Natalie Ludwig
Let us perform an experiment. What do you perceive when I say the following numbers: five, eight and twenty-three? You might think that sounds silly and be inclined to put this piece away to do something more fun. You might, however, also be one of the lucky few to experience a phenomenon called “synesthesia”.
Synesthesia describes a condition where one sensory stimulation elicits another unstimulated sensory output, in addition to the output corresponding to the primarily stimulated sense. In easier words, so-called “synesthetes” (people who experience synesthesia) will connect two unrelated outputs. For example, synesthetes can associate visual information (like seeing a particular letter or number) with specific smells or colours [1]. Interestingly, they do so in a reproducible way, so the same letter will always be associated with the same colour. Additionally, the association seems to be unidirectional, so a letter will be connected to a colour, but not the other way around. The field of synesthesia is vast, and there are numerous subforms, ranging from experiencing colours from sounds, specific letters or numbers, to perceiving tastes from phonetic sounds [2-5].
The ability can be either congenital or acquired due to neurological conditions, like migraine or thalamic strokes, or drug ingestion [6-9]. Synesthesia has furthermore been described in the case of sensory deprivation such as blindness [6]. About 4.4 per cent of the population is thought to have congenital synesthesia, although the prevalence differs between the different subforms [10]. Of note, though, the prevalence of this condition in the general population is probably underestimated, due to stigmas surrounding the idea that an individual perceives inputs that the majority would not [6]. Strikingly, synesthesia seems to be quite common among people in art-related fields. One study, in particular, determined the prevalence of synesthesia among fine arts students using a questionnaire and found it to be as high as 23 per cent[11]. This is supported by anecdotal evidence of synesthesia in several well-known and impactful artists, like Vincent van Gogh and Wassily Kandinsky [6].
But what are the mechanisms behind synesthesia? While we do not know the pathophysiology exactly, it is most likely due to either direct crosstalk between two brain regions or indirect feedback via a third brain region [12]. Which regions are involved depends on the form of synesthesia an individual has. In the case of grapheme, referring to letters or numbers, and colour synesthesia, for example, synesthetes show specific brain activity patterns compared to non-synesthetes. Upon hearing audio recordings of graphemes, they display a significantly increased number of connections between six regions associated with colour perception and three so-called “grapheme regions” (p < 0.024) [13]. Nonetheless, the anatomical basis of synesthesia remains elusive. Further research will be essential to understand the underlying mechanisms fully, as well as to determine the prevalence of the different subtypes of synesthesia in the general populations.
So maybe, next time when you see colours and shapes appearing in front of your eyes while you are listening to music, you will appreciate your super ability a bit more. Remember, you are not going crazy; you are just special.
References:
[1] Grossenbacher, P.G. & Lovelace, C.T. Mechanisms of synesthesia: cognitive and physiological constraints. TRENDS in Cognitive Sciences 5, 36-41 (2001).
[2] Hochel, M., et al. Congruence or coherence? Emotional and physiological responses to colours in synaesthesia. European Journal of Cognitive Psychology 21, 703-723 (2009).
[3] Ward, J. & Simner, J. Lexical-gustatory synaesthesia: linguistic and conceptual factors. Cognition 89, 237-261 (2003).
[4] Russell, A., et al. Chocolate smells pink and stripy: Exploring olfactory-visual synesthesia. Cogn Neurosci 6, 77-88 (2015).
[5] Bergfels Mills, C., et al. DIGIT SYNESTHESIA: A CASE STUDY USING A STROOP-TYPE TEST. COGNITIVE NEUROPSYCHOLOGY 16, 181-191 (1999).
[6] Safran, A.B. & Sanda, N. Color synesthesia. Insight into perception, emotion, and consciousness. Curr Opin Neurol 28, 36-44 (2015).
[7] Podoll, K. & Robinson, D. Auditory-visual synaesthesia in a patient with basilar migraine. J Neurol 249, 476-477 (2002).
[8] Alstadhaug, K.B. & Benjaminsen, E. Synesthesia and migraine: case report. BMC Neurol 10, 121 (2010).
[9] Schweizer Ta, et al. From the thalamus with love: a rare window into the locus of emotional synesthesia. Neurology 81, 509–510 (2013).
[10] Simner, J., et al. Synaesthesia: The prevalence of atypical cross-modal experiences. Perception 35, 1024-1033 (2006).
[11] Domino, G. Synesthesia and creativity in fine arts students: An Empirical Look. Creativity Research Journal 2, 17-29 (1989).
[12] Ward, J. Cognitive neuroscience of synesthesia: Introduction to the special issue. Cogn Neurosci 6, 45-47 (2015).
[13] Tomson, S.N., et al. Neural networks of colored sequence synesthesia. J Neurosci 33, 14098-14106 (2013).
