pathological” synesthesia (Ramachandran & Hubbard, 2001b).
Enhanced cross activations are most likely to occur between adjacent brain regions given that such regions are most likely to be already partly connected to begin with. But this isn’t always true because preexisting connections can also exist (less frequently) between far-flung brain regions that are functionally linked. The synesthesia gene(s) could enhance these connections.
Bi-directionality in synesthesia
In large, synesthetes report these experiences are unidirectional, such that numbers and letters may automatically elicit colors, yet colors will not cause the automatic percept of a number. While these subjective reports have been accepted for a numbers of years, mounting evidence from neuroimaging and behavioral studies suggest that synesthesia may be partially, if not unconsciously, bidirectional in grapheme-color synesthesia. Knoch, Gianotti, Mohr, & Brugger elegantly showed that synesthetes, but not controls, implicitly activated numerical representations when randomly generating colors (2005). In a study recording synesthetes' brain-waves, contextual expectation of a color mediated categorization and discrimination of graphemes as early as 100ms post stimulus onset (Brang, Edwards, Coulson, Ramachandran, 2008). In addition, synesthetes instructed to choose the numerically larger of two numbers (the 5, when 4 and 5 are presented together) showed slowed reactions times if the numbers were colored with a numerically incongruous synesthetic color (4 printed in the color induced by 8, and 5 printed in the color induced by 2; Cohen Kadosh, Sagiv, Linden, 2005).
A color blind synesthete; "Martian colors"
There is at least one color blind synesthete on record who reported that she could see colors in numbers that she couldn't see in the real world; referring to them as "martian colors". Her color anomaly, caused by deficient cone pigments, allowed her to see only a limited range of real colors. But perhaps the color neurons in V4, having been specified genetically, were largely intact and were being indirectly stimulated by cross activation via graphemes. (Ramachandran and Hubbard, 2001b). This negates the memory association theory and supports the sensory cross activation theory.
Top-down and contextual effects5 which is a large 5 made up of little 2s one can use a mental zoom lens to focus on the big 5 or the little 2s and for synesthetes the color is seen to switch correspondingly (Ramachandran & Hubbard, 2001b) which implies that top down attentional focus can modulate the cross modal activation in the fusiform gyrus. 6) used an ambiguous grapheme ("A" or "H") embedded either in between "T" and "E" (as in "THE") or between "C" and "A" ("CAT"). The color of the grapheme then depended on which letters the central letter was grouped with, proving that the linguistic categorization of the grapheme, based on spelling, can determine the induced synesthetic colors (Ramachandran and Hubbard, 2001b). The role of linguistic/phonemic effects has since then been shown by several other groups.
Finally, some synesthetes report that imagined letters (visualized in the mind's eye) are, paradoxically more vividly colored than actual printed ones. For example, when you look at a white printed letter, the bottom–up activity in V4 caused by the white “clashes” with the red color induced by cross activation; but for top down imagined letters even though the activity in fusiform number nodes is weaker the final experienced color is stronger because there is no contradictory bottom-up information (Ramachandran & Hubbard, 2001b).
Additionally, a number of groups have demonstrated the importance of language and semantic meaning in synesthesia. Simner and Ward (2006) showed that individuals who experience tastes in response to words (lexical-gustatory synesthetes) actually perceive the taste before they can say the associated word (while the word is still on the tip of the tongue) - demonstrating this gustatory sensation arises from semantic meaning. The significance of semantic activation was recently shown in grapheme-color synesthesia using event-related potentials to record synesthetes' brainwaves (Brang, Edwards, Coulson, Ramachandran, 2008). In this study, sentences were presented one word at a time, ending with a congruent or incongruent color word ("The sky is BLUE" versus "The sky is RED") OR ending with a congruent or incongruent synesthetic grapheme ("The sky is 4" versus "The sky is 7"). Brainwaves in the grapheme condition showed that synesthetic colors are processed in the brain for semantic meaning and context more quickly (approximately 150ms) than color words.
In addition to semantic effects in grapheme-color synesthesia, the saturation and luminance of the color experienced with each grapheme seems to be modulated by linguistic factors, specifically the frequency of letter and number use in a language (Beeli, Esslen, Jäncke, 2007; Smilek, Carriere, Dixon, Merikle, 2007) and similarly by numerical magnitude (Cohen Kadosh, Henik, Walsh, 2007).
The link between synesthesia and metaphor (Ramachandran & Hubbard, 2001b) has already been alluded to. The nature of the link remains elusive given that synesthesia involves arbitrarily connecting two unrelated things (e.g. color and number) whereas there is a non-arbitrary conceptual connection between Juliet and the sun. One potential solution to this problem comes from realizing that any given word only has a FINITE set of strong first order associations (sun = warm, nurturing, radiant, bright) surrounded by a penumbra of weaker second order associations (sun = yellow, flowers, beach, etc.) and third and fourth order associations that fade way like an echo. The overlapping region between two halos of associations (e.g. Juliet and the sun; both are radiant, warm and nurturing) - the basis of metaphor- exists in all of us but is larger and stronger in synesthesia as a result of the cross-activation gene. In this formulation synesthesia is not synonymous with metaphor but the gene that produces synesthesia confers a propensity towards metaphor. A side- effect of this may be that associations that are only vaguely felt in all of us (e.g. masculine or feminine letters or good and bad shapes produced by subliminal associations) may become more explicitly manifest in synesthetes, a prediction that can be tested experimentally. For instance most people consider certain female names, e.g. Julie, Cindy, Vanessa, Jennifer, Felicia, etc. to be more "sexy" than others e.g. Martha and Ingrid. Even though we may not be consciously aware of it, this may be because the former involve pouting, tongue, lips etc. with unconscious sexual overtones. The same argument would explain why the French language is often thought of as more sexy than German. It might be interesting to see if these spontaneously emerging tendencies and classifications are more pronounced in synesthetes.
Taken collectively, these results show that the different forms of synesthesia span the whole spectrum from sensation to cognition and, indeed, this is precisely why synesthesia is so interesting to study.
In summary, these experiments conducted by several groups in the last decade have spawned a new era of investigation into this strange phenomenon that so intrigued Galton. While the topic has been discussed for over a century, the exact definition of synesthesia and what constitutes a "true" form of the phenomenon remains open to debate. But studies on synesthesia in the last decade have taken us on a journey from genes (affecting S2a receptors, perhaps) to anatomy (e.g. fusiform and angular gyri) to psychophysics (texture segregation / contrast effects/ apparent motion / Mc Collough effect / Stroop interference) to metaphor. They suggest that far from being a "fringe" phenomenon as formerly believed (or indeed that it is purely “conceptual” or associative in nature), synesthesia can give us vital clues toward understanding some of the physiological mechanisms underlying some of the most elusive aspects of the human mind.
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