synaesthesia literally, ‘joining the senses’. People who experience synaesthesia (synaesthetes) inhabit a world slightly, but magically different from that of most people — a world of extra colours, shapes, and sensations. ‘Mine is a universe of black “Is” and pink “Wednesdays”, numbers that climb skywards and a roller-coaster shaped year’ is one description, reported by Motluk (1997). For synaesthetes, the experience of a single sense is accompanied by sensations in other sensory modalities.

Coloured-hearing is the most common form of synaesthesia, in which hearing a word elicits the perception of colour. The colour sensation elicited by a word is often determined by the letters in the word, with the first letter being the most influential. ‘For me “run”, “right”, and “religion” are all black … because the letter R is so strikingly black. … Even the word “red” … is a black word, while the “black” is, because of its B, blue’, said one synaesthete. The experience that words have particular colours can be helpful. Miss Stone, reported by Francis Galton in 1883, said ‘I have always associated the same colours with the same letters, and no effort will change the colour of one letter … Occasionally, when uncertain how a word should be spelt, I have considered what colour it ought to be, and have decided in that way. I believe this has often been a great help to me in spelling, both in English and foreign languages.’ Some composers report experiencing colours in response to chords or notes. For example, Olivier Messiaen stated; ‘I see colours which move with the music, and I sense these colours in an extremely vivid manner.’ Other forms of synaesthesia combine many of the senses. An extreme example was experienced by the patient ‘S’ who was intensively studied by the Russian neuropsychologist, Alexander Luria. Presented with a tone of 2000 Hz at 113 decibels, S said, ‘It looks something like fireworks tinged with a pink-red hue. The strip of colour feels rough and unpleasant and it has an ugly taste — rather like that of a briny pickle.’ In this example a sound is eliciting experiences of colour, touch, and taste.

Synaesthetes are not simply using metaphorical language to describe sensations that are, in reality, no different from those of other people, nor have they simply learned peculiar associations between different senses. The synaesthetic experience is an automatic and involuntary response to certain stimuli. The experience is vivid and consistent: true synaesthetes give precisely the same descriptions of their experiences even when these are separated by months or years. In support of an organic basis, synaesthetes seem to have had their experiences from earliest childhood and certainly before the age of four, and the syndrome seems to run in families. A transient experience of synaesthesia can be induced in non-synaesthetes by drugs such as hashish and mescaline. All these observations suggest that an explanation of synaesthesia needs to be sought at the level of brain function.

The techniques of functional brain imaging (positron emission tomography and functional magnetic resonance imaging) have been used to observe brain activity associated with a synaesthetic experience. As yet, only one fully-controlled study has been reported, by Frith and Paulesu (1997). Volunteers were scanned who experienced colours when they heard words. In comparison to people who did not have such experiences, extra areas of brain activity were identified in the synaesthetes when they were hearing words. This extra activity occurred in regions of the brain normally activated when naming the colours of objects, e.g. reporting ‘yellow’ in response to ‘banana’. Activity was not detected in regions of the visual cortex concerned with earlier stages of colour processing. This study confirms that the experiences reported by synaesthetes are associated with characteristic patterns of brain activity. The same regions are activated in non-synaesthetes when they are having experiences which are, to some extent, qualitatively similar. We remain, however, far from an understanding of the physiological basis of synaesthesia.

The problem is to explain how activity occurs in brain regions concerned with aspects of one kind of sense when the incoming stimulation derives from some other sense. There are essentially two possibilities (though very speculative), both of which involve some kind of ‘miswiring’ in the brain. The crosstalk theory suggests that information within the processing stream of one sensation crosses to another stream, leading to anomalous sensations. The feedback theory proposes that information from one sensory stream reaches a central region (e.g. a region concerned with object identity) and is then fed back towards the periphery, activating regions concerned with another sensory modality. Both ideas imply that, for a synaesthetic experience to occur, neural connections exist, that are not present, or not activated, in the more usual, non-synaesthetic individual. One suggestion is that the brain of an infant is naturally synaesthetic, such that information from any modality activates all sensory regions; activity in the cerebral cortex simply reflecting the amount of sensation, whatever its source. During maturation, responses to different modalities become localized in distinct brain regions. This differentiation may be achieved by the selective death of nerve cells, which is known to occur during infancy. Synaesthesia lasting into adulthood could be the consequence of partial failure of this mechanism.

Although synaesthesia is a relatively rare phenomenon (estimated to be 1 in 2000), understanding its physiological basis is of considerable importance to neuroscience. If we can understand synaesthesia then we might also understand the general mechanism by which neural activity is translated into conscious sensory experience. The main philosophical interest of synaesthesia, however, is the vivid example that it provides of how subjective our perceptions really are.

Chris Frith

Bibliography

Baron-Cohen, S. and and Harrison, J. E. (1997). Synaesthesia: classic and contemporary readings. Blackwell, Oxford.

synaesthesia (sin-is-theez-iă) n. a condition in which a secondary subjective sensation (often colour) is experienced at the same time as the sensory response normally evoked by the stimulus. For example, the sound of the word ‘cat’ might evoke the colour purple.