• Miroslaw Sienkiewicz

Synaesthesia Honours Project: The Induction of Synaesthesia in Non-Synaesthetes

Continuation from reading the book “Sensory Blending: On Synaesthesia and related phenomena” (Deroy, 2017).

I did a quick jump to one of the latest chapters in the book: “The Induction of Synaesthesia in Non-Synaesthetes” as other chapters are not essential for my project. I think that for now I learned enough about synaesthesia and from tomorrow I’ll look deeper into Auditory-Visual Synaesthesia in other books, researches and articles.

This chapter is focused on examination of three methods of induction of synaesthesia: training, posthypnotic suggestion and pharmacological agents in non-synaesthetes.

Criteria’s for synaesthesia:

  1. atypical conscious experience (colour) in response to stimulus (sound)

  2. a high degree of consistency in the inducer-concurrent associations

  3. a high degree of involuntariness (or automaticity) in the coupling of the inducer and concurrent and by which the concurrent breaches conscious awareness and impacts cognition

  4. a high degree of specificity of the inducer and the concurrent (Deroy and Spence, 2013; Ward, 2013; Ward and Mattingley, 2006)

Induced synaesthesia can be similar to early-stage synaesthesia where consistency doesn’t need to be present.

Training: in one coloured grapheme experiment, synesthetes displayed activation in left parietal (Figure 1) and extrastriate visual cortex (Figure 2) (fMRI) and non-synesthetes don’t.

Figure 1 Parietal lobe is yellow coloured. Picture by Henry Vandyke Carter - This is a retouched picture, which means that it has been digitally altered from its original version. Modifications: vectorization (CorelDraw). The original can be viewed here: Gray728.png. Modifications made by Mysid., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1676555

Figure 2Extrastriate cortex is yellow coloured. Picutre: By The original uploader was Washington irving at English Wikipedia. - These images were created using Blender and Matlab.. Originally from en.wikipedia; description page is/was here., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1643737

Trained non-synaesthetes don’t exhibit behavioural effects observed in congenital synaesthesia (Cohen Kadosh et al., 2005).

In Meyer and Rothen (2009) experiment, colour was paired with a startling (thrilling) sound that produced conditioned startle (surprising) response measured by skin conductance response. Training produced small Stroop effect (23 ms) (“The Stroop test measures the ease with which a person can shift his/her perceptual set to changing demands, and critically, to suppress a habitual response in favour of an unusual one“, (Johnstone et al., 2010)) and patients displayed a conditioning effect for colours but not graphemes. One explanation is that in synaesthetes colour activates grapheme representations which are associated with the conditioned stimulus, but similar event doesn’t take place for trained non-synaesthetes (Cohen Kadosh et al., 2005).

Conclusion: Induction of Synaesthesia produce ambiguous results.

Post-hypnotic suggestion (on highly suggestible individuals):

Suggestions in hypnosis include : promotion of minimized awareness of one’s environment, reduced metacognition and perceived effortless attention towards the instructions of the experimenter (Brown, Antonova, Langley, and Oakley, 2001).

An example of a hypnotic suggestion for a visual hallucination may be, “When you open your eyes in a few moments, you will look at the computer monitor in front of you and see a red circle.” Posthypnotic suggestions are suggestions for alterations in a particular function following a hypnotic de-induction. (Deroy, 2007)

One experiment (Cohen Kadosh et al., 2009) was about grapheme-colour synaesthesia for six grapheme-colour pairs. Digits were black with coloured background (either congruent or non-congruent). Both synaesthetes and non-synaesthetes displayed the same pattern.

Another more recent study (Anderson, Seth, Dienes, and Ward, 2014) gave contradictory results to previous experiment but procedure for screening participants was not as rigorous as in typical hypnosis studies.

Results: Post-hypnotic suggestion can reproduce behavioural and phenomenological markers of synaesthesia based on available evidences. Activation of neural mechanisms depends on hypnotic suggestions. Greater activation in V4 (region of brain involves in colour photoisms) is observed in colour hallucinations than in colour imaginary. The problem with this experiment is that it proves that it’s possible to temporary produce synaesthesia which is in contradiction to theories that anatomical connectivity plays a casual role in online occurrence of synaesthesia (or it means that these theories are incomplete). One possibility is that two or more mechanisms are responsible for synaesthesia.

