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By: Cindy Qiu

Scientific inquiry into synaesthesia has only become popular in the past few decades, despite the earliest medically documented cases being recorded in the early nineteenth century [1]. Because of the wide variation in types and degrees of experience within this phenomenon, the term ‘synaesthesia’ was not implemented until a large number of cases surfaced. Many of the first studied cases involved the the perception of colour. There was much interest at the time regarding the relationship between sounds and colours, a topic first brought to light in the seventeenth century by eminent scientists such as Isaac Newton[2]. He specifically believed in a physical law that could relate the seven colours in the light spectrum with the seven octaves of audible pitch [2]. This led to a liberal outlook in the field of cross-sensory perceptions, although no such experiences had yet been recorded. The lack of case studies prior to the nineteenth century could be due to the supernatural beliefs circulating in society at the time, such as witchcraft and the presence of demonic spirits [2], and reports of synaesthesia could be interpreted as hallucinations or delusions. These theories could have easily discouraged people from expressing their experiences since anything outside of the norm could result in the death of the individual. The idea of synaesthesia posed a difficult question in the knowledge of sensory mechanisms in the early nineteenth century. It was believed at the time that each sense was experienced by the connection to its corresponding sensory organ. This was thought to be an exclusive one-on-one connection, and cross-sensory connections were not possible [2].


Historical Perspectives


1. First medical report of synaesthesia


The first report of a synaesthetic experience in the scientific community was self written by Georg Sachs in 1812 [1]. In his attempt to educate his readers about albinism, he also incorporated a portion about his experiences as a synaesthete. His dissertation revealed that letters of the alphabet, musical notes, and even certain ideas had specific associations with colours. Georg Sachs did not attempt to offer an explanation as to how his experiences were taking place, but started the dissertation by speculating that it was a matter of the mind rather than of the eyes [2]. He also held the belief that it was not connected to his albinism and that he was not unique in his perceptions [2].
Specific colours corresponding with specific letters: a possible representation of what Georg Sachs experienced.
Specific colours corresponding with specific letters: a possible representation of what Georg Sachs experienced.


2. Historical theories


i. Informal theories


Sachs did not think of his cross-modal perceptions as a negative occurrence, a view shared (for the most part) by those that read his dissertation.[2]. In fact, one reviewer thought it may have been due to a heightened sense of vision that this colour association with non-visual stimuli [2]. Another suggested that the cause of this specific form of synaesthesia was the overdependency on one of the senses [2]. In Sachs’ case, his dependency on vision was greater than on the other senses, therefore his visual response was active even as a result of other methods of stimulation.

ii. Formal theory


In 1848, Charles-Aguste-Éduoard Cornaz, a well known surgeon, addressed Sachs’ experience as an “abnormalit[y] of the eyes and their appendices” [2]. His take on the form of synaesthesia described by Sachs was what he termed “hyperchromatopsia”, the hyperactivity of colour perception. This proposal focused on this phenomenon at the level of the sensory organ rather than the brain, where all senses are processed and integrated [2]. At the time of the proposal however, this was a reasonable explanation since the mappings of the brain and its neuronal networks had not yet been established. Cornaz may not have had the last word in understanding synaesthesia, but he played an important role in encouraging people to share their synaesthetic experiences so that subsequent research and theories could develop [2].


Current Perspectives


1. Modern theories


Physiological evidence for the existence of synaesthesia has now been shown by researchers involved in everything from diagnostics to neuroimaging [3]. Brain regions that some studies claim to be involved in various forms of synaesthesia include the frontal and parietal cortices, and other higher-order sensory pathways [4].The heightened interest in synaesthesia in the past few decades has demonstrated the usefulness of finding its cause. While curiosity lead researchers in determining the underlying mechanisms behind synaesthesia, discovering more about the senses increases the overall comprehension of how each function on their own, as well as together [5]. Further understanding of how the senses work can thus lead to practical applications.

A well accepted model of synaesthesia in present day highlights the spectrum of cross-modal experiences on which it belongs. Current theories state that the ‘normal’ population – non-synaesthetes – frequently have multiple sensory activation from a single stimulus. Synaesthesia is therefore the extreme on a spectrum of cross-modal experiences [4]. Animals, mostly primates, also exhibit cross-modal perceptions, and can be used to determine whether there are evolutionary purposes in the presence of this phenomenon.

Another theory proposed in 1988 by Maurer and Maurer [6], and further refined by Maurer and Mochland [7] is the idea that infants and young children start off as synaesthetes. As certain cortical connections compete with one another, some die off. This results in fewer cortical associations in adults compared to children. The normal brain after this pruning process contains specific patterns and connections from one brain region to another. The brain of a synaesthete may fail to undergo synaptic pruning, and therefore have persistent connections between different senses that normally would have disappeared.

