Title: Synaesthesia – a convincing example of a genuine effect in psychology.

Synaesthesia is a condition in which stimulation of one modularity leads to unusual activation of different modularity. According to Simner (2007) the most common synaesthesias (ca. 88%) are induced by linguistically related stimuli such as words, graphemes (letters and numerals) and phonemes which trigger visual, gustatory or olfactory experience (e.g. colour, shape, taste, smell). For synaesthetes, in everyday life, reading a newspaper or listening to CD might result in seeing colours or experiencing tastes. For example (Simner, 2007), when ES hears a major sixth tone interval he tastes low-fat cream. Similarly, on hearing F-sharp he sees the colour purple. Such experience is sometimes described as a “merging of senses”. After Galton (1880) carried out his first studies on synaesthesia in the late 19th century not many scientists were investigating the phenomenon treating it instead as a curiosity. Recently, in the light of contemporary cognitive and neuroscience studies the topic of synaesthesia regained interest. Since initially, evidence indicating that synaesthesia is a real and concrete sensory phenomenon was scarce and based mainly on the anecdotal reports, scientists were interested in testing its genuineness. According to Ramachandran and Hubbard (2001) some accounts of the condition stated that it is solely product of imagination or that such experiences are nothing more than childhood memories of coloured magnets. Others considered the condition to be no more than metaphorical speech or an effect of taking drugs. In this essay I will demonstrate how various studies dealt with the problem of proving that synaesthesia is genuine. One of the most popular methods investigating synaesthesia is The Test of Genuineness (TOG) for Coloured-Word Synaesthesia (Baron-Cohen, Wyke & Binnie, 1987) or the Revised Test of Genuineness (Asher, Aitken, Farooqi, Kurmani & Baron-Cohen, 2006), which allows researchers to study the auditory form of the condition. These tests allow scientists to establish whether the unusual experience accompanying stimulation in one of the sensory modalities is consistent over time. It was previously demonstrated that among grapheme-colour synaesthetes test-retest consistency is very high, reaching between 70% and 90%. By contrast, controls score between 20% and 38% (Asher et al., 2006). Experiencing the same photism (a visual experience of a colour) upon seeing graphemes at various times functions as evidence that synaesthetes do not fake the condition or make up images. Initially, checking whether synaesthetic experiences are consistent over time involved using crayons, colour posters or basic colour terms in English (Berlin & Kay, 1969). Since methods using items like crayons were not very precise extensive colour palettes, available today on every computer, helped make the consistency test very reliable. To standardise testing synaesthetic individuals Eagleman, Kagan, Nelson, Sagaram and Sarma (2007) have devised a thorough Synaesthesia Battery. One of the tests in the Synaesthesia Battery focuses on internal consistency, i.e. participants’ results can be checked for consistency not only after time delay but also within a single session. In the study investigating internal consistency, subjects are shown a letter or a digit on a computer screen. Their task is to match their synaesthetic experience accompanying a grapheme ( i.e. A-Z or 1-9) with the colour from the colour palette. Participants have to undergo 108 trials three times in randomised order. Responses are considered to be consistent if a participant chooses the same or a similar colour upon seeing the same grapheme across the trials at different times. Such consistency separates synaesthetes from controls whose task is to create free associations or use their memory to find a certain colour. The colour equivalent for each grapheme (which helps to calculate colour consistency score) is computed with a mathematical formula which represents the geometric distance in the RGB (red, green, blue) colour spectrum. The formula takes into account that some individuals do not experience colours for all graphemes and others have colour associations only for numbers or letters. In the actual study, using the standardised internal consistency test, Eagleman and his colleagues (2007) found out that colour consistency was high enough to reliably distinguish synaesthetes from controls. Interestingly, one of the synaesthetic participants, EF, demonstrated a “colour drift” in the retest. Namely, during initial testing, EF found letter G green in all three trials. After time delay, G appeared consistently brown to him. Such change does not mean that the experience was not genuine. It suggests that grapheme-colour synaesthesia might change over long periods of time. However, given that there is no time limit on the consistency test, in order to fake synaesthesia one could work out a system translating graphemes into colours. In order to control for this, a speeded congruency test was devised (Eagleman et al., 2007) which should be carried out on completion of the internal consistency test. In this task, participants are presented with the coloured grapheme displayed on a computer screen. In only 50% of trials the graphemes are coloured congruently with the synaesthetic