The Synesthesia Tree: a New Resource for the Synesthesia Community


Paper presented for the VII International Congress on Synesthesia, Science and Art, October 2022, Granada, Spain

Pau Sandham Burns


2021 was the year of the joint launch of El Árbol de la Sinestesia and The Synesthesia Tree, two new websites about the different types of synesthesia. Both the Árbol and the Tree have over 100 pages, each addressing a particular type, subtype or manifestation of synesthesia as well as some phenomena that are not synesthesia but which often prompt questions or doubts. Each unit contains a detailed description of the type in question, reports by people with this type, links to relevant research, curiosities and further information. This creates a linked network similar to the roots of a tree, fully accessible via incorporated tools such as an extensive alphabetical list, interactive charts or the Synesthesia Finder resource, where users choose from brief lists of concepts to quickly reach the information on the type they are looking for.

The site is constantly being updated and improved and has received a very positive response within the synesthesia community. It has a dual aim: firstly to help people with a limited knowledge of the topic to discover and learn about their own synesthesia, an aspect now increasingly facilitated by the development of new online technologies, and secondly to complement the excellent synesthesia resources already available – to mention a few, the Battery Test, Sean Day’s website or that of Sussex University, and the studies and books on the subject – and to make it easier for readers to access them. The presentation addresses different aspects of the past, present and future of the Tree.

The Synesthesia Tree exists alongside the version in Spanish, El Árbol de la Sinestesia. As a professional translator, I very much enjoyed the challenge of adapting the site to both languages and reaching a wider audience. The huge potential of the English version of the Tree as compared to its Spanish counterpart, the future path of both languages in today’s technological world and ideas for extending the Tree to include more languages via human or machine translation are also aspects worthy of reflection.

Keywords: Resources, Languages, Translation, Types of synesthesia, Discovering your synesthesia



On 1 February and 4 April 2021 respectively, the two websites El Árbol de la Sinestesia and The Synesthesia Tree were published online. They have an ambitious goal: to provide quite detailed information on all the currently known types and subtypes of synesthesia, with different examples of how each one is experienced and links to find out more. It is a far-reaching project but also a much-needed and very appealing one, and I embarked on it with huge enthusiasm, spending my free time on it and taking around two years to complete it. Over a year has now passed since the launch and I am delighted to see that it has been a success and the two sites – particularly the Tree – are being read by many people, helping to spread knowledge of a topic that is currently at a key point in its scientific development.



The Synesthesia Tree currently has a total of 134 pages (El Árbol de la Sinestesia has 125, as the Tree includes some additional resources). It has the following components, extensively linked throughout the site:

1. 104 pages basically addressing the different types of synesthesia. Each of these pages is about i) a type, subtype or particular manifestation of synesthesia (88 pages); ii) a particular concurrent/inducer or a group of related synesthesia types (10 pages);  or iii) a phenomenon that is not synesthesia but often prompts questions or doubts (6 pages). These pages contain a fairly detailed description (between 70 and 900 words), real accounts where different people who have these types talk about their experience, and also links to studies, references to websites of interest and any other aspect I considered relevant in each case.

2. Four “root” pages serving as a gateway to all the others: i) the Alphabetical List, which is the most widely used (currently with around 350 entries); ii) a table showing the types of synesthesia by combinations of senses or perceptions (85 entries); iii) a list of the different types of synesthesia grouped by prevalence (64 entries); and iv) the Site Map, listing all the pages, ordered by categories.

3. Brief definitions (6 pages), where the most relevant concepts needed for an understanding of this topic are defined in just a few words or sentences: projection and association, inducer and concurrent, etc. (6 pages).

4. General information (8 pages): All about Synesthesia, The Most Common Types of Synesthesia, the Neurological Basis, Books, etc.

5. A page entitled “Do I Have Synesthesia?” where readers can comment on their own particular cases and will receive an answer.

6. A “News” section and a “Seeking Research Participants” section (both in the Tree only).

7. The Synesthesia Finder (also only in the Tree), a function where users choose between brief lists of concepts to find a rapid route to the synesthesia type they are looking for. This tool was designed so that people can identify their own types of synesthesia, or just find information in a few short clicks. The Finder’s original starting point was synesthetic inducers only, but I have recently extended it to also include concurrents as a gateway for the search and it now consists of a network of 98 interlinked pages providing numerous paths to reach the desired information. It is compatible with any device and has proven to be practical and effective.

