SYNAESTHESIA is a neurological condition in which stimulation of one sensory pathway evokes sensations in another sensory modality. This may occur because of abnormal connections between the brain’s sensory systems, or because the flow of information between those systems is not inhibited as usual.
First described in the 1880s by Francis Galton, synaesthesia is known to exist in several different forms. Galton described “persons who almost invariably think of numerals in visual imagery”. This form, now known as grapheme-colour synaesthesia, was experienced by the physicist Richard Feynman, who often said that he could see equations in colour.
The artist Wassily Kandinsky, on the other hand, is believed to have been a tone-colour synaesthete. He perceived musical notes not only as sounds, but also as specific colour hues, and tried to create the “visual equivalents of symphonies” in his paintings.
Now, researchers from the California Institute of Technology have now identified yet another form of synaesthesia. In the journal Current Biology, they report on 4 individuals in whom moving visual stimuli evoke sounds. They have named this condition “hearing-motion synaesthesia”.
Melissa Saenz and Christof Koch identified hgearing-motion synaesthesia by chance, when a visitor to Melissa Saenz’s lab mentioned that he could “hear” a screensaver consisting of a pattern of moving dots. When Saenz questioned him about the sensation, he told her what all synaesthetes say: that it was automatic and that he had experienced for as long as he could remember. Subsequently, Saenz sent the screensaver out in email. Of the several hundred recipients who eventaully saw it, 3 replied saying that they could hear it. They were recruited for the new study, together with the visitor to the lab.
Saenz and Koch devised a task which could be used to objectively confirm the reports of the 4 participants, a task on which they would out-perform non-synaesthetes who do not experience the “extra” sensation. The task involved judging rhythmic patterns – in each trial, the participants were presented with pairs of sequences of either visual flashes or auditory beeps, and then asked if the two were the same.
Typically, non-synaesthetes are much better at judging auditory than visual sequences. But the hearing-motion synaesthetes should be at an advantage when presented with sequences of visual flashes, because they can hear, as well as see, the pattern. This is exactly what was found: the 4 synaesthetes and the 10 non-synaesthete controls performed equally well in the trials of sound sequences, with an accuracy of around 85%. But in the trials with sequences of visual flashes, the synaesthetes remained accurate, with a score of about 75%, whereas the performance of the controls fell to 50%, which is what would be expected by chance.
The synaesthetes in the study were all aged between 23 and 33 years of age, and none had any history of visual, auditory or neurological conditions. They all experience visually-induced sound perceptions, which typically consist of such as beeping, tapping or whirring associated in time with flashing visual stimuli or visual motion.
Saenz and Koch note that there are interactions between the auditory and visual systems in non-synaesthetes, and that a recent paper published suggested that the brain may in fact translate the temporal patterns of visual information by forming auditory representations of it. This would serve to improve the accuracy of judgements based on the perception of different stimuli which are related in time, such as the image and sound of a moving car. They are, therefore, now carrying out neuroimaging experiments to determine if the sounds perceived by these synaesthetes are associated with acitvation of the auditory cortex, and plan to investigate whether hearing-motion synaesthesia is an exaggerated form of the normal interactions between the auditory and visual systems.
Synaesthesia was once thought to be extremely rare, but it may be rather common. Recent research suggests that 1 in 100 people may have one form or another of the condition. (Saenz and Koch have put a hearing-motion synaesthesia test online.) In 2005, Jamie Ward and his colleagues at UCL described, for the first time, mirror-touch synaesthesia, in which one expreiences tactile sensations when they see another person being touched. Thus, hearing-motion synaesthesia is the second form of synaesthesia to be identified in 3 years, and it is likely that other types remain to be discovered.
Saenz, M. & Koch, C. (2008). The sound of change: visually-induced auditory synesthesia. Current Biology 18, R650-R651. DOI: 10.1016/j.cub.2008.06.014