Sounds Like Blue?

in Categories, Current Issue, Life & Physical Sciences, Spring 2011
November 22nd, 2011

It might be synesthesia.

Article by Daniel Roden

Imagine listening to a Mozart concerto and seeing it pass before your eyes in a stream of colors and shapes; imagine reading Proust when each letter has a particular color, each word like a little impressionist painting; imagine sitting in class, trying to pay attention to a lecture but distracted by little black circles that pop into your mind each time the person sitting next to you clicks his pen. Trying to imagine any of this might seem strange, but for a person with synesthesia this type of strange is routine.  Research on synesthesia has accelerated since the 1980s, and has revealed a great deal about different types of synesthetic experiences and their prevalence, as well as the genetic components of synesthesia, and its neural correlates.

What is Synesthesia?

“Synesthesia is a condition in which two or more senses are automatically and involuntarily coupled. Synesthesia is a neurological condition in which two or more senses are automatically and involuntarily coupled. Letters and numbers may be colored.  Simple geometric shapes, colors and even taste or a sense of touch might accompany sounds; and number sequences might fall into specific patterns and orientations in space.

Synethesia was a popular psychological subject in the beginning of the 20^th century, but interest waned during the 1930’s with the rise of behaviorism.  In the 1960s many scientists and review boards considered the phenomenon “simply the result of a hyperactive imagination”, and synesthetes were considered to be people “trying to draw attention to themselves”. Some critics went as far to say that so-called synesthetes were ‘potheads’ and ‘acid junkies’.¹ This type of criticism was tuned to the culture of the time, and in some way may be valid – certain psychoactive compounds have been known to trigger synesthesia-like experiences. ^2,3 Even today it may be hard for people to believe that some people perceive letters or numbers in specific colors. Synesthesia, however, is hard to fake. Over the years several tests and technologies have been developed to verify genuine synesthetic perceptions.

Testing Synesthesia

Synesthesia is characterized by a number of constraints: the experience is involuntary and automatic, lasts a lifetime, and is affect laden (emotional). Early tests of synesthesia used one or more of these constraints to distinguish between synesthetes and non-synesthetes. Some of these techniques are still used. For example, using variations of the Stroop task (a task in which words are colored and subjects are supposed to identify the color or the word without confusing the word and the color) to verify that a synesthetic experience is different from letter-color associations. Other tests, like the Synesthesia Battery Test, check the durability of an individual’s synesthetic experience over several months.  In the past few years, scientists have used brain-imaging technologies such as functional magnetic resonance imaging (fMRI), diffusion tensor imaging (DTI), and electroencephalography (EEG) to study the neural correlates of synesthesia.

Inside the Brain of a Synesthete

To what extent do the brains of synesthetes vary in cortical activity and connections compared to non-synesthetes? In 2002, Nunn et al. found that brain areas specific to color perception in the visual cortex, are activated in the left hemispheres of synesthetes by spoken words, as opposed to control subjects, who showed no such activity, even when asked to imagine colors in response to spoken words and trained on word-color associations.⁴ Researchers have also demonstrated that, compared to controls, there is increased structural connectivity in the brains of synesthetes in areas important for sensory perception like the fusiform gyrus, the left intraparietla suclus (IPS) and the frontal cortex.^5,6,7

One study using EEG, found that synesthetes have marked amplitude differences in early sensory-perception in response to simple visual stimuli. Areas in the brain responsible for transferring color information from the eyes to the visual cortex (the Lateral Geniculate Nucleus) are more active in synesthetes when they are shown colorful stimuli. On the other hand, when stimuli are low-contrast, black and white patterns, synesthetes showed less activity in these early areas than non-synesthetes.⁸ This study along with those mentioned above, suggests that there are extensive differences in cortical circuitry in synesthetes compared to non-synesthetes.

More Connections or Disinhibition?

“This would suggest that a synesthete’s brain has more synaptic connections than a normal brain. Even though the neural correlates of synesthesia are not completely understood, two models have been proposed: more connections or less inhibition.