experience reported previously. Subjects are asked to decide whether the colour is congruent or incongruent with their experience. It was shown (Eagleman et al., 2007) that subjects score very highly on the test being correct at 94% of cases with an average speed of 0.64 ± 0.78s. Non-synaesthetes’ scores are correct only in 64% and their response time is significantly longer, i.e. 0.91 ± 0.87s. It can be concluded that short time display and requiring hurried reaction from subjects satisfactorily discriminates between cheaters and real synaesthetes. Another way of determining how real and automatic synaesthetic experiences are allowing us to make systematic predictions involves a Stroop task. In a standard Stroop task subjects are presented with colour names displayed in a congruent or an incongruent colour (McLeod, 1991). For example, a word PURPLE may be written either in a congruent purple or in an incongruent silver . It was found (Hubbard & Ramachandran, 2005) that when subjects were asked to name the colour of the colour names presented to them their reaction time (RT) was longer in the incongruent condition. Since participants were not asked to read the words but still experienced interference it was concluded that reading is an automatic process which interferes with the actual task. If synaesthesia is genuine, colours elicited by graphemes should cause some interference in a synaesthetic variant of the Stroop task. Indeed, participants’ RT in the “synaesthetic Stroop task” when asked to name the colour of the grapheme which is different from their synaesthetic experience is longer than when they are put in an congruent condition. For example, the colour of number 7 which could elicit an experience of colour green, when presented in orange will take longer to be named. On the basis of such results synaesthesia was also suggested to involve an automatic processing. Interestingly, Dixon, Smilek, Cudahy and Merikle (2000) found that synaesthetic Stroop interference can be elicited merely by thinking or imagining the inducing stimulus which would suggest that synaesthesia can be both perceptual and conceptual in nature. To investigate whether synaesthetic experiences appear on conscious or unconscious level of visual processing Rich and Mattingley (2002) developed a synaesthetic priming task. In a study involving this task, participants were presented with inducers (letters eliciting synaesthetic colours) as primes and they were asked to name a colour patch that appeared subsequently. If participants are really synaesthetic the effects of priming should influence their perception. Indeed, in the condition in which the priming was brief but still long enough (500ms) for participants to see it synaesthetes were found to be susceptible to interference in incongruent versus congruent trials. Conversely, in the condition in which inducer lasted only for 28 or 56 ms and therefore its perceptual processing was very difficult, the synaesthetic Stroop effect was absent. Above findings indicate not only that the participants had genuine synaesthesia but also that inducers need to be made available to consciousness in order to elicit synaesthetic colours. One could also attempt to determine whether synaesthesia is truly perceptual by employing pop-out or segregation tests. These tests rely on people’s tendency to separate stimuli from the background and group it into one mental form. If synaesthetic experiences are genuine, seeing coloured items should enhance the grouping effect. To test this, Ramachandran and Hubbard (2003) presented subjects with the display of mixed numbers, arranging 2’s in a way to form a triangle. As the results showed, synaesthetes unlike controls easily noticed the shape of the triangle up to 90% of the time. It can be concluded that for synaesthetes for whom a digit 2 consistently elicits certain colour, a pop-out task is facilitated by their condition. The human visual system relies on various cues including hue, contrast polarity, luminance and texture in order to make sense of objects in the visual field. As a consequence, individuals with synaesthesia are likely to use form cues such as colour to help their visual perception. For example, a motion coherence threshold for a synaesthete should be much lower due to facilitated segmentation of stimuli. This rule would apply only to stimuli eliciting synaesthetic percepts. In the standard motion coherence task, participants are shown a display that consists of an array of dots composed of noise and signal dots. Whereas signal dots move in a certain direction (e.g. rightwards or upwards), noise dots’ movement is random. In the task, subjects are required to guess the direction of signal dots motion. It was found previously (Snowden & Edmunds, 1999) that when the signal and noise dots were given unique identities (e.g. different colours) motion coherence thresholds were lower than when both types of dots looked the same. In a potential synaesthetic motion coherence task, moving graphemes that elicit unusual experiences could take the place of dots. If noise and signal graphemes induce different colour cues aiding visual segmentation, synaesthetes’ motion coherence thresholds should be lower than controls’. If this prediction is supported, such unusual motion coherence task could be used to distinguish between synaesthetes and non-synaesthetes.