8. Multiple links to other websites of interest: The Battery Test, Sean Day’s site, Richard E. Cytowic, Sussex University Synaesthesia Research, the Synesthesia subReddit, Journey through the Senses, etc. Also, only in the Tree, there is a page listing websites with a large number of links to other synesthesia-related sites (Links to Links).


The main goal of The Synesthesia Tree was to fill a void: at the time of its creation, no list existed with a large number of types and subtypes of synesthesia each linked to a specific, detailed description and with real accounts of how they manifested. The questions I often read on my frequent visits to forums like the Synesthesia subReddit or the various Facebook groups had made me aware that many people were looking for a list of this kind, and in 2018 I embarked on the information-gathering process for the subsequent creation of a website that could meet the needs of all these people. However, before addressing the subject of actually constructing the website itself, I would first like to provide a bit of background: the milestones on the road to the Synesthesia Tree’s creation.

The real starting point was in December 2016, when – already well into adulthood of course! – I discovered my own synesthesia. This discovery is not a particularly life-changing experience for some synesthetes, notably those of the newer generations, but for others it certainly is a key moment, and that was the case for me. As I began to look for more information to attempt to clarify and distinguish the numerous types I am lucky enough to have, I developed a great interest in the subject. I was also interested in sharing this knowledge with others in the same situation, knowing they would greatly benefit from being able to gain a greater understanding of their own experiences and find a name for them.

With this aim in mind, a psychology graduate friend and I prepared and conducted a workshop, “La sinestesia en todos los sentidos”, covering its many different aspects.[1] As part of the preparation for this workshop I made my initial approach to classifying the different types, so that Eduardo and I could provide a clear explanation of them to the people taking part. The workshop was given in Seville in 2018.

The knowledge I accumulated at around this time, and also the fact that 2018 was the year I attended the VI International Congress on Synesthesia, Science and Art in Alcalá la Real, Spain, coming into contact with a network of like-minded people involved in this field, served as further inspiration to continue my engagement with this fascinating subject, which was now of such great importance in my life.

From that time on, purely for fun and the desire to learn, I embarked on a search for a way of classifying all the different types and subtypes of synesthesia as clearly and simply as I could, to put this knowledge across to other people. Given the complexity of the topic and its huge number of manifestations – and the many phenomena that fit part of the definition but are not accepted as such, or which command varying opinions from different researchers – I quickly realised that the original idea of making a flow diagram was simply not feasible, as it would immediately reach monstruous proportions and get completely out of hand. After experimenting with a few different ideas I transferred my efforts to an Excel spreadsheet, with some similarities to Sean Day’s chart on his website[2] but with each cell containing the conventionally accepted names for each type, together with more information: descriptions, links, and so on. The first row and column contained the 5 senses (sight, hearing, smell, touch, taste) followed by a series of additional elements that can be synesthetic inducers or concurrents: cognitive or conceptual concepts like graphemes, time units and other sequences or groups; proprioception/sensation/interoception/thermoception/kinestesia; emotion; people; mathematics; personification; parity; and spatial positioning. Logically some of these combinations gave rise to several different types of synesthesia within the same cell while other cells were empty, but that had no practical significance as the Excel table was not designed to be perfectly balanced or to be published or even shared with anyone: it was a mere reference point for getting all the necessary information together to be able to create the definitive system for “classifying the types” that I could use as a springboard to create my website.

The practicality of the Tree resides in the fact that each layer of elements is hidden behind another: it’s a huge maze, but all we can see is the entry, the interface. That was the key to making such a complex enterprise available for consultation. The invisibility of the roots or branches of the tree was the solution, because the Tree had to be huge by nature: first and foremost I wanted it to be exhaustive, covering all the types of synesthesia, both known and yet to be discovered.

Almost 90 types and subtypes of synesthesia are described in the Tree. Obviously more types exist, but some were left out because although they are theoretically possible, there is no record at present of anyone actually having them. This long list of “rejects” includes possible manifestations such as Pain personification, Grapheme-tactile, Perceived emotion-sound or Own body movements-taste. If any real accounts of these uncommon types emerge I intend to gradually add them to the Tree. They might have few readers, but my aspiration to completeness means they have to be there.

