Dyslexic Neurons

According to French neuroscientist, Dr. Stanislas Dehaene, it is not surprising that some readers have difficulty reversing letters when they read, the particular neurons we must use for reading are designed to be “dyslexic”. In the natural environment this left/right scrambling of objects is an asset, but to read we humans must re-train a part of the brain that is wired for a different visual purpose in our illiterate evolutionary ancestors. Other primates couldn’t care less if they see a “b” and a “d” as the same thing; in fact, it is important in the natural environment that mirror images of shapes are perceived as identical!


When you think about it, mirror-image “dyslexia” is an amazing synthesis. Why should completely different patterns on our retina (b and d, for example) be perceived as the same object? How is that possible?


As Dr. Dehaene explained in his November 12, 2009 lecture to the public on “Reading in the Brain”, held in Melbourne, Australia, reading is a very new skill in evolutionary terms, which developed only about 5000 years ago. Reading requires rapidly recognizing specific shapes (letters or characters) and associating them with a specific meaning. Recognizing the alphabet utilizes areas of the brain that are designed to recognize specific objects in our natural environment.


When you see a chair you recognize it instantly whether it is facing right or left, even though the shape of the image focused on your retina is entirely different if the chair is oriented differently in space. Whether it is a wooden kitchen chair, an upholstered Lazy-boy, or a molded plastic patio chair, we still recognize these objects instantly and effortlessly as a chair. Imagine programming a computer to identify a chair in any orientation or style in a complex scene among the jumble of all the other objects. This difficult discrimination is far too sophisticated to be accomplished in the retina, or even in the lower visual centers of our brain (the cerebral cortex at the back of our head). Enormous computation and analysis are required to recognize an object no matter how it is oriented or varied in design.


This higher-order visual computation takes place in a specific part of the cerebral cortex. Here individual neurons become specialized to recognize a particular object, no matter where it is in the visual field or how it is oriented. By recording the electrical responses of individual neurons in this part of the brain, scientists can see a single neuron fire in recognition of a cup and nothing else. No matter how you present the cup–handle on the left or right, a coffee mug or a beer stein–this neuron will fire in recognition. A neighboring neuron may be wired to recognize a fork, but be blind to the cup, and so on.


This spot is located in the same place in everyone’s brain, no matter what culture you may have been nurtured in, what language you speak, or whether you read left-to-right or right-to-left. It is located in a small area in the left cortical hemisphere in the temporal region. Damage to this spot of the brain caused by small strokes renders people unable to read a word. Humans have had to “recycle” neurons in this part of the brain, which is shared by monkeys and other primates, to now recognize specific letters of the alphabet.


The problem is that these neurons recognize the mirror image of an object as being identical. This creates a problem in reading, where the same visual circuits are wired to respond equally to mirror images of objects. In learning to read we must un-do the intricate wiring that took years of evolution to enable these neurons to see mirror images of objects as identical. Now this neuron must recognize the letter b, regardless of font, handwritten or typed, capitalized or lower case, yet suppress the natural tendency of these neurons to see the mirror image “d” as identical. This is one reason learning to read takes so long, and it explains why children normally reverse letters in reading and writing until they have reprogrammed these visual circuits to defeat the mirror image recognition that is vital to survival in the “real world.”


After his lecture I asked Dr. Dehaene if people with dyslexia have some compensatory abilities as a result of their weaker capacity to discriminate mirror images. He told me that in fact there is evidence that dyslexics are superior at tasks requiring mirror imagery. For example, dyslexics are over-represented among astronomers, artists, and mathematicians.


For more on this subject see Dr. Dehaene’s new book Reading in the Brainwww.readinginthebrain.com

Readers of this article may also be interested in my article  “Watching the Brain Learn” on the Scientific American website  http://www.scientificamerican.com/article.cfm?id=watching-the-brain-learn


  1. stephen m on May 26, 2010 at 3:08 pm

    Proust And The Squid: The Story And Science Of The Reading Brain by Maryanne Wolf

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