2. Synaesthesia-related experiences in animals


Animal studies in synaesthesia help to uncover certain evolutionary topics within human development. The association between high pitch and light colours versus low pitch and dark colours is found to exist in both chimpanzees and humans (in this case, non-synaesthetes).

chimp.square.jpgIn a study by Ludwig, Adachi, and Matsuzawa (2011)[8], a white square and a black square would appear beside each other on a screen. For each trial, the participant (human or chimpanzee) was required to pick out either the black or white stimulus. Meanwhile, a high pitched noise or a low pitched noise which the participant was instructed to ignore would be played in the background. When the pitch and the required stimulus was congruent – for example, a low pitched noise was in the background while the participant was asked to select the dark stimulus – both humans and chimpanzees reacted faster than in incongruent conditions. Chimpanzees were faster in responding in congruent conditions, but with less accuracy than humans.chimp.accuracy1.jpg

The presence of this mild form of synaesthesia in both species can be used to formulate hypotheses regarding the development of language and vocabulary. For example, some words have certain connotations, or are used to describe things that are different from their literal meaning. There is the possibility that there may be a natural basis for the usage of particular words.

3. Synaesthesia-related experiences in the normal population


The full complexity of the underlying mechanisms of a synaesthete’s brain may not yet be fully uncovered, but the road in reaching that complexity may require first the understanding of milder experiences on the cross-sensory spectrum. In a study by Bien et al (2012) [4], fourteen ‘normal’ participants were required to perform a congruency test. The congruencies in this experiment involved the pairing of high pitch with small object, versus low pitch with large object. In each trial, either a large or small circle would appear on the right side or left side of the screen. This was accompanied by a high or low pitch, originating from either the right or left side. The participant was then asked to indicate which side the sound originated from. The congruent condition would entail the small circle residing on the same side from which the high pitched sound emanated (Figure 1).pitch.size.jpg

After these ordinary trials, the participants would undergo the same trials with continuous TMS pulses
inhibiting parts of their parietal lobe for 40s. Areas of interference included the intraparietal sulcus which has been known to play a role in spatial functioning.








Without TMS stimulation, accuracy was lower for judging the source of sound if it was congruent from the size of the circle than if it was incongruent. This is because an incongruent condition would produce less of a cross-modal experience and therefore easier to distinguish that in the congruent condition. With the effect of TMS however, accuracy for the congruent condition significantly increased (Figure 2).tms.jpg

This may be due to the fact that interference from TMS prevented the activity of certain cross-modal pathways, and therefore a reduction in synaesthetic experience lowered the association of size to pitch. Relationship between specific areas of the brain were also revealed through the use of EEG and ERP (Event-related potential) technology. The ERP of the frontal cortex was similar to the ERP of the parietal cortex when only the parietal cortex was stimulated with TMS (Figure 4). This indicated a relationship between the two regions of the brain when presented with the size-pitch cross-modal perception.

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Many other studies have dotted the field of synaesthesia with normal population research. Toddlers have been shown to have innate biases towards which shapes and colours letters of alphabet embody (a type of colour-grapheme synaesthesia) [9]; higher pitch is often related to sour- or sweet-sounding food names[10]; and higher pitch is associated with actual sweet or sour tasting foods whereas bitter or umami correspond to lower pitches [11].

4. Impacts on other cognitive functions


Having more connections between the senses may be an interesting topic of study, but does it hold advantages in daily life? Synaesthetes have been known to report feelings of negative affect when incongruencies within a type of synaesthesia is too great [12]. One of the benefits of being a synaesthete however seems to be an enhanced memory, as is often reported by those experiencing synaesthesia [13]. On the other hand, while synaesthetes excel in particular memory tasks, they are also known to do worse than the controls in other memory tasks [14]. To test recall ability of synaesthetes who reported to experience certain colours with certain letters or words, a series of experiments were conducted by Radvansky et al (2011) [15].

Experiment 1
The participants of the study included eight synaesthete and a group of non-synaesthete controls. Some of the words were congruent with the synaesthetic experiences while others were not. The participants (both control and synaesthetes) were then asked to recall as many words as possible in any order. The performance of the control group was not affected by the colour of the words, therefore the colour does not increase or decrease the capacity for memory. For synaesthetes, performance of recall was better than the control group in congruent conditions, but worse when the conditions were incongruent. The decrease in recall ability was arguably due to multiple modalities struggling to coexist in the given condition, and thus took away from the mental processes of the working memory.

