Hearing, touching and tasting in color

I don't know about you, but to me Wednesday is sun-shiney yellow. Tuesday is hunter green, Thursday purple-ish blue and Friday a deep red. Monday is white, a blank slate and a chance for a new week, whereas Saturday is sparkly black. Sunday is gray, the depressing slouch towards the beginning of the work-week, but also a convenient mix of Saturday and Monday.

This color-word association is not a figment of my imagination or an indication that I'm going crazy, but is instead a recognized neuropsychological phenomenon called synesthesia. Synesthesia can be thought of as a crossing of the senses, where one perceptual experience simultaneously expresses another. It is estimated to occur in some form in roughly 20% of the population, with color-grapheme being the most common (1). Here, letters or numbers are experienced as taking on unique and varying shades of color. Apparently the experience that I have is also quite common, wherein units of time - such as days of the week - are also expressed in color. There are up to 60 different forms of synesthesia, including sound-color, grapheme-shape and, my personal favorite, word-taste. In this last one, words or names are experienced as different flavors in the individual's mouth. Reported palates range from buttered toast to berries and everything in between. Think of it as chewing a different flavored jellybean for every person that you know.

How does this happen? Synesthesia occurs when there is additional "cross-talk" between neurons in different sensory regions of the brain. Initial activation in one area "spills over" into neighboring regions, creating a sensory effect from the stimulation of those neurons. For example, seeing a word or letter activates the aptly named visual word form area, or VWFA, located towards the back and bottom of the brain where visual information is processed. The VWFA is adjacent to area V4, another visual part of the brain responsible for experiencing color. In people with grapheme-color synesthesia, greater connectivity between VWFA and V4 can result in the incidental firing of V4 neurons when they see letters, giving them the sensation of perceiving color at the same time (2).

Scientists have set out to study what exactly is happening in the brain during these moments of synesthesia. One researcher, British neuroscientist Dr. Julia Nunn, is using functional magnetic resonance imaging (fMRI) to try and get some answers. As you may have read before, fMRI maps blood flow in the brain to identify areas that "light up" or are active during a particular cognitive process. Using fMRI, Dr. Nunn confirmed that when grapheme-color synesthetes saw letters or numbers, neurons in V4 lit up in addition to VWFA (3). Importantly, this activation is not elicited when people think of or visualize a color, only when they perceive it. We know from previous studies that you have different activation in your brain when you're thinking about a color compared with when you're actually seeing it, so Dr. Nunn's research critically shows that people with synesthesia are not merely imagining color when they see a letter, but are actually experiencing it.

This cross-talk between brain regions is due to white matter connectivity. There are two primary types of tissue in our skulls: gray matter and white matter. Gray matter is the actual neurons, whereas white matter is the tracts that run between these cells, carrying signals across different parts of the brain. Integration between brain regions is essential for us to be able to synthesize different sensory input, like combining perceptions of sight and smell to help us differentiate a rose from a daffodil. Individuals with synesthesia have been shown to have greater amounts of white matter connectivity, particularly between sensory areas of the brain, causing these regions to activate together even when they are not directly stimulated (4).

But why do some people have more of these connections than others? Until late adolescence, our brains are constantly growing, generating new neurons and forging new tracts between them. However, alongside this cell production is a process called "pruning". This is where unessential neurons and connections, ones that are rarely used, die off. This stage is equally critical as it allows the brain to focus on the most important processes, strengthening and making them more efficient. Thus, by the time we reach adulthood we have fewer brain cells and connections than we had as children, but the ones we do have are much faster, allowing us to think and react quicker. However, in the brains of synesthetes it is possible that this process is somewhat suppressed, leaving greater amounts of white matter between key sensory regions. Synesthesia, then, may be evidence of an over-connected brain, lacking in pruning.

If you weren't able to retain your synesthetic connections from birth though, don't despair. Research suggests that some forms of synesthesia can also develop through sensory deprivation. People who are blind often share a form of touch-sight synesthesia, their fingertips acting as eyes and creating activation in the visual cortex through touch. This effect can even be induced in people who have been blindfolded for just two days, suggesting that rather than having to generate new connections between regions, existing ones are merely lying dormant, ready to re-activate if need be.

No word yet on how to get the jellybean-name connection to kick in though.

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1. Cytowic R.E. & Eagleman D.M. Wednesday is Indigo Blue. Cambridge, MA: MIT Press, 2009.

2. Hubbard E.M. et al. Individual differences among grapheme-color synesthetes: brain-behavior correlations. Neuron 45, 975-985 (2005).

3. Nunn J.A. et al. Functional magnetic resonance imaging of synesthesia: Activation of V4/V8 by spoken words. Nature Neuroscience 5,371-375 (2002).

4. Rouw R. & Scholte H.S. Increased structural connectivity in grapheme-color synesthesia. Nature Neuroscience 10, 792-797 (2007).

Dana Smith

PhD student in Experimental Psychology at the University of Cambridge