  1. Synaesthesia is a result from a disruption of cortical ingibition, producing conscious awareness of visual information, that is normally inhibited (Cohen Kadosh and Walsh, 2008; Eagleman and Goodale, 2009).

  2. In congenital (“real”) and induced (simulated) synaesthesia, excess connectivity is a by-product of the repeated binding of inducer and concurrent representations. A consequence rather than cause of synaesthesia (Cohen Kadosh and Walsh, 2008).

After all, these experiments can be questioned because mechanisms of hypnosis itself are poorly understood.

Pharmacological agents:

This method has advantages over previous two: synaesthesia can be elicit in short period of time and it may be effective on large proportion of population (in contradiction to post-hypnotic suggestions experiments). In some cases, neurotransmitter systems targeted by particular recreational drugs are well knowns (e.g. tryptamine psychedelics like psilocybin). Drugs like LSD, Mescaline and Psilocybin Mushrooms have been reported to induce synaesthesia experience.

In experiment with Mescaline (Simpson and McKellar, 1955) both authors were in non-synesthete group and congenital synaesthetes group was also made of two participants.

Four doses (0.3-0.5 mg) was taken during different occasions. Synesthetic impressions were noted but no attempts for train or repeated experiences were made. Eight types of synaesthesia have been recorded e.g. inducers varied but concurrent was typically visual (most common: auditory-visual). Drug has also enhancing effect on synaesthetes.

Another experiment (Hartman and Hollister, 1963) was more rigorous and include mescaline, psilocybin and LSD. Eighteen participants who were blind to the type of drug. They listened sixteen pure tones at four set frequencies between 500 and 4000 Hz at relatively equal amplitude at baseline. Visual effects like shattering of patterns, brightening of visual field were observed as more enhanced while under mescaline and LSD. Effect of psilocybin has non-significant increase in these experiences. Recent research (Luke et al., 2012) proved that LSD is the most prevalent inducer of synaesthesia from these three drugs. Auditory-Visual synaesthesia “effect” increase with dosage of psilocybin (Griffiths et al., 2011; Studerus, 2013). Relatively equal doses of psilocybin, ketamine and MDMA cause decreasing prevalence of this type of synaesthesia. Subjective experience of synaesthesia has been reported in tests with Ayahuasca and Salvia Divinorum (visual somatic synaesthesia).

Lack of consistency for synaesthesia effects has been reported in this method of induction of synaesthesia. Exception is display consistency via a texture segregation behavioural test (Brang and Ramachandran, 2008).

Auditory-Visual synaesthesia (sound-visual) is most frequent type of the phenomena observed during drug-induced experiments.

Disinhibition theories predict a reduction in GABA in visual cortex as a possible neurochemical mechanism underlying congenital synaesthesia. (Deroy, 2017)

Theorizing propose:

Serotonin-2A subtype agonism is fundamental to drug-induced synaesthesia (Brang and Ramachandran, 2008). LSD operate largely via 5HT2A agonism.

5HT2A inhibitors: Prozac and Wellbutrin cause synaesthesia.

Melatonin induce grapheme-colour synaesthesia via chemical switch via 5HT1 inhibition and subsequent 5HT2A disinhibition.

Whole-gene linkage scan and family-based linkage analysis did not proved evidence of a 5HT2A-linked gene for synaesthesia.

Lack of serotonergic action in Salvia Divinorum challenge serotoninergic hypothesis but it is possible that kappa receptors regulate the serotonin system (Bruchas et al., 2011), giving rise to secondary serotonergic effects.

Recent research suggests that LSD-induced synaesthesia does not meet standard criteria for synaesthesia (Terhune et al., 2016).

It’s too early to draw final conclusions regarding the authenticity of drug-induced synaesthesia.


Anderson, H. P., Seth, A. K., Dienes, Z., & Ward, J. (2014). Can grapheme-color synesthesia be induced by hypnosis? Frontiers in Human Neuroscience, 8, 220. doi: 10.3389/fnhum. 2014.00220.

