Is synaesthesia a high-level brain power?
17 December 2009 by Ewen Callaway
Gallery: A window into the synaesthetic mind
THE word "synaesthesia" derives from the phrase "joining of the senses", but the phenomenon might not be the uncontrollable perceptual mishmash that this implies. Instead, the condition may be the result of a special ability in the "higher" brain areas used for language and attention.
Earlier experiments found that people with colour-grapheme synaesthesia, who link numbers and letters with certain colours, are incredibly speedy at a certain task. That is identifying hidden shapes formed out of one number or letter that are embedded in a sea of different, similar-looking characters: a pattern made up of "2"s on a background of "5"s, for example. It was assumed that they automatically imbue the numbers with different colours, causing the hidden pattern to "pop out" as soon as they glance at the display.
Now Jamie Ward at the University of Sussex in Brighton, UK, and colleagues offer a different explanation. They had 36 colour-grapheme synaesthetes sit a similar test. When given just 1 second to identify the hidden shape, the synaesthetes were more likely to spot it than controls. But they still only found it about 40 per cent of the time. Volunteers' descriptions of the trial offer insights into why this is. "I only see the colours in the part that I am looking at," said one. "I have to attend to the symbols," said another (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2009.1765).
Ward's explanation is that colours don't "pop out" at synaesthetes automatically. Instead, they only see numbers in colour if they fall within their focus of attention. So when volunteers happened to aim their attention at the part of the display in which the shape was hidden, he reasons, they found it quickly, but when their focus captured none, or just a portion, of the shape, they performed no better than controls (see diagram). If Ward is right, this implies that higher aspects of the brain are unusual in synaesthesia, not just those involved in automatic perception.
Ward's study isn't the only one to suggest this. A group led by Danko Nikolić at the Max Planck Institute for Brain Research in Frankfurt, Germany, taught 16 colour-grapheme synaesthetes to equate characters from an ancient Slavic script none had seen before with letters and numerals they already associated with colours.
The researchers found that the volunteers transferred the colours from their native alpha-numerical system to the newly learned one within minutes of practice. These connections are based on meaning, making them unlikely to be the result of automatic, lower-level processes, says Nikolić. He suggests "abandoning" earlier theories that synaesthesia is simply a result of cross-wired perceptual areas (Journal of Vision, DOI: 10.1167/9.12.25).
Not everyone interprets the results in this way. Edward Hubbard at Vanderbilt University in Nashville, Tennessee, who performed the first hidden-shape experiments with synaesthetes, says perceptual areas are involved too. He notes that synaesthetes seem to activate a colour-processing brain area too quickly after seeing a character for higher areas to be driving the process.