The Tree is subject to a constant process of updating and correction, and I acknowledge that it will never attain perfection. Since it was published I have continued to make new discoveries and a few new pages[3] have been created. Needless to say I have a long list of ideas awaiting conversion into future pages or additions to those that already exist, and the only thing stopping me from progressing as I wish is a lack of time.


I have identified several categories of people who could find the Tree useful.

As I have already mentioned, the core idea was that it could help people seeking an explanation of their own experiences: people wanting to know or confirm whether they had synesthesia. I believe it is particularly useful for people who have the less common types, as up to now there has been a near-total absence of any kind of specific, meaningful description of these types online. The Tree was also conceived for people who already know they have one or more types of synesthesia but who would like to find out whether they have any further types and what their names and characteristics would be; and it can help others with experiences that are not synesthesia but which are often confused with it (some examples being the correlation between smell and taste, misophonia, or emotions and memories being triggered by smells and tastes).

I have also confirmed that the Tree benefits users who help others by answering their questions on forums such as Reddit or Facebook. As a moderator of the Synesthesia subReddit, I have the satisfaction of seeing that the general level of knowledge is increasing and the use of the correct terms for each type of synesthesia is now a little more widespread, partly thanks to the Tree. It can also be an aid to people who are not synesthetes themselves but want to understand or help others who are: their children or other family members, their SO, friends or colleagues, students or even patients.

Additionally, it has proven to be a good source of information for people writing about synesthesia: journalists wishing to write a general article on the subject for online magazines but without much prior knowledge of the topic; fiction writers and video creators. I would be happy if this could contribute to avoiding some of the inaccurate, outdated information often found in informative articles whose authors draw on a multiplicity of sources, many of which are unreliable but difficult to distinguish when the writer has insufficient prior knowledge to do so.

Finally, I would like to mention the case of researchers who sometimes seek starting points to approach synesthesia types that are little studied or scarcely known and who might find a resource of this type useful, despite not being a scientific study in itself. An example of this is the recent collaboration with Ninghui Xiong and James Wannerton on their presentation “Bringing Synaesthesia To Life: A study of synaesthesia in ancient Chinese culture”.[4]

And I would naturally like to add to these categories all people with a general interest in the subject, whatever the reason may be.

4.1 How can it help improve people’s knowledge of synesthesia in general?

This is one of the main goals of The Synesthesia Tree. I believe the process has already begun and I hope to see it continue. In a little over a year, both the Árbol and the Tree have achieved a high ranking in the search engine results pages and I am pleased to see how they are progressively overtaking some general articles that had previously cornered some of the top spots despite containing errors and outdated information.

My aims are therefore as follows:

-To provide clear, detailed descriptions of all the existing types of synesthesia, most of which were not previously available in other online resources.

-To supply information that is easy to read but serious and well-grounded, seeking a midway balance between general-interest articles and scientific papers. Although I am aware it will never be perfect, I have made a major effort to find a style that will be basically acceptable for all readers, from people making their first contact with the subject to users already well-versed and knowledgeable.

-For the Tree to be complete, covering all types of synesthesia.

-For there to be different levels of detail: brief overviews, but also a network of links so that readers can access a more in-depth description of each aspect if they wish.

-To provide concise and clarifying explanations of items about which there continues to be some confusion, e.g. which types of synesthesia are really the most common, what subtypes of musical synesthesia exist or what the word ideasthesia means.

-For it to be accessible: easily found in search engines and compatible with different devices.

-To accurately name all the different types of synesthesia and to include all the alternative names habitually used to describe them. This information figures at the top of each page.

-To provide correctly written information in two languages, freeing users from having to resort to the unreliable texts produced by machine translation or non-professional human translators with little knowledge of the subject.

-To keep the site constantly updated, and to add any new types or manifestations of synesthesia if they arise.

In short, as the title of this presentation suggests, The Synesthesia Tree is designed to be a new resource, useful to the Synesthesia Community in many different ways.


