Owen Gildersleeve
I used to be stupid. It was a long time ago and I was very young – too young to question teachers who said I was a bit dim. Besides, wasn’t there proof? I had been slow to learn to read, my spelling was atrocious and my aptitude for climbing trees wasn’t fooling anyone. So, I believed them. Then, aged 7, everything changed. For the first time, I had a teacher who saw beyond my literary ineptitude. She told me I was smart – and, suddenly, I was. Thank you, Mrs Kennedy!
That was more than 50 years ago. Back then, dyslexia already had a long history, but news of it hadn’t reached the classroom – at least none I found myself in. Children like me usually weren’t given a diagnosis or special help. Far from it. At the convent school I attended next, spelling tests were a ritual humiliation, where the nuns lined us up against a wall in the order of our results and shamed those who failed to perform.
Things have changed since then. Teachers are fully aware of dyslexia. Children with this, the most common of learning disabilities, are no longer labelled “stupid” or “lazy”. Huge volumes of research have been published. Yet, fierce disagreements have also raged about what causes dyslexia and how best to diagnose and treat it.
Now, finally, we are pinning down what makes dyslexic brains different. We may even have found the key neural pathway that drives the condition. Better yet, this research is pointing to novel, practical and fairer ways to help all children who struggle to read.
What if the idea of the autism spectrum is completely wrong?
Humans invented writing around 5000 years ago – far too recently to have evolved an adaptation for deciphering the written word. Besides, for most of our history, the vast majority of humanity has been illiterate. So, when we learn to read, our brains must co-opt areas specialised for other functions. That this isn’t straightforward for everyone has long been recognised. The word dyslexia (from the Greek for “disordered words”) was coined in 1883 to describe a curious condition in which experiencing a stroke had left a person unable to read or spell. But, within a few years, it was being used for a far larger group – children who find it hard to acquire these skills.
The Epic of Gilgamesh, one of the oldest surviving literary works, is over 4000 years old. But in evolutionary terms, writing is a very recent inventionAlbum/Alamy
Developmental dyslexia is often described as a significant and persistent difficulty with reading and spelling. Traditionally, the definition has also included the idea that these difficulties are unexpected given the child’s other cognitive abilities – although the recently updated definition issued by the International Dyslexia Association has finally dropped this controversial “discrepancy” idea. We know that both genes and environment play a role, that dyslexia is a spectrum and that it can manifest in different ways, such as spelling difficulties (a particular problem in alphabetic language systems), slow reading and writing speeds, or both.
All this makes its prevalence hard to quantify. Most estimates put it at around 10 per cent of people globally; some say twice that. What’s more certain is that dyslexia is a universal human condition found in all languages and writing systems.
Reading is cognitively complicated. It entails three key components: understanding how sounds relate to squiggles on a page (phonology); mastering the patterns of written words and the rules of spelling (orthography); and making sense of what you see (semantics). To do this, your brain recruits networks evolved to process vision and speech and links them into a new “reading circuit”. This is located primarily in the brain’s left hemisphere and also comprises three main components. The occipitotemporal region, located low in the back of the brain, instantly recognises those squiggles as words, hence its common name: the “visual word form area”. Word shapes are matched to sounds in the temporoparietal junction, above and behind your left ear. And further forward, the inferior frontal gyrus (Broca’s area) handles speech production and meaning.
A widespread network of brain pathways is active during reading, reflecting the cognitive complexity involvedBrian A. Wandell; Rosemary K. Le
The underlying circuitry is the same, regardless of the language or script – even for writing systems that don’t look anything alike, such as those for English and Chinese.
Clearly, there is a lot that can go wrong, not just in the reading circuit, but in other aspects of reading such as attention, memory, processing speed, logical thinking and knowledge of vocabulary. So perhaps it shouldn’t surprise us that, over the years, researchers have come up with a wide range of ideas to explain the condition.
One of the most prominent is the phonological deficit theory, which asserts that the problem lies in poor awareness of speech sounds and difficulty relating these to written symbols. “This is now considered to be a primary deficit,” says Maggie Snowling, a psychologist at the University of Oxford who has long been a proponent of the idea. Phonological deficit and a second problem known as rapid automatised naming (RAN) – slowness in identifying letters, numbers and even pictures – constitute the “double-deficit hypothesis”.
