Brain Activation Patterns During Different Types of Synaesthesia
By: Tina Yan

Synaesthesia is an uncommon heritable neurological condition that attributes to involuntary assimilation of two or more senses in the brain of synaesthete individuals.1 As a result of having increased structural connectivity; synaesthetes have a different level of cognitive perception and are thought of as excellent comparison models for the normal cognition pathway of non-synaesthetes. Consequently, synaesthetes have the potential to help better understand how sensory information is processed in the brain. Reports of over 60 different types of synaesthesia has been made.1 However, only three of the more common types of synaesthesia are frequently studied and are better understood, which include grapheme to colour, phoneme to colour, and number form synaesthesia. It is known that all three types of synaesthesia activate the fusiform gyrus, or the visual cortex, during synaesthetic experiences.2 With the help of functional neuroimaging, analysis for synaesthetic specific regions that are active during synaesthetic experiences, are able to be verified.2

1.1 Grapheme-Colour Synaesthesia:

Grapheme-colour synaesthesia is the most common type of synaesthesia that affects at least 1% of the world’s population.3 Grapheme-colour synaesthetes report a higher level of cognition compared to non-synaesthetes, because they experience an association of letters or numbers to different colours, referred to as synaesthetic colours, while reading. Combinations of colour associations to different numbers and letters vary for each synaesthete.3 As a result; general group scales are unable to be generated. Also, a study has shown that grapheme-colour synaesthesia is bidirectional.4 This means that synaesthesia, grapheme-colour and colour-grapheme, can be induced through a coupled grapheme or a colour stimulus that is specific to each synaesthete.4

Figure 1.0 fMRI of ventral surface activation in synaesthetes and non-synaesthete controls during a grapheme stimulus. The hV4 region is shown in purple and the grapheme area is shown in blue on the models. The grapheme area was equally active for both synaesthetes and controls, whereas the hV4 region was highly active only in synaesthetes. Image adapted from Price, C.J., et al. (1994).

Functional magnetic resonance imaging (fMRI) and retinotopic mapping techniques would be used to generate a map of active brain regions, in response to different grapheme stimulus in synaesthetes and non-synaesthetes for comparison. The map would help to determine brain activation patterns that are grapheme-colour specific. When non-synaesthetes read aloud or silently, the left middle and superior temporal regions of the brain would be activated, and only the grapheme would be perceived.5 Using the non-synaesthete response as a standard for comparison, researchers believe that examining the brain activation patterns of synaesthetes can help develop a better understanding of how words and numbers are processed in the brain.

Current research on grapheme-colour synaesthesia has noted that the human V4 (hV4) region in the fusiform gyrus is consistently more active in synaesthetes than in non-synaesthetes, when presented with colourless grapheme stimuli. This was evident in fMRI studies, where activation of the hV4 region was greater in grapheme-colour synaesthetes than in non-synaesthete controls.6, 7, 8 Likewise, in a study that used diffusion tensor imaging (DTI) technique for measuring white matter clustering levels, greater clustering in the inferior temporal lobe near the fusiform gyrus was seen in grapheme-colour synasethetes.9

The grapheme area is also located in the fusiform gyrus, so it is within close proximity to the hV4 region that could lead to cross-activation of the regions .9 However, activity levels in the grapheme area was found to be the same for both grapheme-colour synaesthetes and non-synaesthetes in an fMRI study (Figure 1.0), which means the grapheme area is not grapheme-colour specific.6 There have also been studies that speculated that the hV8 and early visual cortex (V1) are grapheme-colour specific, because activity was seen in these regions on the fMRI of synaesthetes.6, 8 Even though positive activity was detected in these areas, the results varied for each synaesthete, which lead to controversies in the claims of studies for the hV8 and V1 regions.6

