Cognitive Science: An Introduction/What the Senses Have in Common

Introduction to SynesthesiaEdit

Synesthesia is a condition in which a person experiences abnormal responses to stimuli, in addition to the typical responses those stimuli would evoke [1]. In simpler words, synesthesia is a condition where one sense is experienced simultaneously but involuntarily with a second sense. While this is a very broad definition, it is difficult to get any more specific since there are over 60 different manifestations of this condition. The most frequent forms are graphemes to colours, time units to colours and sounds to colours [2]. Even with the vast variety of forms, this condition remains difficult to identify. It requires the same, if not more, detailed line of assessment as any other psychological condition. In the same light, the development of synesthesia is considerably complex. Various theories and hypotheses are attempting to explain where it all starts. The research on this condition has been evolving for the last 200 years and continues to develop today.


Like many other concepts in science, synesthesia is a constantly developing topic. It was first introduced in the late 18th century by German poet and philosopher, Johann Gottfried Herder. In his Treatise on the Origin of Language, in 1772, he termed it simply as an “obscure feeling” [2]. This was the first of many titles the condition was given over the following century. In 1812 Georg Tobias Ludwig Sachs, a synesthete himself, published his work on the albinism of his sister and himself, along with a mention of some strange phenomenon he was experiencing [3]. His publication did not include a name nor a description of the condition however it did line up with the previous ideas of Herder. At this point, the research on all known synesthetes came down to having strange experiences with colour sensations, therefore the terms used were often derived from the word ‘colour’. The first real clinical term was given by Charles-Auguste-Édouard Cornaz in 1848 and he called it hyperchromatopsy (perception of too many colours) [2]. In 1864 it was named pseudochromesthesia by Ernest Chabalier who put more focus on it being a vision interference issue [2].

It was Jules Antoine Millet who coined the term synesthesia in 1892. However, some research suggests another scientist, Frederic W. H. Myers, who used the term at the same time, may have been the first. From then on, the term synesthesia moved through different languages and was widely accepted everywhere [2].


Currently, the research on the development of synesthesia is inconclusive. The literature still has not pinpointed an exact reason or cause for the condition. There are genetic theories that imply it is heritable, however, there have been cases with sudden onset of the condition in children and some have it appear in adulthood [4]. It has even been speculated that synesthetic experiences can be learned by training.

Immune HypothesisEdit

This hypothesis describes how some components of the immune system and central nervous system play an important role in the development of synesthesia. As claimed by this hypothesis, the genes that tend to normal cortical development are also involved in the development of synesthesia. These genes have dual functions for both immune function and altering connectivity [4]. The cross-activation model and the disinhibited feedback model are used to explain how the developmental processes that are tied to cortical connectivity must be largely dependent on the immune system [5].

Serotonergic Hyperactivity HypothesisEdit

This hypothesis, in contrast to the immune hypothesis, leads on the basis that excess serotonin in the brain is a cause for synesthesia later in life. This serotonin build-up can happen in different ways. It can be acquired following a brain injury or induced by psychedelic hallucinogens [4]. There is a lot of evidence supporting that certain psychedelic hallucinogens that are serotonin agonists will often induce auditory-visual synesthesia [6].

Neonatal HypothesisEdit

The neonatal hypothesis assumes that the synesthetic associations between shapes and colours are present in infancy, along with ordinality and sound as additional factors of these associations [4]. This hypothesis states that synesthesia is present at birth in all individuals but fades over time due to neural pruning and differentiation among synaptic connections [7]. A study on the impact of ordinality, shape, and sound as factors that would determine the colour of a grapheme, enforced the importance of ordinality because of its superiority when learning the graphemes in childhood [4].


Lastly, It has been observed that synesthesia runs in families as about 40% of synesthetes reported having a first-degree relative with the condition [8]. Preliminary studies suggested it was through an X-linked dominant mode of inheritance but this was shortly rejected when further large-scale studies were conducted [9]. When considering the number of forms of synesthesia you could assume that each type is expressed by specific genes, however, when the form varies within the same family, it would be more likely that the genes only gives vulnerability to the condition rather than the expression itself [8].

