What are you doing right now? I’m no psychic, but I can say for certain one thing that you’re doing: reading. You’re reading this sentence, word by word, and extracting meaning from little black lines of orthography, a fancy term for the rules of written language. If you really think about it, what you’re doing right now is quite difficult. What are the neural processes that enable us to read?
Reading is a cultural invention and uniquely human cognitive process. Around 3500 B.C., our ancestors began using symbols to convey meaningful sounds. While 3500 B.C. might seem like eons ago, this date actually isn’t that far back if we consider the amount of time needed for a cultural invention like reading to exert pressure on biology. So, unlike the basic sensory systems of vision and hearing—which are not cultural inventions—reading hasn’t been around long enough in the evolutionary timeline to demand a functionally specialized brain region. Yes, reading got left out of evolution because the brain learned how to do it a little too late. Dissimilar to other learned skills such as driving or juggling, reading and literacy are tools that are almost necessary to success for the overwhelming population of humankind. Surely, from brain to brain, a scientist might expect to see that reading makes consistent use of the same brain regions across the population. But which circuits? And why? These became important questions for scientists: What is the mechanism that allows culturally invented skills like reading to become acquired by the brain?
The most popular hypothesis to address this question is neuronal recycling. This phrase (unlike many things in science) is quite straightforward. Proposed by researchers at the College de France specifically to tackle the issue of reading, neuronal recycling theorizes that existing neurons have been reused to perform processes necessary for reading. To help illustrate this, let’s return to thinking about what you’re doing right now. As you are reading, you are performing three very specific processes: 1) seeing a word, 2) determining the meaning of a word, and 3) sounding out and speaking a word.
Luckily for reading, there are already regions in the brain dedicated to these specific processes. Importantly, these are processes that are not unique to reading, but rather, are necessary during other tasks that require similar neuronal computations. First, the visual word form area (VWFA) is situated in temporal-occipital cortex, a region where neurons are able to compute low-level visual stimuli, like the lines that make up the letter “g”. Evolutionarily, the VWFA is believed to have evolved to help identify low level, naturally occurring shapes such as the horizon in the environment. The VWFA thus helps identify the shapes that form letters. Evidence that this region is important for identifying words in literate individuals comes from lesion studies— these show that damage to the VWFA causes difficulties with reading but not with the processing of more general shapes or visual stimuli. Second, two areas in the temporo-parietal cortex – the supramarginal gyrus (SMA) and angular gyrus – are involved in the perception and interpretation of language. They are tools for determining what a word means. Reading thus makes use of these gyri in temporo-parietal cortex to build the link between a meaning and a word form.(For example, this part of the brain contains neurons that help you read this sentence and link the word “neuron” to “brain cell”!) Third, your brain sends now-meaningful word forms to the inferior frontal gyrus (IFG). The IFG is a well-known language hub of the brain and is critical for motor control of language. Here, the IFG performs the final processes required to analyze a word’s sound components and articulate them out loud. During the tiny amount of time it took you to read the word “neuron,” these three parts of your brain worked way harder than you might imagine.
The three regions we just described—1) VWFA, 2) temporo-parietal gyri, and 3) IFG—comprise the canonical “reading network,” which is essentially a hijacked set of neurons. These parts of the brain don’t come reading-ready. Remember: According to “neuronal recycling,” in order to acquire literacy, reading behavior teaches the brain to make use of the unique processes that these regions of the brain were born to perform. Functional imaging studies support this idea. Researchers from Georgetown University showed that during development, learning how to read hijacks these systems. Hijacks?, you read, That sounds a little mean. Well, in a way, it is. The reading network is more or less dormant before we learn to read but is then forced to become active as reading skills are acquired. Patterns of brain activity increase from childhood to adulthood in the reading network. Importantly, brain activity in these regions is linked to phonological abilities, a key component of reading. Phonological abilities are how you know that the “i” in “ski” is pronounced differently than the “i” in “glide.” The fact that these abilities are reflected in brain activity means that the better you get at reading, the more active your reading network becomes during reading tasks.
What does the reading network look like in people who never learn how to read? We know from a landmark study that illiterate individuals have less gray matter (think: number of neurons) in multiple regions, including part of the reading network within the temporo-partietal gyri, where meaning is mapped onto form. Additionally, the study showed that illiterate individuals also have less white matter (think: the brain’s wiring) between the left and right hemispheres, which might be due to a decreased need to process reading in the left hemisphere.
People with reading disabilities, such as dyslexia, can also take advantage of the way reading hijacks the brain. An intervention study provided evidence that children with dyslexia can increase their gray matter both within and outside of the reading network by practicing intensive reading skills for a minimum of eight weeks. Interestingly, part of the gray matter increases observed in dyslexic readers during the intervention were within the hippocampus, a region of the brain crucial for learning and memory. These findings in reading disability add to our scientific understanding of reading as a learned skill that changes the brain in unique ways.
So what’s next for reading research? Scientists have begun testing novel ideas relevant to this cognitive process. For example, while we know that factors such as socioeconomic status and intellectual abilities impact reading abilities, we don’t know how these factors contribute to reading’s relationship to the brain. Another exciting new field of science is concerned with dyscalculia, a math-specific disorder that affects many individuals with dyslexia. Some scientists are beginning to test whether math and reading share the same networks. Do dyscalculia and dyslexia both hijack the brain in a special manner during development? Finally, and perhaps one of the biggest mysteries of reading, we still don’t know whether dyslexia is a cause or consequence of abnormalities in the brain’s reading network.
Does your brain feel hijacked yet? Probably not. Years of learning to read have made this neural process feel innate, though it’s important to remember: reading is a learned skill that forever benefits from practice. While your reading network may be fully formed, there is no harm in acquiring new words and new knowledge. To keep reading, click here for more content on the brain!
Written by Gabrielle Torre
Illustrated by Jooyeun Lee
Translated by Adriana Pérez
Carrieras, M., Seghier, M.L., Baquero, S., Estevez, A., Lozano, A., Devlin, J.T., & Price, C.J. (2009). An anatomical signature for literacy. Nature.
Dehaene, S., & Cohen, L. (2007). Cultural Recycling of Cortical Maps. Neuron.
Krafnick, A.J., Flowers, D.L., Napoliello, E.M., & Eden, G.F. (2011). Gray matter volume changes following reading intervention in dyslexic children. Neuroimage.
Turkeltaub, P.E., Gareau, L., Flowers, D.L., Zeffiro, T.A., & Eden, G.E. (2003). Development of neural mechanisms for reading. Nature Neuroscience.