Figure 2.0 Spatial representations of fusiform gyrus and posterior parietal cortex (PPC) in the brain. The colour-selective hV4 is indicated in red, and the visual word form area (VWFA), also known as the grapheme area, is indicated in green. The PCC region that includes SPL and intraparietal sulcus (IPS) is indicated in blue. Grapheme-colour synaesthesia results from cross-activation of grapheme and colour areas, where signals are processed in the PCC region. Image adapted from Hubbard, E.M., (2007).
In a temporary lesion study that used transcranial magnetic stimulation (TMS), lesions the superior parietal lobe (SPL) of grapheme-color synaesthetes and non-synaesthetes were made, and they were both unable to recognize the grapheme stimulus and synaesthetic colours were not perceived by synaesthetes (Figure 2.0).10, 11, 12 The SPL integrates sensory information, so a temporary lesion would cause both synaesthetes and controls to not be able to perceive grapheme stimuli, because signals will not be processed.10 More specifically, synaesthesia will not be experienced.

2.1 Phoneme-Colour Synaesthesia:

Phoneme-colour synaesthesia, or coloured-hearing, is the second most common type of synaesthesia where synaesthetes associate a colour to different spoken phonemes.1 Like grapheme to colour associations, phonemic to colour associations are specific to each synaesthete. Two areas are activated when non-synaesthetes hear words, activation bilaterally of the superior temporal and left of the inferior frontal gyrus.13 An explanation for coloured-hearing is cross-activation of language areas on the left hemisphere to the left hemisphere of the temporal parieto-occipital (TPO) junction in the brain.14 Moreover, topology maps of synaesthetes and non-synaesthetes are compared to determine phonemic-colour specific brain regions. Studies would present spoken words versus tones and colours versus Mondrian patterns stimuli to subjects, while recording brain activity levels. It was also found that phonemic-colour synaesthesia is bidirectional, where colour-phonemic synaesthesia also exists.15

Figure 3.0 Activation maps of synaesthetes and controls combined with colour activation mappings. The yellow indicate activated regions in response to spoken words stimuli and the blue are colour activation mappings. Overlapping yellow and blue regions are indicated in red. The right side of the image corresponds to the left hemisphere of the brain. STG, superior temporal gyrus. IFG, inferior frontal gyrus. The STG is active for both synaesthetes and controls. Only synaesthetes had V8 activity and IFG that are boxed in red. Image adapted from Nunn, J.A., et al (2002).

Through neuroimaging studies, phonemic-colour specific regions are found to be on the left TPO that included the hV4 and VWFA.13 In one of the earlier studies using positron emission tomography (PET), greater activation was found at higher order visual cortices in the TPO junction of synaesthetes.14 Significant activity was also found at the lower visual areas that included V1, V2, or V4 regions after hearing spoken words.14 PET has low resolution, which limited the accuracy in determining coloured-hearing specific areas. However, in a more recent study using fMRI, a better resolution was obtained, and it was found that the left hV4 region was activated in phonemic-colour synaesthetes and not controls (Figure 3.0). 13 Also, there was congruence from both of the above-mentioned studies that the primary visual cortex, V1 and V2, was not necessary for synaesthetic experiences to happen.13, 14 Greater activity in the left hemisphere of the temporal cortex in the posterior ventrolatoral region was present in both studies.13, 14

Furthermore, a percentage of phoneme-colour synaesthetes reported to that have alien colour effect (ACE): hearing colour names will elicit colours that are different from what was named.16 To determine activation differences, an fMRI study that compared clustering maps of non-synaesthetes, synaesthetes without ACE, and synaesthetes with ACE was done. From the clustering maps, both synaesthetes had activity in the posterior ventrolateral region of the temporal cortex on the left hemisphere.16 More distinctively, the highly active region was the VWFA, otherwise known as the grapheme area, that was absent in non-synaesthete controls.16

3.1 Number Form Synaesthesia:

Number form (NF) synaesthetes create specific spatial representation of numbers, mainly ordinal numbers that describe its position in a sequence, which differs for each synaesthete.17, 18 The numbers would either have a specific location in space or be part of a number line (Figure 4.0). Studies have found that colour synaesthetes are more likely to have NF synaesthesia compared to non-synaesthetes.17, 18 It was also noted that this type of synaesthesia is unidirectional.17 Normally when the brain processes numbers, the right fusiform gyrus and the bilateral anterior IPS are activated.17 But for NF synaesthetes, the left and right posterior IPS is activated during an ordinal number stimulus.17 This was found in fMRI studies of NF synaesthesia, where the above mentioned region was active in synaesthetes but not in the non-synaesthete controls.17, 19

Figure 4.0 Illustration of how NF synaesthetes visualise numbers. The numbers are usual in a number line with a specific position in space. Image adapted from Tang, J., Ward, J., & Butterworth, B., (2008).

