Dec 18 2009

The Nature of Synaesthesia

Fans of the show Heroes are familiar with a recently introduced character whose mutant power is to see sounds. She is deaf, making the ability more interesting for her, and discovered that loud sounds, like music, produce beautiful colored lights.

This is a real phenomenon known as synaesthesia – an uncommon neurological condition in which people perceive one kind of sensory input as if it had properties of another – for example numbers have color or shape, or visual stimulation producing the perception of sound. The current interpretation of synaesthesia is that one sensory area in the brain is leaking electrical signals to an adjacent brain area – visual signals are stimulating auditory cortex resulting in the perception of sound.

However recent evidence suggests that synaesthesia may be more complex than just leaking signals between adjacent brain areas. Perhaps it results, at least in some cases, from higher levels of cortical processing involved in attention and concentration.

Jamie Ward and colleagues at the University of Sussex in Brighton, UK, have been studying color-grapheme synesthetes – those who see numbers or letters as colors. Such synesthetes have been shown to perform better on certain kinds of hidden shape tests. If a shape is made out of  the number “2” in a field of “5s” it can be very challenging to perceive the shape. But color-grapheme synesthetes perform much better on such tasks, because the numbers “2” and “5” have different colors, so the shape pops out.

However, when Ward performed this test on a number of color-grapheme synesthetes and normal controls and gave them only 1 second to see the shape, the synesthetes performed better than controls but were only able to see the shape 40% of the time. Some of the subjects reported that the numbers had colors only where they were looking – that portion of the puzzle on which they were focusing their attention. This suggested to Ward that perhaps the phenomenon of synaesthesia is not a simple leaking from one sensory area to another but results from higher-order processing involved in attention.

This could explain the results of this study – if the synesthetes happened to be looking at the portion of the puzzle where the shape was hidden in the 1 second provided to them, the shape popped out – but not if they were not looking directly or mostly at the shape.

There is other evidence to support this view. Another study performed at the Max Plank Institute found that color-grapheme synesthetes could transfer their color associations. They told subjects to associate characters from a Glagolitic language they did not previously know with their existing letters or numbers. They found that the color associations transferred to the new Glagolitic characters.

Transfer of color associations to new languages has been previously described, but this study showed that it could occur after only 10 minutes. The association was confirmed with a Stroop test – which means the researchers were not relying on subjective report. This shows  a high level of plasticity for synaesthesia, but also that synesthetic associations can be made at higher conceptual areas – at the level of abstract meaning, not just simple sensory input.

Still another line of evidence for this view comes from neuroanatomy. Researchers have found that synesthetes have increased gray matter volume, and that they have novel brain connections that are widely distributed – not just in sensory areas.

Although the speed with which some synesthetes can perform the hidden-shape test, argues Edward Hubbard, a synaesthesia researcher, is too fast for complex cortical processing, so simple sensory processing must also be involved.

What all of this means is that synaesthesia is more of a complex neurological phenomenon than simple sensory leakage. It involves a novel pattern of hard wiring in many brain areas, involving attention and abstract concepts in addition to sensory cross-over.

This is interesting on many levels, and researchers hope that it can be exploited to learn more about the anatomy of brain connections.

But also I think this is an excellent example of variation in nature. We tend to think of species as archetypes, but they are much more varied than most people assume – and this includes humans, and specifically the pattern of hard-wiring of the human brain. Nature is always “experimenting” with new patterns, and sometimes those novel patterns produce unexpected results. It seems likely that most of the time these new patterns will be inconsequential or neutral – different but not necessarily better or worse, depending on context. Sometimes they may be detrimental. And on occasional new patterns that emerge by chance may produce a beneficial new ability.

Whether or not synaesthesia is “beneficial” is an interesting question in itself, and likely depends on context. Perhaps if it can be exploited to enhance intellectual performance in useful tasks (not just psychological tests) it can be beneficial. Some color-grapheme synesthetes are math wizards, because they can see the numbers and this helps them manipulate them.

Perhaps the central premise of Heroes is not that far off – new heritable genetic variations can arise in the human population producing new and interesting abilities. Although I doubt this will produce people who can fly anytime soon.

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