As a professional translator, I have been involved in linguistic pursuits since I received my degree in Modern Languages from Sheffield University many years ago. I wrote El Árbol de la Sinestesia in Spanish, although I had always intended there to be two sites existing alongside each other, one in Spanish and the other in English. Adapting and translating El Árbol to the English version was a challenge I greatly enjoyed. Producing an informed, heartfelt translation with a good degree of freedom but anchored at the same time to the roots of a tree that was my own creation enabled me to embark on another journey of learning, more relaxed and straightforward than the first but just as enriching on a personal level. So what has my experience been with having two websites, identical (or very similar at least), in two languages, with two audiences, reaching two worlds with many similarities but also many differences? And what can this experience tell us about the situation of languages online today?

I’ll begin by making some reflections on the traffic each site receives. The Tree has approximately 8 times more traffic than El Árbol. Bearing in mind that the Tree includes an additional resource, the Synesthesia Finder, accounting for up to 10% of the total traffic, the final ratio is around 1:7 in favour of the Tree, so the imbalance is still substantial. It is clear that the Tree receives more mentions and more links from an English-speaking online synesthesia community with a lot more members, and some of these links are on influential websites with a huge number of visits such as the Synesthesia subReddit, but the vast majority of the traffic to both sites is from Google (and to a lesser extent other search engines like Bing, Yahoo or DuckDuckGo). Both The Tree and El Árbol have similar search engine rankings and are both highly positioned: running a search for specific synesthesia types, a high percentage of their pages are either in first position (in “position zero” heading the list or as the first result after the featured snippet), or within the top 5 or 10.[5] Therefore, on an equal SEO footing, the ratio between the two suggests that the English version has a natural audience of much greater proportions than the site in Spanish.

Although it is difficult to establish the exact prevalence of each language on the Internet in general, a recent study[6] indicates that 62.2% of all websites are written in English and 3.8% in Spanish. These figures suggest that despite being an important online language – the fourth in the ranking, after English, Russian and German – Spanish has a mere 6% of the representation achieved by English (1:16). As to the number of users in each language, those using English potentially account for many more, not only due to the large numbers of websites available to them but also because in addition to the native speakers of both languages there are approximately 7.5 times as many non-native English speakers as non-native Spanish speakers,[7] and it can therefore be assumed that most speakers of the world’s other languages, in the case of not having access to reliable pages in their mother tongue on a specific subject, would choose to read (or translate with the aid of a machine translation engine) a website written in English rather than in Spanish. Another interesting figure is that the Wikipedia, existing in 280 different languages, has a ratio of approximately 1:7 between the readers of its Spanish and English language versions,[8] which is fully in line with the proportion for El Árbol and The Tree.

What cannot be found, of course, are figures showing how many people fail to find what they are looking for because they are searching in a language other than English. I am convinced that this has been a problem for many Internet users when attempting to glean information on synesthesia, although luckily advancements have been made that point to a solution. In my own case, I came up against this difficulty in around 2013 - 2015, when looking online to find an explanation for my atypical sensory experiences but running the searches in Spanish rather than English. Filling the search box with phrases such as veo los olores en el aire (“seeing smells in the air”) or formas visuales con los olores (“visual shapes with smells”) only led me down blind alleys and it was not until late 2016 that I decided to have another hack at it, but in English this time, and was able to find the explanation for my manifestations of olfactory-visual synesthesia – after having travelled various convoluted paths of unrelated articles, with hundreds of (albeit interesting) comments. However, with the exponential increase in the number of websites existing in Spanish – the total number of sites in all languages has doubled in the last 7 years[9] – today it is much more likely for this type of search to yield relevant results. I feel proud of having “added my brick to the building” in this regard with El Árbol, and that since 2021 Spanish-language googlers will be taken straight to its specific and credible information without first being dragged through digital mazes and unconnected forums, even if their synesthesia-related search terms are vague and incomplete. So that’s “mission accomplished” in Spanish, and contribution made in English. However, many people are functional monolinguals in languages other than English (or Spanish) or do not master it sufficiently to read articles and get information, while others simply prefer to run their searches in their non-English mother tongue. So what happens if someone is searching in, say, Turkish, Swahili or Albanian?