A long-overlooked writing system from 5000 years ago is still largely undeciphered, but could mark the moment humans first represented their speech with written words
Then there are explanations that focus on the brain itself. Temporal processing theory, for instance, points to an underlying problem of timing in the way the auditory system processes sounds, which has profound knock-on effects for reading and writing. The magnocellular hypothesis, in contrast, places an emphasis on visual attention. It stems from findings that magnocellular neurons, which process fast-moving visual information, are less active and less numerous in people with dyslexia.
In short, decades of research have generated plenty of ideas, but not much consensus. A major roadblock has been that studies were small and not easily compared. This is now being addressed thanks to the availability of large-scale longitudinal datasets and improved methods. That may not sound dramatic, but it has been a game changer.
Take genetics. Clearly, dyslexia runs in families: I have my father to thank for mine. “The heritability you see in twin studies is about 70 per cent,” says Michelle Luciano, who studies behavioural genetics at the University of Edinburgh in the UK. That’s massive – but comes with a big caveat. “It’s not just the genes; it’s the gene-environment correlation that’s really important,” says Snowling. The idea is that factors such as low birth weight and whether literacy is encouraged at home play a key part in shaping the chances of developing dyslexia in those with a genetic susceptibility.
There is also no single dyslexia gene – many tiny genetic changes interact to generate the condition in all its guises. And tracking them down is fiendishly difficult. Traditional methods of genetic analysis identified several “candidate” genes that people with dyslexia appear to share. Now, geneticists can scan full genomes looking for variants linked with complex traits – and do this at scale. The first such genome-wide association study (GWAS) on dyslexia was published in 2022.
Using information from the 23andMe genetic database, a team led by Luciano analysed the genomes of 51,800 people with a dyslexia diagnosis and over 1 million controls. This revealed something surprising about those previously identified candidate genes. “A lot of these actually haven’t turned out to be that important,” says Luciano. The analysis did, however, identify 42 genetic variants linked with a risk of dyslexia. “Many of these genes had already been reported as being associated with either general cognitive ability or educational attainment,” she says. “But we also found several novel ones that may be more specific to language and reading.”
We’re finally pinning down the mechanisms that drive obsessive-compulsive disorder, revealing a complex combination of imbalanced brain networks, the immune system and even gut microbes
The study highlights the genetic complexity of dyslexia. It also confirms observed links with some neurodevelopmental conditions such as ADHD – but not others. “For autism, for instance, we found no genetic overlap,” says Luciano. She and her colleagues have since worked on an even larger dataset of more than 1.2 million people, identifying a further 13 genes related to dyslexia that implicate early brain developmental processes in the biological underpinning of reading. “Genetic studies give us a possibility to bridge the gap between cognitive theories and neuroscience,” she says.
In fact, these genetic findings have already provided a springboard for a giant leap in revealing the neuroanatomy of dyslexia. A team led by Clyde Francks at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands, used them to calculate the genetic predisposition to dyslexia of 35,231 people in the UK Biobank database. They then identified brain regions that differed in people whose genes put them at risk of dyslexia. (I must now declare an interest: I have been in the UK Biobank project from the start, done all the tests and had my genome sequenced and my brain scanned, so one of those 35,231 people is me.)
Francks and his colleagues found that the genetic risk of dyslexia was associated with widespread reduction in brain volume compared with people not at genetic risk, especially in language-related and motor areas. (Yikes!) Again, there were correlations with ADHD and other neurodevelopmental conditions – although reductions in the motor cortex were specifically linked with dyslexia. That might seem strange, but motor control could affect reading in a variety of ways, says Francks. “For example, infants with better motor skills might be able to learn language-related skills more quickly through better coordination when producing speech sounds, pointing to track letters and words or keeping time during chants and songs.”
Meanwhile, earlier this year, another impressive large-scale study finally nailed a long-standing debate in the field over the contribution of visual difficulties, not just phonological ones, to reading problems. “We and others have long held the view that dyslexia is quite heterogenous, and phonological problems are only one part of the story,” says Jason Yeatman, who specialises in the neurobiology of reading at Stanford University.
Around one in ten children have dyslexia, and early intervention is crucial to help struggling readersGuerilla/Getty Images
He and his colleagues carried out tests on 400 children in kindergarten and first grade (typically aged 5-7) at California schools, measuring how quickly they processed visual information such as identifying the direction of dots moving on a computer screen. This revealed that around 15 per cent of the children had problems with visual-spatial attention, and these were associated with reading difficulties a year later. “These kids aren’t picked up by any of the conventional screening processes,” says Yeatman. What’s more, the research identified a hidden subgroup whose weak visual processing was masked by strong phonological skills.