A girl with grapheme-colour synesthesia tries to
explain what she experiences when she sees numbers.

Synesthesia is the phenomenon in which activation in one sensory area results in the activation of another sensory area. A classic example is the grapheme-colour synesthesia, in which seeing a letter (e.g., A) leads to also seeing a colour (e.g., red). Instances of synesthesia were reported as early as 200 years ago, but it did not receive substantial research interest until much later in time. The three major characteristics of synesthesia are: synesthesia experiences are often reflect common associations (e.g., seeing light colours when hearing a high pitched tone), associations are persistent, and for visuospatial forms synesthetes, associations have a definite position in space [1]. Various theories have been proposed to explain why synesthetes are able to see what they see or hear they hear. The most prevalent has been the activation of neural areas associated with the specialized sensations. For example, for grapheme-colour synesthesia, it has been shown that there is an activation of the V4 colour area along with the grapheme area (right inferior temporal gyrus) in the synesthete brain [2]. Till now, cases of synesthesia are still fairly rare, and often come in many different variants combining various modalities. It is an interesting paradigm to further understand how different modalities in the brain can interact via neural connections, and how our behaviour and perception can be changed due to our brains. Furthermore, increase in research in synesthesia due to the advent of technology can further the usage of concepts of synesthesia in sensory prosthetics. This is called intentional synesthesia technology, and it aims to apply synesthetic concepts to replace an impaired sense with a functional one to better the quality of life for those that are missing one sense or another.

1. J. Simner. Defining Synaesthesia. British Journal of Psychology. (2012) 103(1): 1-15.
2. T.M. van Leeuwen. “How one can see what is not there”: Neural mechanisms of grapheme-colour synaesthesia. (2011) Sept 14 – Epub.

Auditory Visual Synesthesia

main article: Auditory Visual Synesthesia
author: Yun Liu

Auditory Visual Synesthesia
Image Unavailable
Auditory visual synesthesia allows synesthetes to experience
anomalous sensation in which sounds, chords, or pitch could elicit
different visual experiences [4] (image adapted from

Auditory visual synesthesia is a neurological phenomenon in which a sound stimulus can elicit visual experiences [4]. Unlike auditory-induced visual image experienced by non-synesthetic population, auditory visual synesthesia is involuntary and consistent[1]. Auditory visual synesthesia is further categorized by the type of audio stimuli paired with specific visual responses such as sound colour synesthesia and coloured music synesthesia[1]. Previous studies stated that auditory visual synesthesia could be the consequence of excessive multimodal integration [2]. Recent studies involving the double flash illusion, however, have observed reduced multimodal integration [3]. Recent researches also discovered the anomalous activation of the cortical areas such as V4 and parietal lobe that could lead to potential auditory visual synesthetic experiences [4]. The study of synesthesia is important as it is a rare neurological condition, and understanding of synesthesia can provide insight regarding to the mechanisms of perception, cognition and communication between cortical regions [1].

This video illustrates the synesthetic experiences by a synesthete, Jane Mackay
1. Chiou R, et al. Beyond colour perception: Auditory- visual synaesthesia induces experiences of geometric objects in specific locations, Cortex. (2012), doi:10.1016/j.cortex.2012.04.006
2. Hubbard, E.M., et al. The cross activation theory at 10. Journal of Neuropsychology. (2011) 5(2): 152-177.
3. Neufeld, J., et al. Reduced audio-visual integration in synaesthetes indicated by the double-flash illusion. Brain Research 1473. (2012) 78-86.
4. Neufeld, J., et al. The neural correlates of coloured music: a functional MRI investigation of auditory- visual synaesthesia. Neuropsychologia. (2012) 50:85-89

Lexical-Gustatory Synesthesia

main article: Lexical-Gustatory Synesthesia
author: quresh95

Lexical-Gustatory Synesthesia
Image Unavailable
Image source:

Lexical gustatory synesthesia is a type of synesthesia in which people involuntarily experience food tastes when they hear, read or say a word; words acts as an inducer while taste is the concurrent. Lexical-gustatory synesthesia can be more accurately named as lexical-flavour as gustatory implies that the experience of a synesthetic being limited to sweetness, sourness, bitterness etc. whereas studies show that the triggered concurrent involves a complex array of flavours, much like one would find when eating a meal [1]. 
Studies regarding lexical-gustatory synesthesia have ranged from the factors, such as the structure of a word, that elicit a specific taste to the brain regions and subsequent mechanisms involved in this particular type of synesthesia. Moreover, it has been greatly reiterated, through various tests, that lexical-gustatory synesthesia is a genuine experience and not a purely psychological one however, concurrents have been shown to be shaped by experience, and food eaten during childhood have a higher likelihood of being concurrents. Furthermore, studies have shown that genes also play a role in this phenomenon, thus synesthesia could be hereditary [2]. (See Genetics)
Studies of lexical-gustatory synesthesia have resurfaced as the neural models of this type of synesthesia can be used to understand integration and separation of different cortical regions and the rules that apply to the wiring and interaction of these different regions.