The “more connections” model proposes that synesthesia is caused by an outgrowth of neurons in an individual’s brain that is insufficiently “pruned”. This would suggest that a synesthete’s brain has more synaptic connections than a normal brain, leading to enhanced cross-modal representation. The ‘less inhibition’ model assumes that the amount of synaptic connections and cross wiring in synesthetes is similar to those of non-synesthetes. According to the second model, synesthesia may be the result of faulty or diminished inhibition between certain brain areas. According to this theory, normal brain excitation in cross-sensory networks is balanced by inhibition, but in the synesthetic brain inhibition is diminished, resulting in an increase of coupled activity.

The two hypotheses are not distinguishable by behavioral measures, physical testing, or neural imaging; nor are they mutually exclusive.⁹ If there is increased connectivity in synesthetes, it may in fact result from a decrease in inhibition between brain areas. The disinhibition model is favored by a number of scientists who point to cases of “acquired” synesthesia, in which non-synesthetes experience forms of synesthesia under certain conditions. These experiences may occur during meditation, as non-synethetic individuals fall asleep, or after ingesting psychoactive drugs that effect serotonin or dopamine levels in the brain.

Who’s Got It?

Exactly how many people have synesthesia? It depends on which study you look at. A 1993 survey of two nonrandom London populations came up a prevalence of 1 in 2,000 and 1 in 2,500. In Germany surveys put the likelihood of having synesthesia at 1 in 300, and classroom surveys in the United States initially put the number at 1 in 200.  However, in 2005 Julia Simner and colleagues in Edinburgh assessed a large random sample of two populations, a university and a science museum, and found the prevalence to be 88 times higher than previously assumed: 1 in 23 for any type of synesthesia.¹⁰

In 2008, Simner et al. studied Synesthesia in context of developmental questions. They found a significant number of synesthetes in the childhood population, even in children as young as six-years-old.  Their study put the prevalence of grapheme-color synesthesia (seeing letters in colors) in children at 1.3%, over 170,000 children (0-17) in the UK, and over 930,000 in the USA.¹¹ Are you one of 930,000?

If you are, chances are that one of your family members also experiences synesthesia, so ask them. In 2005, Ward and Simner looked at inheritance patterns of synethesia in seventy-two families and proposed that it may be inherited on the X chromosome from the mother. The X-linked claim has been contested, but scientists are still interested in finding the genetic components that cause synesthesia.¹²

Synesthesia and Aesthetics

The fact that a number of important artists were synesthetes (Kandinsky and Leonard Bernstein are just two) and the fact that these multimedia experiences are so engaging, suggests that synesthesia have very different insights about aesthetics. One study, headed by Dr. Jamie Ward or London X, implies that the sensory coupling that occurs in synesthetes may explain our tendency to like certain aspects of movies. Ward et al. found that non-synesthetes were significantly more likely to enjoy animations when they were based on the experiences of synesthetes than when they were based on random or non-synesthete experience.¹³ Interestingly, one study in 2010 showed a higher prevalence of synesthesia in a group of art students than in a control group.¹⁴

Shaping the Future

What is the future of synesthesia research? There is a lot of evidence that synesthesia is a product of cortical disinhibition, and it is likely that future experiments will try to validate this and incorporate it into more general theories of multisensory cognition. For example, a region of the brain called the claustrum, located in the basal ganglia, has been implicated in multisensory integration and synesthesia, but what is the function of the claustrum? Future research on synesthesia is also likely to focus on the cognitive effects of hypnosis, since hypnosis may reveal important aspects of disinhibition.

It is important to recognize that studies of synesthesia in cognitive neuroscience represent a desire to move toward understanding individual experiences. It is important to remember in neuroscience that each brain is a little different, and it is necessary to pay attention to those individual details in order to better understand synethesia and human cognition more generally.

References

¹Ramachandran, V.S. & Hubbard, E.M. (2001), ‘Synaesthesia: A window into perception, thought and language’, Journal of Consciousness Studies, 8 (12), pp. 3-34.