Yet another way of testing whether synaesthesia is a concrete and perceptual phenomenon is to study brains of synaesthetes using neuroimaging devices such as functional magnetic resonance (fMRI) or positron emission tomography (PET). If the images coming from the synaesthetic brains are different from the ones coming from a control group, one could conclude that the condition is indeed genuine. Additionally, neuroscience methods aid scientists to study the physiological nature of the phenomenon, i.e. its neural correlates. They also attempt to establish possible overlap between brain areas responsible for synaesthetic perception and general visual perception. Most neuroimaging studies so far investigated grapheme-colour synaesthesia. The results of the studies are somehow inconsistent. In the first neuroimaging study using PET by Paulesu and colleagues (1995), cerebral blood flow was examined when participants were listening to tones or words. The PET pattern revealed that tones and words elicited activation in language areas in both synaesthetes and non-synaesthetes. Yet, in synaesthetes unlike in controls additional firing in the visual associative areas including inferior temporal cortex and parieto-occipital junction was reported. Suprisingly, there was no activation in V1,V2 or V4 which stands in conflict with more recent studies. In 2006, Steven, Hansen and Blakemore with the help of fMRI showed that left V4 and bilateral V1 areas in the primary visual cortex were activated in the brain of a blind synaesthete who listened to words inducing colours for him such as days or months. By contrast, the same regions were inactive when the individual was involved in colour imagery. In study employing fMRI technique (Sperling, Prvulovic, Linden,Singer & Stirn, 2006) it was found that achromatic graphemes that induced “coloured” experiences led to activated V4 area as opposed to achromatic graphemes that elicited “colourless” experiences ( i.e. greys, whites or blacks). Inconsistencies in the results might owe to variability in the spatial and temporal resolution of neuroimaging techniques. These methods often do not allow for precise anatomical localisation. It is also likely, that an absence of activation is not really non-existent but rather invisible to the limited abilities of neuroscience devices or results from methodological differences between studies. Most importantly, the difference in brain activation between synaesthetes and controls is a very convincing argument that the condition is real. Additionally, it is worth remembering that interviews, self-reports and structured questionnaires might also be very valuable for determining the genuineness of synaesthesia because they aid researchers to establish the nature and developmental history of the condition. Such methods also help scientists to find out whether synaesthetes are aware of their condition. Self-reports of synaesthetes are often similar and as a result reveal patterns in the condition. For example, it was found (Harrison & Baron-Cohen, 1994) that most synaesthetes told their friends about their extraordinary experiences at an early age. Usually, after having seen their friends’ lack of understanding they kept their synaesthesia secret until something or someone has mentioned the condition again. Impressively, even though their condition was not “fed” in any way, their experiences remained involuntary, vivid and hard to suppress. Such individuals are also convinced that their synaesthetic percepts have not changed since they acquired their first memory of the condition, typically around the age of four. They believe they have had the condition “for as long as they can remember” (Harrison & Baron-Cohen, 2004). To assess individuals’ synaesthesia by directly asking them about it, Rich, Bradshaw and Mattingley (2004) carried out a study using a questionnaire in which one could find questions not only investigating demographic information but also items designed to characterise synaesthesia. Some of the questions asked “how long have you had synaesthesia” and “do you remember anything in the past that may have triggered the development of your synaesthesia”. It was found that 96% of the synaesthetic participants reported having always experienced synaesthesia, many of which, however, did not know that their percepts were extraordinary until later stage of life. Additionally, one of the items investigated whether any members of subjects’ immediate family experience synaesthesia. Importantly, it was previously suggested (Galton, 1880) that the condition runs in families and that more women than men experience it (Harrison & Baron-Cohen,1994). Direct evidence about the role of genetics in synaesthesia development would make such experiences even more plausible. In summation, given all the standardised approaches described in this essay one does not need to rely on anecdotal reports anymore to believe in synaesthesia. Variants of the Test of Genuineness (Baron-Cohen et al., 1987), various experimental tasks as well as neuroimaging studies and interviews are all research methods grounded on scientific evidence. As a consequence they seem to be very good at determining the genuineness of the condition and as a result at distinguishing between synaesthetes and non- synaesthetes. These method are also proof that standardised, research-based and sometimes very simple procedures are excellent for detecting what is or is not a genuine phenomenon.
References:

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