Assessment and DiagnosisEdit

As mentioned earlier, there are over 60 different forms of synesthesia, and while each one has different and specific characteristics, there are some common features among all synesthesia cases that are essential for accurate identification. First is automaticity, then reliability, and lastly consistency [1]. Automaticity implies that synesthetic experiences are automatic. They can not be directly inhibited by the individual. Instances where the synesthetic association is unpleasant highlight the fact that they occur without much control [10]. Synesthetes show high reliability for a response when presented with a triggering stimulus. The last characteristic describes how individuals will display consistent associations over time. Synesthesia does not have a clinical diagnosis, however, there are tests available to assess the degree to which an individual makes associations with their senses. To ‘have’ synesthesia the previously mentioned characteristics must be present.

The Synesthesia Battery was introduced by David M. Eagleman and colleagues as an online information and testing portal. Its assessment uses a questionnaire and multiple online tests. This resource contains tests that target a magnitude of the different forms of the condition. It includes tests for colours to weekdays and months, comparing coloured weekdays in English and foreign alphabets, and matching synesthetic pitches with moving stimuli [11]. The validity of this test was never tested against other long-term tests however, it soon was validated as a reliable method for assessing synesthesia [12].

Before the Synesthesia Battery, Baron-Cohen and colleagues developed the Test of Genuineness (TOG) in 1987 to measure the consistency between stimulus-response linkage [13] . This test involved a test-retest procedure where synesthetes were asked to report their response to certain stimuli, then were retested at least a year later [1]. The TOG focuses on the characteristic of consistency in its assessment. If someone does have synesthesia, their retest results will match their initial test.

The Stroop task is often used to measure someone's selective attention and predict the automaticity of written word processing [14][15]. The Stroop effect is essentially a slower reaction time between conflicting and non-conflicting stimuli. A synesthete will display this effect when shown conflicting stimuli (e.g., a synesthete says the letter C is yellow, they will be shown the letter C in another colour). Their delayed reaction demonstrates that they cannot control the synesthetic response and therefore highlights the automatic characteristic we described earlier [1].

With the variety of tests and tools available, it is important to understand that no single test is better than the other. Each one has its limitations that do not take into account certain characteristics of synesthesia. To obtain a comprehensive assessment it would be most ideal to combine multiple tests to observe the condition from every possible angle.

Types of SynesthesiaEdit

Graphemes to ColoursEdit

Grapheme-color synesthesia is the association between letters or digits and colours. People with this type of condition will say the letter A is red or the number 5 is green. Synesthetes have reported experiencing the colour “in the mind's eye” or the colour was seen in external space [16]. These associations are quite specific and remain relatively consistent across an individual's lifespan. For a specific synesthete, each grapheme will correspond to a specific colour [17]. In an attempt to explore the development of this condition over time, Simmer and Bain, 2013 conducted a longitudinal study that examined child synesthetes and non-synesthetes over time. Their results were able to show that these synesthetic associations start as chaotic pairings, but mature over time.

Time-Space SynesthesiaEdit

Those who experience this form of synesthesia will see external images when thinking of time units. In some cases, neighboring time units, like months, are linked, forming bizarre patterns. In other cases, specific time units appear as highly specific colours [18]. A common and consistent description from time-space synesthetes is that they experience months as circles extending outside of the body [19]. There is less research on this form however some studies have attempted to assess how synesthesia is cognitively useful. Results from Mann et al., 2009 study concluded that time-space synaesthesia may expedite the cognitive manipulation of time-based information.

Sound to ColourEdit

Sound-color synesthesia or chromesthesia is a category that encompasses all the associations between musical or non-musical sounds and colour. It had been studied by focusing on individual tones that elicited a colour response, however, it now includes a mixture of timbre, tempo, emotion, and differing musical styles [20]. For example, when a synesthete hears a middle C on the piano, they might see orange.

A Clinical Connection: Synesthesia and AutismEdit

The prevalence of synesthesia is estimated at 4% of the population and the prevalence of autism is estimated between 1 and 2.6% [21]. It has been observed that synesthesia is much more prevalent in people with Autism Spectrum Disorder [22]. Many studies have been conducted to identify the common traits between these conditions and if the neurodevelopmental mechanisms associated with them are similar. The DSM-5 describes autism with impairments in social communication, narrow interests and repetitive behaviors, and Hyper- or hypo-reactivity to sensory input [23]. As previously described, synesthesia is classified as unusual sensory experiences. This sensory sensitivity was thought to be a commonality among both conditions. Multiple studies have been conducted by administering questionnaires and tests to individuals with synesthesia, autism, and control individuals with neither condition. The main results from these studies conclude that perceptual and attentional processes function similarly in both conditions [24][25].


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