Synesthesia is a rare and interesting neurological phenomenon in which one sensory modality crosses over into another. Synesthetes therefore may see sounds, or taste colors. Abstract concepts may also evoke sensory experiences. In color-graphemic synesthesia letters or numbers evoke the perception of a specific color (for each individual the same number will always evoke the same color). In ordinal linguistic personifcation, days of the week or months of the year convey a specific personality. Other forms of synesthesia involve spacial relationships and size. Over 60 forms of synesthesia have been reported.

What is happening inside the brain in synesthetes is currently under study, but early evidence suggests that their brains are hardwired for the experience – literally there is a cross-wiring where one type of sensation or information processing leaks over into another.

A recently identified form a synesthesia is visual motion to sound synesthesia – people can hear visual motion or flickering images. While the overall prevalence of synesthesia is estimated at less than 1%, visual sound synasthesia may be more common. On a personal note, I have experienced this myself on several occasions when I was profoundly sleep deprived. Every time I blinked my eyes or scanned my eyes across different levels of lighting I would hear a distinct “whooshing” sound.

Charles Spence, a professor of experimental psychology, performs crossmodal research at Oxford University. He is looking at the effects of processing multiple sensory modalities simultaneously in the normal population – and he thinks that to a certain degree we are all synasthetes. On his website he writes:

Synaesthesia is a rare condition in which people report, for example, ‘seeing’ a colour when they hear certain words, like the days of the week, or numbers. Everyday language also uses cross-modal correspondences to describe a variety of sensory experiences – tastes can be ‘sharp’ and colours can be ‘loud’, for example. But can such synaesthetic correspondences be demonstrated in normal individuals for simple stimuli, such as brightness, size, colour, motion etc. This project investigates a number of questions in this line.

He explores things like, for instance, what name people think fits an abstract visual image. For example, which of these two shapes do you think is called “bouba” and which is called “kiki?”Most people will say the purple shape is bouba and the orange shape kiki. Why?

He argues that everyday speech is full of such multimodal associations. We might describe a bright color as “loud” or the taste of a food as “sharp.” The question is, do these associations have anything to do with synasthesia?

Even given Spence’s research findings, I am inclined to think that these are discrete phenomena and are not analogous to synesthesia. For example, what names we would give shapes and tastes may be a predilection hard-wired into our language cortex. Words do sometimes sound like what they mean. The sounds of words can sometimes convey emotion or give a clue as to what the word might mean. Is it a pleasant word or a harsh one? Such built in associations would help us understand and remember language – it gives us a feel for a language we would not otherwise have.

Yes, this involves visual information being accessible to our language cortex – which is also necessary for reading. The different parts of the brain do communicate with each other. I think, however, we should no conflate all such processing with synesthesia, otherwise the concept of synesthesia will lose its specific meaning.

I like the term multimodal processing better. It suggests a higher order processing that brings together multiple modalities of thought or sensation and combines them into a unified experience, or makes associations among them. Synesthesia, on the other hand, is more of a direct involuntary connection between primary sensory or processing modalities, causing one to actually see a color in a number, or hear a sound in response to visual stimuli.

In other words, the evidence (and everyday experience) does support the notion that we all engage in multimodal processing. Charles Spence’s research is exploring how this type of processing is used to enable us to navigate our complex sensory world. For example, he talks about putting a voice and a face together in a crowded and noisy room. Mere temporal and spacial coordination is not enough, there needs to be a multimodal connection linking the sound of a person’s voice with their image. This is a plausible hypothesis for the advantage of such multimodal processing.

But this is not the same thing as synesthesia. At least I don’t think so, but I may be wrong. Research, including that of Charles Spence, will likely shed light on this questions. Spence thinks that normal multimodal thinking is a tiny bit of synasthesia, and synesthesia is an exagerated version of this more typical processes. This may turn out to be correct, and may explain why synesthesia occurs at all. Many neurological phenomena are simply exaggerated or atypical versions of more typical hardwiring.

It is therefore a common question in neurology – is a specific neurological syndrome an entirely new type of phenomenon or is it an exaggerated version of an existing typical phenomenon? An anxiety disorder may simply represent adaptive anxiety unhinged. Or is it a pathological state – wiring that does not normally exist, or a mutation in the gene for a neurotransmitter?

What about the fact that I will display one type of synesthesia only when sleep deprived. This suggests the potential is always there, and only manifests when my brain is stressed. Is this typical, or am I in the 1%? Maybe the 1% figure would be much higher if more people were assessed for sleep-deprived manifestations.

At present it is an open question whether we are all mild synesthetes or if synesthesia is truly a discrete variant of hardwiring. I don’t think Spence’s research has yet settled this question, although it is very interesting – in a very lavendar way.