Brang, D., & Ramachandran, V. S. (2008). Psychopharmacology of synesthesia: The role of serotonin S2a receptor activation. Medical Hypotheses, 70, 903–4

Brown, R. J., Antonova, E., Langley, A., & Oakley, D. A. (2001). The effects of absorption and reduced critical thought on suggestibility in an hypnotic context. Contemporary Hypnosis, 18, 62–72.

Bruchas, M. R., Schindler, A. G., Shankar, H., Messinger, D. I., Miyatake, M., Land, B. B., Chavkin, C. et al. (2011). Selective p38alpha MAPK deletion in serotonergic neurons produces stress resilience in models of depression and addiction. Neuron, 71, 498–511. doi: 10.1016/j.neuron.2011.06.011.

Cohen Kadosh, R., Henik, A., Catena, A., Walsh, V., & Fuentes, L. J. (2009). Induced cross-modal synaesthetic experience without abnormal neuronal connections. Psychological Science, 20, 258–65. doi: 10.1111/j.1467-9280.2009.02286.x.

Cohen Kadosh, R., Sagiv, N., Linden, D. E., Robertson, L. C., Elinger, G., & Henik, A. (2005). When blue is larger than red: Colors influence numerical cognition in synesthesia. Journal of Cognitive Neuroscience, 17, 1766–73. doi: 10.1162/089892905774589181.

Cohen Kadosh, R., Henik, A., Catena, A., Walsh, V., & Fuentes, L. J. (2009). Induced cross-modal synaesthetic experience without abnormal neuronal connections. Psychological Science, 20, 258–65. doi: 10.1111/j.1467-9280.2009.02286.x.

Cohen Kadosh, R., & Walsh, V. (2008). Synaesthesia and cortical connections: Cause or correlation? Trends in Neurosciences, 31, 549–50.

Deroy, O., & Spence, C. (2013). Why we are not all synesthetes (not even weakly so). Psychonomic Bulletin & Review, 20, 643– 64. doi: 10.3758/s13423-013-0387-2.

Deroy, O. (2017). Sensory blending. 1st ed. Oxford: Oxford University Press, p.349.

Eagleman, D. M., & Goodale, M. A. (2009). Why color synesthesia involves more than color. Trends in Cognitive Sciences, 13, 288–92.

Griffiths, R. R., Johnson, M. W., Richards, W. A., Richards, B. D., McCann, U., & Jesse, R. (2011). Psilocybin occasioned mystical-type experiences: Immediate and persisting doserelated effects. Psychopharmacology, 218, 649–65. doi: 10.1007/s00213-011-2358-5.

Hartman, A. M., & Hollister, L. E. (1963). Effect of mescaline, lysergic acid diethylamide and psilocybin on color perception. Psychopharmacologia, 4, 441–51.

Johnstone, E., Owens, D., Lawrie, S., McIntosh, A. and Sharpe, M. (2010). Companion to psychiatric studies. 8th ed. Edinburgh: W.B.Saunders, pp.121-140.

Luke, D., Terhune, D. B., & Friday, R. (2012). Psychedelic synaesthesia: Evidence for a serotonergic role in synaesthesia. Seeing and Perceiving, S1, 74.

Meier, B., & Rothen, N. (2009). Training grapheme-colour associations produces a synaesthetic Stroop effect, but not a conditioned synaesthetic response. Neuropsychologia, 47, 1208–11. doi: 10.1016/j.neuropsychologia.2009.01.009.

Simpson, L., & McKellar, P. (1955). Types of synaesthesia. Journal of Mental Science, 101, 141–7.

Studerus, E. (2013). Psilocybin-induced altered states of consciousness: Tolerability, assessment, and prediction. Saarbrücken, Germany: Südwestdeutscher Verlag für Hochschulschriften.

Terhune, D. B., Luke, D. P., Kaelen, M., Bolstridge, M., Feilding, A., Nutt, D., Carhart-Harris, R., & Ward, J. (2016). A placebo-controlled investigation of synaesthesia-like experiences under LSD. Neuropsychologia, 88, 28–34. doi: 10.1016/j.neuropsychologia.2016.04.005.

Ward, J., & Mattingley, J. B. (2006). Synaesthesia: An overview of contemporary findings and controversies. Cortex, 42, 129– 36.


Deroy, O. (2017). Sensory blending. Oxford: Oxford University Press.

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