What colour is your breast-stroke? Or why synaesthesia is more about ideas than crossed-senses

People with synaesthesia experience odd sensations that make it seem as though their neural wires are crossed. A certain word might always come served with the same particular taste, or a letter or numeral might reliably evoke the same particular colour. But an emerging view among experts is that synaesthesia is grounded in concepts, not crossed senses. By this account, it's certain ideas, regardless of which sense perceives them, that trigger a particular concurrent experience. The latest evidence for this comes from Danko Nikolic and his colleagues at the Max-Planck Institute for Brain Research. They've documented two synaesthetes, HT and UJ, who experience different swimming strokes, whether performing them, watching them or merely thinking about them, as always being a certain colour.

HT and UJ, both now aged 24, began swimming competitively at an early age and the sport continues to be an important part of their lives. The first test that Nikolic's team performed was to present the pair with four black and white close-up photos of different swimming strokes and have them say which colour the strokes triggered using a book of 5500 colour shades. This was repeated four weeks later for HT and three weeks later for UJ. Three non-synaesthete control participants, all swimmers, were recruited for comparison. They similarly reported which colours the photos made them think of and they repeated the exercise after just a two-week gap.

The clear finding was that the difference from the first test to the second test in the precise colours chosen for each stroke by the synaesthetes was eight times smaller than the test-retest difference shown by the controls, thus supporting the synaesthetes' claim that different strokes always provoke the same colours.

Next the researchers administered a version of the Stroop test: the synaesthetes and controls were presented with the same swimming stroke photos as before, but this time they were shown with different coloured tones, for example in blue or yellow. The participants' task was to name the colour. If certain swimming strokes really do evoke particular colours for the synaesthetes then their colour naming ought to have been affected by the precise stroke/colour pairing on any given trial, such that you'd expect them to be quicker if the photo's colour matched the colour evoked by the stroke shown in the image. That's exactly what was found - UJ, for example, was 101ms slower when naming incongruent colours versus congruent ones. No such effect was observed for two control participants.

According to the classic view of synaesthesia as cross-wiring between senses, you'd think that swimming-style synaesthesia would require the act of swimming (via proprioception) to evoke a concurrent experience, but this study suggested it was enough to merely activate the concept of the different swim strokes by looking at pictures. This is consonant with past research showing, for example, that letter/number-colour synaesthesia can be triggered merely by imagining the necessary letter or number. Other research has documented synaeshetic experiences devoid of any particular sensory element, including so-called time-unit-space synaesthesia, in which units of time are experienced as existing in particular locations relative to the body.

"Hence, the original name of the presently investigated phenomenon syn + aesthesia (Greek for union of senses) may turn out to be misleading in respect of its true nature," the researchers said. "The term ideaesthesia (Greek for sensing concepts) may describe the phenomenon much more accurately." For more detailed discussion of how, when and why synaesthetic triggers and their concurrent experiences are acquired, it's worth checking out the full-text of the article.
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Nikolić, D., Jürgens, U., Rothen, N., Meier, B., and Mroczko, A. (2011). Swimming-style synesthesia. Cortex, 47 (7), 874-879 DOI: 10.1016/j.cortex.2011.02.008

This post was written by Christian Jarrett for the BPS Research Digest.