5.1 Machine translation: closing a door and opening a window?

Running a few searches on Google in different languages to assess the situation, my attention was drawn by an article on synesthesia written in Russian in December 2021 by the journalist Artyom Luchko based on information provided by Anton Sidoroff-Dorso.[10] The same article can be found online in a total of… 14 different languages! (English, Spanish, Portuguese, German, Indonesian, Norwegian, Swedish, Slovakian, Croatian, Hungarian, Swahili, Tagalog and Cambodian). With over 16,000 words, it is a long article and therefore fairly accessible in the search engines, even in response to queries using vague or idiosyncratic terms. At first sight it would seem that people running searches in all these languages have found an open door to detailed, quality information on the subject of synesthesia. But is that really the case? Sadly, it is not. Apart from the problem of the scientific level of this specific article not being easy to understand without a prior knowledge of synesthesia or neurology, the non-human machine translation unfortunately even further diminishes the chances of the average person being able to learn from it, comprehend it or enjoy it. Although some parts of the translation are quite readable or even good, others are seriously deficient. The English translation can be reliably taken as being one of the best of the bunch, even though it is naturally much worse than an informed human translation, but the situation for the other languages is not as bright. Today’s neural machine translation, accessible via online translation tools such as Google Translate, DeepL, Bing Microsoft Translator or Yandex Translator[11], depends for its success on learning and huge amounts of practice in the language pair in question, and it is still rudimentary when working with two languages that are not often paired (“zero-shot languages” or those with which the machine translation system has never been trained). It is likely that combinations such as Russian-Indonesian, Russian-Tagalog or Russian-Swedish fit into this group. Another challenge is to achieve a high-quality translation in target languages that are morphologically rich such as Hungarian, Finnish or German, or those with profound structural differences from the source language like Chinese, Japanese or Korean. As a result, these language pairs are under-developed in comparison with others such as English and Spanish, complicating the job of understanding them even more. In these languages, a human post-edition of the text would be necessary at the very least to make any article understandable and pleasant to read, except for those written using only simplistic phrases and basic terminology.

This brings us to a few more general reflections on the advent of machine translation. It has improved exponentially in recent years. In just a decade and a half we have progressed from the fun and laughter provoked by the strangely opaque and mind-boggling sentences served up to us by Babelfish and other platforms of the time to texts that even professional human translators can learn from, even though we hate to admit it. Here is an example of the difference:[12]

Original text in Spanish:

Hola a todos los aficionados a la escúter. Mi nombre es José y vivo al este de España, en la región de Murcia y en un pueblo pequeño llamado Los Alcázares, a orillas del Mar Mediterráneo.

Translation by Babelfish, August 2006
Hello to all the fans when escuter. My name is alive Jose and to the east of Spain, in the region of Murcia and a small town called the Palaces, to borders of the Mediterranean Sea.

Translation by Google Translate, May 2022
Hello to all scooter fans. My name is José and I live in the east of Spain, in the Murcia region and in a small town called Los Alcázares, on the shores of the Mediterranean Sea.

Machine translation is clearly set to become part of knowledge dissemination in the field of synesthesia: if only for practical reasons, we cannot expect there to be much first-hand information in all of the less-spoken languages, proficiently written or translated to a high level by native translators. Today we need professional post-editors of machine translations – whose job is to minimise the errors and turn the texts into something readable and correct – and this speciality does already exist, despite its initial rejection by much of the translator community.[13] However, even without post-editors, if the accomplishments in this field continue at a similar pace to those of the last few years, in the next decade we are likely to see tremendous progress in the availability of readable material for anyone who cannot, or who does not want to, look for their information in English.

In the specific case of The Synesthesia Tree, a future page of the Tree expressly designed to be translated into multiple languages could be very useful. This would mean the text would be specifically written with a view to its machine translation, with simple grammar and vocabulary, avoiding any expressions that could give rise to misunderstandings, ambiguous terms, restrictive cultural references or any mentions of people that might confuse the machine with regard to their gender. Given the bilingual nature of the Árbol/Tree, a choice could even be made between translating this purposefully-created page using either English or Spanish as a source language, according to its structural similarity to the target language or how closely related it is (although tests would have to be carried out to determine whether this was the best criteria, as the great track record of English in machine translation might mean this latter language is always preferable to others). Another strategy could be to select a limited number of key target languages, commission a proficient human translation into each one and use these excellent translations as a basis for subsequent machine translation into all their related languages. I believe that this idea to globalise and make available at least some of the information in the Tree would be an interesting development, provided a good level of quality was achieved. Maybe some future support for this hybrid machine/human translation project could be obtained, to counter the idea that for many people, reading and looking for information in their native language is little less than a luxury. To conclude my thoughts in this section, I would like to say that machine translation is likely to close some doors but will also open many, many windows, in the field of synesthesia and in countless others, and we should therefore assume and embrace it as we await the flood of interesting advancements that is sure to arrive in forthcoming years.