A Case of Lexical-Gustatory Synesthesia
James prefers his girlfriends' to have a nice tasting name 
original video source:
1. Spence C. and Gallace A. Tasting shapes and words. Food Quaility and Preference (22), 290-295 (2011)
2. Brang D. and Ramachandran V. S. survival of the Synesthesia Gene: Why Do People Hear Colors and Taste Words. PLoS Biology (9), 1-6 (2011)

Synesthesia Technology

main article: Synesthesia Technology
author: sungkhl

Figure 1. Brain-Computer Interface Device
Image Unavailable
Diodato, B. (2008) Brain-computer Interface Device. [Photo]
Retrieved March 28, 2013 from

Synesthesia is a neurological phenomenon where an individual experiences simultaneous stimulation of two different sensory pathways by only one stimulus[1]. Today, scientists have used synesthetic concepts to develop new technologies that converts one sensory input into a different sensory output. One of the main purposes for synesthetic technology is sensory prosthesis where a severely damaged sense is replaced by a functional one. An example would be reading Braille, which is a form of writing language using patterns of raised dots, so that the blind can “read” the words with their fingers instead of their eyes. There are also synesthesia technology that enhances certain sensory functions such as detection of toxins in chemical workplaces, gaming control, and medical diagnosis of certain diseases[2][3]. A major research subfield is the Brain Computer Interface (BCI) technology that links the human brain to a computerized device. An example of such a device is the BrainPort System developed by the Wicab Lab in Wisconsin, USA[4]. It uses a camera to pick up images of one’s surroundings and an Intra-Oral Device (IOD) to convert it for transfer to the brain via the stimulation of the tongue[4]. Another synesthetic technology is the Optoelectronic Nose that a lab from the chemistry department at University of Illinois created[2]. The Optoelectronic Nose transfigures olfactory input to visual output using a colorimetric sensor array with chemically responsive dyes that changes color with changes in their chemical environment[2].

1. Simner, J. Defining synaesthesia. British Journal of Psychology 103, 1-15 (2012).
2. Suslick, K. S. Synesthesia in science and technology: more than making the unseen visible. Current Opinion in Chemical Biology 16, 557-563 (2012).
3. Liao, L. D. et al. Gaming control using a wearable and wireless EEG-based brain-computer interface device with novel dry foam-based sensors. Journal of NeuroEngineering and Rehabilitation 9, 1-12 (2012).
4. Danilov, Y. & Tyler, M. Brainport: An alternative input to the brain. Journal of Integrative Neuroscience 4, 537-550 (2005).

Visually Induced Synesthesia

main article: Visually Induced Synesthesia
author: Carrie ChangHua Chen

Visually Induced Synesthesia
Image Unavailable
Visually induced synesthetes may
perceive particular colours when they see
letters/numbers, or elicit specific locations
when thinking about numbers or time
adapted from:

Visually induced synesthesia is the most common form of synesthesia in which a certain type of visual stimuli is perceived with an additional sensory modality. Although there are many subtypes of visual synesthesia, the most studied classical form is grapheme-colour synesthesia (including number-colour synesthesia) characterized by the involuntary and autonomic association of letters and colours. Each synesthete experiences a distinct coupling that persists over lifetime (e.g. the letter “a” may elicit the colour blue in one individual whereas others may perceive it as red). [2] The novel difference in neural activation between grapheme-colour synesthetes and control is the hyper-connectivity between the left inferior parietal and primary visual sensory area in the parietal cortex. [1] Even to this day, the mechanism for synesthesia is still under investigation. It is also interesting to address the questions regarding the extent to which attention is required to evoke synesthetic perception and the effects on which the synesthetic sensations have in memory encoding and retrieval. In addition, the ongoing research in synesthesia that investigates the phenomenon of how the vivid synesthetic visual experiences alter neurocognitive processing and enhance visual memory can potentially assist students with synesthesia in learning by way of further utilizing the unusual paring abilities.

1. Sinke, C., et al. Inside a synesthete’s head: A functional connectivity analysis with grapheme-color synesthetes. Neuropsychologia. (2012) 50: 3363-3369.
2. Simner, J. Defining Synaesthesia. British Journal of Psychology. (2012) 103(1): 1-15.

Why Synesthesia Occurs

main article: Why Synesthesia Occurs
author: Patty Hsu

Figure 1. Synesthetic Experience
Image Unavailable
What letters may seem like to a
grapheme-colour synesthete[5].

Synesthia is a specialized phenomenon in which activation in one sensory modality leads to the activation of another sensory modality. Multiple combinations of different modalities lead to a diverse range of synesthesic possibilities. The first reported case of synesthesia was over 200 years ago, but the mechanisms as to how it occurs is still under debate today. Several opposing theories have been proposed to try and explain the mechanisms as to how these individuals can experience these unique sensations. The two most prominent theories are the cross activation theory[1] and the disinhibited feedback theory[2] and each have different pieces of evidence supporting their claims. Recently due to the advent of new technology, researchers have also been able to discover genes that are thought to contribute to how synesthesia occurs[3][4]. However for those that are not fortunate enough to have the genes involved with synesthesia, acquired forms of synesthesia such as learned synesthesia as well as other forms of drug induced and injury induced synesthesia have been reported, which further adds to the debate over the mechanisms of synesthesia.

1. Ramachandran, V.S. & Hubbard, E.M. Psychophysical investigations into the neural basis of synaesthesia. Proc Biol Sci 268, 979-83 (2001).
2. Grossenbacher, P.G. & Lovelace, C.T. Mechanisms of synesthesia: cognitive and physiological constraints. Trends Cogn Sci 5, 36-41 (2001).
3. Asher, J.E. et al. A whole-genome scan and fine-mapping linkage study of auditory-visual synesthesia reveals evidence of linkage to chromosomes 2q24, 5q33, 6p12, and 12p12. Am J Hum Genet 84, 279-85 (2009).
4. Tomson, S.N. et al. The genetics of colored sequence synesthesia: suggestive evidence of linkage to 16q and genetic heterogeneity for the condition. Behav Brain Res 223, 48-52 (2011).

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