How the brain learns to read is an evolutionary mystery—a new study offers clues

Reading hasn't been around for long, evolutionary speaking. Instead, the brain recruits systems evolved for other purposes

For an experienced reader, recognising the words on this page feels effortless. Yet, evolutionarily speaking, your brain shouldn’t be able to read. Reading has existed for too short a time to have shaped a dedicated centre in the brain.

Therefore, the brain has to learn to read using systems that evolved for other purposes. But it has long been unclear exactly which systems it recruits to do so and how.

This is important, as differences in these pre-existing systems could shed light on why some of us find reading easier than others. It could also explain how people who are deaf, blind or have dyslexia learn to read in remarkably different ways. In my new paper, published in Cerebral Cortex, I have come up with some answers.

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In my study, I investigated the relationship between individuals’ reading skills and their fine-grained brain structure. I also compared twins with different degrees of genetic overlap to see whether shared genetic factors contribute to the links between brain structure and reading skill.

These patterns offer clues about which parts of the distributed “reading network” might provide the initial conditions for reading, and which might respond more to experience.

I used high-resolution brain scans from the Human Connectome Project, a large study of healthy young adults. The participants did not read while their brains were being scanned. Instead, they completed a range of language tests separately. The most important of these tests for my study was an adaptive reading test in which participants pronounced written words aloud. I used their scores on this test as a measure of reading ability.

I discovered that several brain areas contribute to the skills needed for reading, and that some show heritable links with it. To be clear, heritability does not mean that we are born a certain way and cannot change. The brain is, in fact, constantly changing. Rather, we begin with a partly inherited lump of clay that we continuously shape and refine throughout our lives.

Let’s begin with auditory abilities. When we learn how to read in school, we typically begin with identifying sounds and putting them together in our minds. In fact, problems with perceiving the sound structure of language are commonly cited as an underlying factor in developmental dyslexia.

To read, children learn to map each letter to a sound and join strings of letters into words: the letter s to the “sss” sound, i to the short “i”, and t to the “t” sound, so that s-i-t is pronounced “sit”. Breaking spoken words into individual speech sounds starts at the syllable level, dividing sitting into sit|ting. This requires the skill to identify slowly developing changes in loudness. Most syllables start quiet with a consonant like s-, then grow louder at the vowel (-i-), with its singing character, before going down in intensity again (-t).

Before we’ve learnt how to read properly, the brain can use auditory cues like these to detect syllables and their internal structure in spoken language and begin its road to literacy.

Particularly suited for this task is a region next to the primary auditory cortex at the top of the left temporal lobe, which sits underneath the temple and upper ear. It is called the medial belt area and is tuned to slow, coarse changes in sound. Strikingly, I found that a well-organized structure of its nerve cells is linked with reading ability.

The relationship had a genetic component, suggesting that the organization of this perceptual system might affect how easy we find it to learn to read. Such differences could contribute to variation in literacy that does not simply separate people with and without dyslexia, but span the whole population.

Source: Read the original article on www.scientificamerican.com