An interview of Heather Man, a student from the University of Waterloo, about her
grapheme-colour synaesthesia and number form synaesthesia.

See Also

Mechanisms Underlying Synaesthesia
Diagnosing Synaesthesia
Current Research and the History of Synaesthesia


  1. Sagiv, N & Ward J. Crossmodal interactions: lessons from synesthesia. Prog Brain Res. (2006). 155:259-71.
  2. van Leeuwen, T.M., den Ouden, H.E., & Hagoort, P. Effective connectivity determines the nature of subjective experience in grapheme-color synesthesia. J Neurosci. (2011). 31(27):9879-84.
  3. Simner, J., et al. Synaesthesia: The prevalence of atypical cross-modal experiences. Perception. (2006). 35:1024–1033.
  4. Gebuis, T., Nijboer, T.C., & van der Smagt, M.J. Of colored numbers and numbered colors: interactive processes in grapheme-color synaesthesia. Exp. Psychol. (2009). 56(3):180-7.
  5. Price, C.J., et al. Brain activity during reading. The effects of exposure duration and task. Brain. (1994). 117(6):1255-69.
  6. Hubbard, E.M., Arman, A.C., Ramachandran, V.S., & Boynton, G.M. Individual differences among grapheme-colour synesthetes: brain-behaviour correlations. Neuron. (2005). 45(6):975-85.
  7. Hangii, J., Wotruba, D., & Jancke, L. Globally altered structural brain network topology in grapheme-color synaesthesia. J Neurosci. (2011). 31(15):5816-28.
  8. Sperling, J.M., et al. Neuronal correlates of colour-graphemic synaesthesia: a fMRI study. Cortex. (2006). 42(2):295-303.
  9. Rouw, R., & Scholte, H.S. Increased structural connectivity in grapheme-color synaesthesia. Nat Neurosci. (2007). 10(6):792-7.
  10. Rouw, R. & Scholte, H.S. Neural basis of individual differences in synesthetic experiences. J. Neurosci. (2010). 30(18):6205-13.
  11. Esterman, M., Verstynen, T., Ivry, R.B., & Robertson, L.C. Coming unbound: disrupting automatic integration of synesthetic color and graphemes by transcranial magnetic stimulation of the right parietal lobe. J Cogn Neurosci. (2006). 18:1570 –1576.
  12. Hubbard, E.M. A real red-letter day. Nature Neuroscience. (2007). 10(6):671-2.
  13. Nunn, J.A., et al. Functional magnetic resonance imaging of synaesthesia: Activation of V4/V8 by spoken words. Nature Neuroscience. (2002). 5:371-75.
  14. Paulesu, E., et al. The physiology of coloured hearing a PET activation study of colour-word synaesthesia. Brain. (1995). 118:661-76.
  15. Goller, A.I., Otten, L.J., & Ward, J. Seeing sounds and hearing colours: an even-related potential study of auditory-visual synaesthesia. J Cogn Neurosci. (2009). 21(10):1869-81.
  16. Gray, J.A., et al. Evidence against functionalism from neuroimaging of the alien colour effect in synaesthesia. Cortex. (2006). 42(2):309-18.
  17. Tang, J., Ward, J., & Butterworth, B. Number forms in the brain. (2008). J Cogn Neurosci. 20(9):1547-56.
  18. Sagiv, N. et al. What is the relation ship between synaesthesia and visuo-spatial number forms? Cognition. (2005). 6(6):432-48.
  19. Hubbard, E.M., Piazza, M., Pinel, P., & Dehaene, S. Interactions between number and space in parietal cortex. Nat Rev Neurosci. (2005). 6(6):432-48.