The Synesthesia Tree has many possible futures ahead of it. Only time – both chronological and available – will tell whether they materialise or not, but I can see a huge array of possibilities waiting to be developed and explored, on my own account or in collaboration with other people.

The most vital, immediate advancement is to continue updating, correcting and complementing the pages of the Tree with new information, and adding new pages where the case may be. But I would also like to make a few more general reflections on the future of the Tree and stress some aspects I believe to be important:

- As synesthesia is a relatively young science, its study is booming, with new discoveries being made each passing year.  The Tree has to keep up with these new findings and change over time.

- As the grower of the Tree I feel responsible for contributing to raising the level of knowledge about synesthesia in general, and perhaps more ways can be found to achieve this improvement, with new pages, more links to relevant sources of information and other novel strategies such as creating specific content geared to rapidly achieving a high SEO ranking to displace some of the more sensationalist, error-ridden articles that can still be found among the top search positions, although it is clear that the situation is improving.

- Uniting. Centralising. Growing in size. The Tree could become a vehicle for people to access many dependable sources. It could be the trunk that gives way to infinite branches, not just those that grow on the Tree itself but all the branches existing in the wood we call the Synesthesia Community. This would mean improving the current resource linking system and perhaps devising a new one implicit in the pages themselves making it very easy to add information and links.

- Emerging new technologies could be used to find other ways of getting this information to its audience. New apps, new digital platforms, better accessibility, a more powerful system of voice commands, the use of new formats very different from the ones we use today. The trend towards passivity and zero effort on seeking information prevails… and it is a path well worth exploring.

I am attracted by the idea of us working together in future as the community we represent, and my role could be to power some of my own projects and also form part of some of yours, so that we can share reliable, accurate information on synesthesia with an even greater audience. Collaborating in this way, the future of the Tree will be to improve, deepening its roots and expanding its branches, growing towards the light to be more useful to all of us, both inside and outside the Synesthesia Community.


Alonso, E., Vieira, L.N. (2017). The Translator’s Amanuensis 2020, in The Journal of Specialised Translation, n. 28, pp. 345-361.

Alonso, E., Vieira, L.N. (2020). “Una introducción a la posedición”. United Nations - Spanish Translation Service, Geneva.

Esteban Hernández, S. (2020). “Evaluación de la calidad de la traducción de motores de traducción automática neuronal en textos del ámbito jurídico”, Faculty of Translation and Interpretation, Universitat Autònoma de Barcelona, Spain.

Fan, A. et al. (2021). “Beyond English-Centric Multilingual Machine Translation”, in Journal of Machine Learning Research, n. 22, pp. 1–48.

Maučec, M. S., Donaj, G. (2019). “Machine Translation and the Evaluation of Its Quality”, in A. Sadollah, & T. S.  Sinha (Eds.), Recent Trends in Computational Intelligence, IntechOpen, London.

Wiesmann, E. (2019). “Machine translation in the field of law: a study of the translation of Italian legal texts into German”, in Comparative Legilinguistics, Vol. 37, pp. 117-153.

[1] La sinestesia en todos los sentidos, Pau 365 and Eduardo Ruiz, Seville, Spain, June 2018.

[4] Xiong, N. and Wannerton, J. (2022) “Bringing Synaesthesia To Life: A study of synaesthesia in ancient Chinese culture”. VII International Congress on Synesthesia, Science and Art, October 2022, Granada, Spain.

[5] On the basis of 40 Google searches for names of different specific types of synesthesia in late May/early June 2022, the sites ranked in the top 10 search results in 90% (Tree) or 95% (Árbol) of cases. The percentages for rankings in position 0 or 1 were 73% (Tree) and 72% (Árbol).

[6] Based on the top 10 million websites by Source: W3 Techs. 2022. 

[8] Finances Online/Wikipedia. 2022.

[12] From the online Suzuki Burgman USA Forum, August 2006.

[13] See, for example, Alonso, E. and Vieira, L.N. (2020). “Una introducción a la posedición”. United Nations - Spanish Translation Service, Geneva.

No comments:

Post a Comment