Stimulus-parity synesthesia

Stimulus-parity synesthesia involves perceiving a wide range of concepts as being either odd or even. The term was coined in 2014 in a study by R.C. White and A. Plassardof Oxford University, who noted that the phenomenon had already been observed as long ago as 1893 in the works of French psychologist and researcher Théodore Flournoy.

The stimuli or inducers are a long list of concepts: letters, words, colours, shapes, weekdays, people, faces, school subjects, foods and even just all things in general.

(Note: “All things in general” is not usually considered a synesthetic inducer, although it seems to be a characteristic of stimulus-parity synesthesia, often being mentioned.)

It does not appear to be very common, but there have been very few studies on it so far so it is possible that many more synesthetes have it without having realised this fact. It seems to be much less frequent in response to sensory stimuli like smells, tastes or sounds than with conceptual inducers like sequences and other learned categories (letters, time units, etc.). It can therefore clearly be categorised as a conceptual type of synesthesia.

Curiously, for some people with stimulus-parity, numbers are perceived as being odd or even… but do not always agree with the parity they are conventionally assigned (1=odd, 2=even etc.), instead following the synesthete’s own idiosyncratic system, which is stronger in their mind than the original. The words that designate the numbers can also be odd or even, meaning they might conflict with the numbers themselves (17 is odd but the word seventeen could be even, for example).

The parity associations are highly consistent and appear to be so throughout the synesthete’s entire lifetime.

Here are some accounts of stimulus-parity synesthesia:

“It was hard to work out the reason behind this, it’s just the way things are, but thinking about it it’s become clear to me that even things tend to be regular, stable and dry while odd things have a flow to them. Most objects have parity for me. Colours do too. Red and blue are clearly even, while yellow, green and brown are odd. I remember often saying when I was little that you couldn’t colour a drawing without using yellow because the picture would be thirsty. It’s the distinction between what’s dry and what’s liquid. Odd flows like water.”

(Pau 365, my own experience)

“R says that odd things give a dark feeling, whereas even things give a warm and light feeling, and M says that odd things are intricate, whereas even things are full and wellrounded.”
(White and Plassardin their 2014 study.)

“Even letters: a, c, d, e, h, i, l, m, o, p, r, s, u, w, C, D, E, H, I, K, M, O, P, S, U, Z. 

Odd letters: b, f, g, j, k, n, q, t, v, x, y, z, A, B, F, G, J, L, N, Q, R, T, V, W, X, Y.”

(In this post on the online debate platform Reddit/Synesthesia. 2019.) 

“A subject who displays many phenomena of this kind used to see all the faces he encountered as odd or even, according to the length of the nose, etc., and still finds that lettuce and rhubarb are odd, rice and pasta even, and so on.”

(Théodore Flournoy, in his book Des phénomènes de synopsie (1893), p223. Quoted by A. Plassard and R.C. White in this study in 2017. Original in French.)

Are there variants of this type of synesthesia? Duality synesthesia

The Oxford University researchers in the study on stimulus-parity synesthesia also observed a similar phenomenon where a synesthete used the identifiers “positive and negative” rather than “odd and even”.

Others have said that they classify concepts by weight: they are either heavy or light. Another manifestation is hot or cold, and there are many more. This phenomenon could be called Duality synesthesia.

Go to the page on duality synesthesia

Gender is also assigned by some to concepts such as letters, numbers or colours, which are considered masculine or feminine.

Go to the page on gender as a synesthetic concurrent

The gender perception variant could be related to both duality and the personification types of synesthesia.

Go to the page on personification synesthesia

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