Experiment 2
The von Restorff isolation effect occurs when out of a list of words, one word stands out. For instance, if out of a list of words written in black ink one is written in red, that words will be isolated and will be more likely to be recalled. When synaesthetes and the control group were asked to recall from a list of words exhibiting the von Restorff isolation effect, the control group was more likely to remember the isolated word that synaesthetes. This was thought to be caused by the pre-exhisting relationship between words and colours already held by the synaesthetes. Their own perceptions of colour drowned out the uniqueness of the isolated word and thus made it no easier to recall than any other word on the list.

Experiment 3
In Experiment 2, the isolation effect did not work for synaesthetes because the colour incongruencies may have overwhelmed the goal of the effect. However if the word itslef is different from the others, for example semantically rather than just its colour, the isolation effect then has a chance of working. What was found was that making a word stand out based on its meaning also did not increase the performance of synaesthetes like it did with the control group. This was because synaesthetes focus more on the physical characteristics of the word rather than its semantics.
Experiment 4
The final experiment was a DRM false memory task. This involved showing a list of words with high association to each other as well as to a word that was not on the list. Embedded in the list was also a list of control words that had little or no association with most of the words on the list. The test was to see whether the false memory word would be recalled. The results showed that synaesthetes were less likely than control group to recall the false memory word, and also more likely to recall a control word, possibly because once again less attention was paid to the semantics rather than the physical attributes of the words.




References

1. Jewanski, J., Day, S.A., & Ward, J. A Colorful Albino: The First Documented Case of Synaesthesia, by Georg Tobias Ludwig Sachs in 1812, Journal of the History of the Neurosciences: Basic and Clinical Perspectives 18:3, 293-303 (2009).

2. Jewanski, J., Simner,J., Day, S.A., & Ward, J. The Developmen of a Scientific Understanding of Synesthesia from Early Case Studies (1849–1873), Journal of the History of the Neurosciences: Basic and Clinical Perspectives 20:4, 284-305 (2011).

3. Hochel, M., & Milán, E.G. Synaesthesia: The existing state of affairs. Cognitive Neuropsychology 25:1, 93- 117 (2008).

4. Bien., N., Oever, S., Goebel, R., & Sack, A. The sound of size Crossmodal binding in pitch-size synesthesia: A combined TMS, EEG and psychophysics study. Neuroimage 59, 663-672 (2012).

5. Holcombe, A.O., Altschuler . E.L., & Over, H.J. A developmental theory of synaesthesia, with long historical roots: A comment on Hochel & Milán (2008). Cognitive Neuropsychology 26:2, 227-229 (2009).

6. Maurer, D., & Maurer, C. (1988). The world of the newborn. New York: Basic Books.

7. Maurer, D., & Mondlach, C. J. (2005). Neonatal synaesthesia: A re-evaluation. In L. C. Robertson &S. Sagiv (Eds.), Synaesthesia: Perspectives from cognitiveneuroscience (pp. 193–213). New York: Oxford University Press.

8. Ludwig, V.U., Adachi, I., & Matsuzawa, T. Visuoauditory mappings between high luminence and high pitch are shared by chimpanzees (Pan troglodytes) and humans. Proceedings of the National Academy of Sciences of the United States of America 108, 20661-20665 (2011).

9. Spector, F., & Maurer, D. The Colors of the Alphabet: Naturally-Biased Associations Between Shape and Color. Journal of Experimental Psychology: Human Perception and Performance 37:2, 484-495 (2011)

10. Crisinel, A.-S., & Spence, C. A sweet sound? Food names reveal implicit associations between taste and pitch. Perception 39, 417-425 (2010).

11. Crisinel, A.S., & Spence, C. As bitter as a trombone:Synesthetic correspondences in nonsynesthetes between tastes/flavours and musical notes. Attention, Perception, &Pyschophysics 72:7, 1994-2002 (2010)

12. Callejas, A., Acosta, A., &Lupianez, J. Green love is ugly:Emotions elicited by synesthetic grapheme-color perceptions. Brain Research 1127, 99-107 (2007)

13. Yaro, C., & Ward, J. Searching for Shereshevskii: What is superiorabout the memory of synaesthetes? Quarterly Journal of Experimental Psychology 60, 681– 695 (2007).

14. Smilek, D., Dixon, M. J., & Merikle, P. M. (2003). Synesthetic photisms
guide attention. Brain and Cognition, 53, 364 –367

15. Radvansky, G.A., Gibson., B.S., & McNerny M.W. Synesthesia and memory: Color Congruency, von Restorff, and False memory Effects. Journal of Experimental Psychology 37, 219-229 (2011).