²Cytowic, Richard E., and David Eagleman. Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia. 215

³Ramachandran, V.S. & Hubbard, E.M. (2001), ‘Synaesthesia: A window into perception, thought and language’, Journal of Consciousness Studies, 8 (12), pp. 3-34.

⁴Nunn JA, Gregory LJ, Brammer M, et al. (April 2002). “Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words”. Nat. Neurosci. 5 (4): 371-5

⁵Ward, Jamie, Ryan Li, Shireen Salih, and Noam Sagiv. “Varieties of Grapheme-colour Synaesthesia: A New Theory of Phenomenological and Behavioral Differences.” Consciousness and Cognition 16 (2007): 913. www.sciencedirect.com. 27 Nov. 2006. Web. 27 Apr. 2010. <http://home.comcast.net/~sean.day/Ward%20et%20al%202007.pdf>

⁶Hubbard, Edward M. “A Real Red-letter Day.” Nature Neuroscience 10.6 (2007): 671-72. www.nature.com/natureneuroscience. Nature Publishing Group.

⁷Rouw, R. & Scholte. H.S. Nat. Neurosci. 10. 792-797 (2007)

⁸Barnett, Kylie J., Foxe J. John, Molholm Sophie, Simon P. Kelly, Shani Shalgi, Kevin J. Mitchell, and Fiona N. Newell. “Differences in Early Sensory-perceptual Processing in Synesthesia: A Visual Evoked Potential Study.” NeuroImage 43.3 (2008): 605-13. www.sciencedirect.com. 28 July 2008. Web. 27 Apr. 2010.

⁹Avidan, Nili, Anand K. Sarma, Rejnal Tushe, Dainna M. Milewicz, Molly Bray, Suzanne M. Lea, and Eagleman M. David. “The Genetics of Colored Sequence Synesthesia: Evidence of Linkage to Chromosome 16q and Genetic Heterogeneity for the Condition.” Nature Preceedings (2009). 20 Nov. 2009. Web. 27 Apr. 2010. <http://precedings.nature.com/documents/3987/version/1>.

¹⁰Simner, Julia, Mulvenna, Catherine, Sagiv, Noam, Tsakanikos, Elias, Witherby Sarah A., Fraser, Christine, Scot, Kirsten, Ward, Jamie Synaesthesia: The prevalence of atypical cross-modal experiences Perception 2006 volume 35(8) pages 1024-1033http://www.perceptionweb.com/abstract.cgi?id=p5469

¹¹Simner, Julia, Harrold, Jenny, Creed, Harriet, Monro, Louise, Foulkes, Louise Early detection of markers for synaesthesia in childhood populations Brain 2009 132(1):57-64;doi:10.1093/brain/awn292 http://brain.oxfordjournals.org/cgi/content/full/132/1/57

¹²Avidan, Nili, Anand K. Sarma, Rejnal Tushe, Dainna M. Milewicz, Molly Bray, Suzanne M. Lea, and Eagleman M. David. “The Genetics of Colored Sequence Synesthesia: Evidence of Linkage to Chromosome 16q and Genetic Heterogeneity for the Condition.” Nature Preceedings (2009). 20 Nov. 2009. Web. 27 Apr. 2010. <http://precedings.nature.com/documents/3987/version/1>.

¹³Ward, Jamie, Samantha Moore, Daisy Thompson-Lake, Shireen Salih, and Beck Brianna. “The Aesthetic Appeal of Auditory-visual Synaesthetic Perceptions in People without Synaesthesia.” Perception 37 (2008): 1285-296. Www.perceptionweb.com. Pion, 4 Aug. 2008. Web. 27 Apr. 2010.

¹⁴Rothen, Nicholas, and Beat Meier. “Higher Prevalence of Synesthesia in Art Students.” Perception 39 (2010): 718-20. Www.perceptionweb.com. Perception, 10 Mar. 2010. Web. 5 Apr. 2011. <http://www.perceptionweb.com/perception/editorials/p6680.pdf>.