This past week I read a 2013 New Yorker piece by one of the most notable neurologist’s of the modern era – Oliver Sacks – titled “A Mind’s Eye.” The premise of this piece was the way in which blind individuals experience their blindness; how even if the visual cortex isn’t active in the classic sense of what society considers “seeing,” they have compensatory sensory experiences that are equally as vivid. It was an incredibly eloquent account of his research, and I highly recommend it as a quick read for anyone and everyone!

There were several topics he mentioned that I felt compelled to research further, but today I’ll be talking about a sensory condition he briefly mentioned called “synesthesia”. It was relevant to Sack’s piece because of the sensory integration component: synesthesia is an “anomalous blending of the senses in which the stimulation of one modality simultaneously produces sensation in a different modality” (Scientific American, 2002). In other words, for a person with synesthesia, the five senses of sight, smell, sound, taste and touch are inherently intermingled in an unpredictable but consistent manner. There are several synesthetic associations, ranging from taste-touch to sound-color. For example, chances are, you reading this will look at the number “8” and see a black digit that you logically know follows “7” and precedes “9”. Someone with synesthesia, however, may see the number “8” and associate it with fluorescent lime green, or vividly taste ripe strawberries. These simultaneous associations come naturally to them, automatically without any conscious effort.

So what are the neural areas that contribute to this perceptual phenomenon? There must be a biological basis, given that the perception of any somatosensory sensation requires neurological activation of the brain. I’d heard about synesthesia on a few separate occasions in several different courses, but I’d never taken the time to try and understand the contributing mechanisms before. In my research I came across a 2013 fMRI brain-imaging study by Tomson et al., where researchers looked at brain regions in both synesthetes and controls when potentially synesthetic activity was induced. They looked at a variety of different types of synesthetic associations (i.e. grapheme-color, sound-touch, etc) and looked at anatomical activation differences between the two groups. Most specifically,  they found that grapheme-color activation induced differences in the bilateral posterior temporal gyri. There were also several processing differences found; synesthetes clustered visual regions during grapheme stimulation, whereas controls clustered in the frontal and parietal regions. The literature around parietal cortex hyperactivity in synesthesia is controversial, and the authors concede that despite their lack of support for the argument, several researchers prior have found support for it (Van Leeuwen et al., 2010. Neufeld et al., 2012).

(Above is a figure from Tomson et al.’s fMRI study showing activational differences in grapheme-induced synesthetic activation!!!!)

If we’re being completely honest, a lot of this paper went right over my head. The most important thing I took away from it is that there are irrefutably differences in the activation of synesthetes. Also, that the production of synesthetic experience is a complex interaction of brain regions and areas that function collaboratively (which I should have guessed, given the fact that nothing in the brain acts completely independent of anything else.)

Another expert suggested that synesthesia arises from anomalous cross-wiring between brain areas that are typically clearly distinct (Van Campen, 2010). For example, cross wiring between the digit and letter processing areas and the color processing areas in the visual cortex may communicate more heavily in synesthetic rather than in non-synesthetic brains.

The causes of synesthesia remain unknown – some scientists have suggested that everyone is born synesthetic, but as the brain develops anatomical differentiation and pruning decrease the highly interconnected neural circuits resulting in “normal” sensory processing. A genetic component is evident, because the condition is known to run in families. Finally, a strong sex difference is empirically supported, and women are six times more likely to express synesthesia than their male counterparts.

I can’t imagine a world in which the color blue consistently smells like dandelions, or a situation where the letter C elicits the sound of a D#. But that is the everyday reality for many individuals. Synesthesia provides research opportunities for psychologists interested in all areas spanning from cognitive to developmental to biological basis, and as such I’m interested to see what the research continues to tell us from here on out.

References:

Palmeri, T.J., Blake, R.B., Marois, R. (2006). What is synesthesia? Scientific American.

Sacks, O. (2003). The mind’s eye. New Yorker, 28, 48-59.

Tomson, S. N., Narayan, M., Allen, G. I., & Eagleman, D. M. (2013). Neural networks of colored sequence synesthesia. Journal of Neuroscience, 33(35), 14098-14106.

Van Campen, C. (2010). The hidden sense: Synesthesia in art and science. Mit Press.

Van Leeuwen, T. M., Singer, W., & Nikolić, D. (2015). The merit of synesthesia for consciousness research. Frontiers in psychology, 6.

Watson, M. R., Akins, K. A., Spiker, C., Crawford, L., & Enns, J. (2014). Synesthesia and learning: a critical review and novel theory. Developing Synaesthesia, 109.