Ticker-tape synaesthesia reveals links to dyslexia
New study finds brain areas involved in perceiving speech as ‘subtitled’ bear striking similarity — yet contrasting activity levels — to those involved in difficulty reading and writing.
02 January 2025
By Emma Young
Synaesthesia comes in a wide array of forms; so many, in fact, it's impossible to list them all. Some people perceive colours when they hear music, for example, whereas others perceive every word as having its own unique taste. There are even those with what's known as 'ticker-tape synaesthesia' (TTS) who, when they hear speech, perceive it as subtitled in their minds.
Recent research on 17 people with this particular form of unusual sensory processing has now revealed insights into why it happens. The results suggest that, in some ways, TTS is the opposite of dyslexia, a disorder involving difficulty in reading and writing that affects an estimated one in ten people in the UK.
In an earlier paper, Fabien Hauw at the Sorbonne University and colleagues reported a study of a 69-year-old French engineer with ticker-tape synaesthesia. 'MK' told the team that for as long as he could remember, whenever he has heard someone else or himself speaking, he's simultaneously perceived the words in written form, in black letters, "in his head". MK said he was unable to stop this from happening, and explained that if people nearby were talking, he found it quite tricky to read. This form of synaesthesia, as the team describe it, is effectively the opposite of reading: instead of visual letters being translated into speech inside our heads, with TTS, the opposite happens.
For their new, larger-scale study, they recruited at additional 16 people with TTS, alongside MK, plus matched controls, and expanded their tests. All of the participants had their brains scanned using fMRI while they listened to spoken sentences, words, numbers, or made-up words, and while they viewed images and written words. The team also scanned their brains while they were at rest.
These experiments revealed what the team call the 'TTS network' — the brain areas and the patterns of activity and connectivity that together distinguish the brain of someone with TTS from the brain of someone without it.
Hauw and his colleagues found, for example, that when people with TTS were listening to words, there was an over-activation of language areas in a region called the left perisylvian cortex, and also of the occipitotemporal visual word form area (VWFA). This tiny part of the brain develops as we learn to read, and specializes at recognising letters. The team also found that, compared with the controls, people with TTS showed a larger overlap in speech-related and reading-related brain areas. They propose that TTS happens when there are "exceptionally powerful and automatic top-down influences" from perisylvian areas onto the VWFA.
Even when the people with TTS were not hearing speech, their brain scans revealed some differences compared with the controls. For them, the team observed greater connectivity between two particular regions (the left prefrontal and bilateral occipital regions), which they think reflects a reduced threshold for conscious access to visual representations.
To put it another way, this greater connectivity might mean they consciously experience visual imagery that for most people stays subliminal. The team suspects that this particular difference might make someone predisposed to developing some form of synaesthesia with a visual element.
This work throws new light not only on TTS, but also its relationship with dyslexia. "The regions over-activated in ticker-tape synaesthesia overlap with regions under-activated in dyslexia," they observe in their paper. It's possible, they believe, that TTS and dyslexia both result from atypical development of the reading system in the brain; while with dyslexia there's a reduction in 'bottom-up' activation by written words, with TTS, 'top-down' activation from speech is increased.
The team would now like to see studies involving regular brain scans of children as they learn to read. Research adopting this design could help reveal why and how some children develop TTS, and why others develop dyslexia.
Read the paper in full:
Hauw, F., Béranger, B., Cohen, L. (2024) Subtitled speech: the neural mechanisms of ticker-tape synaesthesia. Brain, 147(7), 2530–2541, https://doi.org/10.1093/brain/awae114
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