Synaesthesia and anomalous perception

What it is

A condition in which stimulating one sense or concept automatically and consistently produces an experience in another.

The commoner forms:

• Grapheme-colour — letters and numbers have colours. The commonest studied variety.
• Chromesthesia — sounds have colours.
• Ordinal-linguistic personification — numbers, letters, days and months have personalities and genders. Tuesday is brisk and slightly unkind; seven is a serious young man.
• Spatial sequence — the months of the year occupy a definite shape in the space around the body.
• Mirror-touch — seeing someone touched produces a felt touch on one's own body.
• Lexical-gustatory — words have tastes.

Estimates run to about four per cent of the population for at least one form. It runs in families, is more often reported by women, and synaesthetes are typically surprised to learn that other people do not experience the world this way.

Where it came from

Francis Galton described it in 1880 and it was studied seriously for a generation. Then behaviourism arrived, introspective reports became scientifically unrespectable, and synaesthesia disappeared from the literature for most of the twentieth century — dismissed as metaphor, as attention-seeking, or as drug-related.

It came back because someone found a way to test it.

The test is consistency. Ask a synaesthete which colour the letter J is, wait months or years without warning, and ask again — the answer matches at rates far above what non-synaesthetes achieve even when told in advance they will be retested. Later work added a Stroop-type measure: a synaesthete shown a letter printed in the "wrong" colour is measurably slower to name it, which is not something anyone can fake.

Neuroimaging then found the correlates. In grapheme-colour synaesthesia, colour-processing areas including V4 activate on seeing black letters. Structural differences in connectivity have been reported. It is now ordinary cognitive neuroscience, with journals, conferences and a large literature.

How it is used

Not much, deliberately — it is not a practice. It appears here because of what it demonstrates.

What we can and cannot say

We can say it is real and was wrongly dismissed for a century. That is the point of this article. A large body of first-person reports about inner experience, consistent across cultures and centuries, was rejected by science because it could not be measured and because the reports sounded fanciful. Then someone designed the right test, and the reports turned out to be accurate descriptions of something that was actually happening.

That is a caution that runs in the sceptical direction, and it belongs on this site as firmly as the cautions running the other way. It does not license believing anything; it licenses distinguishing not yet tested from tested and failed. Most of the claims elsewhere in this section are in the second category. Synaesthesia was in the first.

We can say it bears on the correspondence traditions without validating them. The idea that letters have colours and numbers have qualities is fundamental to numerology, to the Hebrew letter mysticism of Kabbalah, and to the colour-and-planet correspondences of Western occultism. Synaesthesia demonstrates that some people genuinely perceive such correspondences — which explains where the conviction comes from, and does not make the systems true. The crucial finding is that synaesthetic associations are idiosyncratic: two synaesthetes rarely agree on what colour A is. The experience is real; the correspondences are personal, not cosmic. That distinction is doing a great deal of work.

We can say it is relevant to the hard problem. Synaesthesia is a case where the same physical input produces reliably different conscious experience in different people, with identifiable neural differences — a natural experiment on the relationship between mechanism and quality, and one of the more useful handles anyone has on the hard problem of consciousness.

We can say related anomalies of perception are equally real and equally ordinary: aphantasia, the absence of voluntary visual imagery, described only in 2015 and present in perhaps two to five per cent of people; hyperphantasia at the other end; misophonia; tetrachromacy. Human perceptual experience varies far more between people than most people assume, and much of that variation was invisible until someone thought to ask.

We cannot say synaesthesia supports any metaphysical claim. What it supports is a methodological one: that a report of inner experience which cannot currently be verified is not thereby false, and the correct response is to find a test.

Further reading

• Richard Cytowic and David Eagleman, Wednesday Is Indigo Blue (2009).
• Julia Simner's work on prevalence and on childhood development of synaesthesia.
• Adam Zeman's papers on aphantasia, for the parallel case.