Thursday, July 8, 2010

The neural basis of synesthesia

Wikipedia has a page on the neural basis of synesthesia, but not yet described there is a new study in press by Vilayanur S. Ramachandran’s group that provides interesting insights.

Synesthesia is a neurological condition in which affected individuals experience one sense (e.g. hearing) as another sense (e.g. visual colours). Ramachandran’s latest study investigated grapheme-colour synesthetes who experience specific colours when they view specific graphemes (i.e., letters and numbers). The results demonstrate that two brain areas – for grapheme and colour representation respectively – are activated at virtually the same time in the brains of synesthetes who are viewing letters and numbers. On the other hand, normal controls viewing the same thing exhibit activity in the grapheme region but not the colour region.

This is the first study of synesthesia to demonstrate simultaneous activation of the two brain areas, known as the posterior temporal grapheme area (PTGA) and colour area V4 (pictured below in the brain of a representative synesthete). The finding was made possible because the researchers used a neuroimaging technique called magnetoencephalography (MEG) to measure weak magnetic fields emitted by specific areas of the brain while the subjects viewed graphemes. Compared to other neuroimaging techniques, such as fMRI and EEG, MEG offers the best combination of temporal and spatial precision in measuring brain activation.

If you read the Wikipedia page, you know that there are two main theories that attempt to explain how synesthesia occurs in the brain: the cross-activation theory and the disinhibited feedback theory. Let’s call them Theory 1 and Theory 2 for simplicity. Theory 1 posits that the grapheme and colour brain areas are ‘hyper-connected’ such that activity in the grapheme area evoked by viewing a letter or number immediately leads to activity in the colour area and conscious perception of colour. Theory 2 maintains that there are ‘executive’ brain areas that control the communication between the grapheme and  colour areas, and in synesthetes this control is disrupted. To reiterate, Theory 1 says that normal brains are anatomically different than synesthete brains, whereas Theory 2 says that normal brains are the same as synesthete brains but the two brains act differently.

The results of Ramachandran’s group support Theory 1, the cross-activation theory, since this model predicts that the colour and grapheme areas should be activated at roughly the same time in synesthetes looking at graphemes.

This is perhaps the strongest evidence for the cross-activation theory of synesthesia to date. But to complicate things, Ramachandran’s group proposed a new theory called ‘cascaded cross-tuning model,’ which is essentially a refinement of the cross-activation model (let’s call it Theory 1.1).

According to Theory 1.1, when a synesthete views a number, a series of simultaneous activations lead to perception of a colour. First, a subcomponent of the grapheme area responds to features of the number (e.g. the “o” that makes up the top of the number 9). This leads to activity in other subcomponents of the grapheme area representing possible numbers that the feature is part of (e.g. the “o” could be a component of the numbers 6, 8, or 9) as well as the colour area V4. At this point however, colour is not consciously perceived. Next, when the grapheme area identifies the number 6 (based on monitoring by other brain areas), activity in V4 is triggered, leading to conscious perception of the colour associated with the number 6.

Cool theory? Cool theory.

Note, however, that it only applies to ‘projector’ synesthetes who see colours in the outside world when they see numbers, but not ‘associator’ synesthetes who perceive the colours in the “mind’s eye.” Also, it doesn’t yet apply to other forms of synesthesia, such as acquired synesthesias (e.g. synesthesia for pain).

Yeah, it’s only a matter of time before Theory 1.2 takes over.


Brang D, Hubbard EM, Coulson S, Huang M, & Ramachandran VS (2010). Magnetoencephalography reveals early activation of V4 in grapheme-color synesthesia. NeuroImage PMID: 20547226


  1. Neat. Thanks for the informative post.

    Their "cascaded cross-tuning" theory seems like it's not really that innovative to me - cortical activity associated with normal visual perception is already thought to be composed of many parallel and interacting "cascades" of activity that get lots of input from executive-type controllers elsewhere in the brain. Why would we have ever thought that synaesthetic perception was any different?

  2. Good point - I guess the problem with the old theory is those 'controller' influences weren't accounted for... the new theory can't be described as 'cross-activation theory' anymore because it involves more than just cross-activation of two brain areas.

  3. Interesting.
    It strikes me that synaesthesia is pretty much what what happen to someone if they got a metal wire stuck in their head making random (but fixed) connections between two sensory areas. So I've always been a fan of anatomical theories.

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  5. The team's recent publication is titled -

    Similarly shaped letters evoke similar colors in grapheme-color synesthesia.

    it is here -

    which sounds like an enormous flashback to the early 1910s or so with all this behavioural over-analysis of hooks and circles.

    There are at least three problems with this -

    First, not all synaesthesiae are modality-specific it means in many cases reactions are not limited to colours but shapes, locations, texture, taste, etc. So, neither the anatomical substrate nor cascading could explain it. Only the curtailed description of synaesthetes' subjective experience does.

    Second, take a look at the "archetype" synaesthetic ABC here -
    or here -

    O and Q, R and P, W and M, E and F - any similarities are hardly noticeable and correlations might be very loose unless you apply "large-scale analysis" which Mark Twain so much praised.

    Third, synaesthesia transfer (when synaesthetes study a foreign language) works quite differently in different cases. My own observations with about two dozen synaesthetes revealed that some of them "borrow" the phonetic aspects (letters that sound similar look similar) while others the graphemic aspects - but still others develop unique qualities.

    Therefore, to my mind, the CCT (or former cross-activation) is not any closer as the authors claim, to further highlighting the relationship between conceptual and perceptual factors in this phenomenon but just keeps reviving a dinosaur.