Numbers as a synesthetic concurrent

In the past, sequences of cultural symbols such as numbers, letters or time units used to be considered a valid inducer of synesthesia but not a synesthetic concurrent (except in the case of types like spatial sequences or ticker tape). This meant that expressions such as number-colour or number-sound would be seen as types of synesthesia, while colour-number or sound-number would not. However, in relatively recent times researchers have acknowledged the possibility of numbers being a synesthetic concurrent, normally as a manifestation of bidirectional synesthesia.

What form do these types of synesthesia take?

1. Bidirectional synesthesia with numbers as a concurrent

In the vast majority of cases synesthesia is a one-way process, meaning that a synesthete who perceives colours on listening to music would not normally also hear musical sounds when looking at coloured surfaces. But some synesthetes do in fact have two-way perceptions, so for instance each number makes them perceive a particular colour but they also have an automatic, consistent mental representation of that number or amount (magnitude) when they see or think about each colour (“6 is red… and red is 6”).

Sean Day explains this phenomenon in his book Synesthetes (2016):

“Whereas ‘colored letters and numbers’ synesthesia has been long known, with hundreds of studies done on different letter and number sets, there emerged an idea that this type of synesthesia must, by default, be one-way(1) – that is, seeing a letter or number might synesthetically evoke a color, but seeing a color wouldn’t evoke a letter or number. We now know that such is not the case: two-way synesthesia, while extremely rare, does exist(2), and there are rare cases of synesthesia in which, indeed, seeing a specific color will synesthetically evoke a number or letter.”

(1) See, for example, Mills et al. 1999.
(2) See, for example, Brugger et al. 2004; Cohen-Kadosh et al. 2005; Gebuis et al. 2009.

Here are some scientific studies that have addressed the topic of bidirectional synesthesia. All of them mention the case of numbers:

Numbers, Synesthesia, and Directionality by Roi Cohen Kadosh and Avishai Henik, in the Oxford Handbook of Synesthesia, 2013. (The link is to Google Books. The chapter starts on page 103 and the part about bidirectionality on page 111).

Priming Letters by Colors: Evidence for the Bidirectionality of Grapheme–Color Synesthesia, Peter Horst Weiss et al, 2008

Effects of synaesthetic colour and space on cognition. Clare Jonas, University of Sussex, 2010. (Papers 4 and 5, p.146.)

2. Unidirectional synesthesia with numbers as a concurrent

In this case, an inducer – a colour, a geometric shape, a musical note, a sound or a taste – evokes a number or magnitude, but these numbers or magnitudes do not elicit any perception of their corresponding colour, shape, sound or taste.

“Stimulus-parity” synesthesia, in which many different concepts create the strong, involuntary sensation that they are either odd or even, could perhaps be considered a very basic variety of this type of synesthesia, as each concept has a binary value somehow related to the concept of number or amount.

Go to the page on stimulus-parity synesthesia

Here are some accounts by people who experience numerical perceptions in response to different inducers:

With colours

“[I]f I see bright grass green, I get a sense of 3; a stop sign is 4.5; butterscotch yellow prompts 2.56.”

(Source: Philly Markowitz, quoted in the book Synesthetes, 2016, by Sean Day, p.21)

With people's personalities

“She emanated a spherical field of crimson 2s with magenta highlights, assertive and feminine, lined underneath with shy violet 3s.”

(Source: Joel Salinas, talking about meeting Rosie Doherty, in his book Mirror Touch, 2017, p.222.)

With geometric shapes 

“For years now, I have associated numbers with shapes. For example, circle is one to me. Sometimes, if I'm trying to say the shape, I'll accidentally say the number instead, which sounds very weird.”

(Source: This post on Reddit/Synesthesia. 2020.) 

“When I'm playing on a console and I have to press square or triangle, I always think of them as five and six respectively in my head.”

(Source: This comment on Reddit/Synesthesia. 2020.) 

With musical sounds (notes and musical genres/sequences)

“The notes in music each have a number. So when I play a g-chord on piano, it isn’t G-B-D-G. It’s 0-4-7-12.”

However, they offer a possible explanation of their perceptions:

“I think it is in part my way of memorizing chords and notes. I can’t read music, so I’ve always remembered the notes by what number they’d be based on the key of the song (i.e. if the song is in the key of G, G is 1, G sharp is 2 and so forth).”

(Source: This post on Reddit/Synesthesia. 2016.) 

"It's not for all types of music but only certain genres. Classical, progressive, and a lot of instrumental music is what's most common for me genre wise (although on a rare occasion I can get a song that isn't of those genres.)

Basically I'll listen to a song and then, on what I can only describe as some sort of "white board" in my brain, I will consciously see a sequence of numbers for each part of a that song.

At first I thought it was like trying to figure out a math problem (which freaked me out because my entire life I have never been good at math) but after having it occur more frequently I realized it was just a string of numbers and each number was associated with a section of the song.

Sometimes the sections are small, sometimes they're large and more like an entire part of a song, and sometimes they're single notes of a song.

The only numbers I can associate with music are the numbers 1-9 and haven't come across any other numbers than those.

When this happens it's not in any way a sensory overload, it's more just sort of there and not a bother in any way. In fact I've found that when this happens for those certain songs they turn out to be my favorite (it's very relaxing to focus on both the music and its associated numbers.)”

(Source: This post on Reddit/Synethesia. 2016.) 

Go to the page on stimulus-parity synesthesia (concepts are odd or even)

Go to the page on number-form synesthesia (numbers have a spatial location)

Post first published: 28 February 2022