Wednesday, May 12, 2010

Reading with recycled neurons (Books - Reading in the Brain by Stanislas Dehaene)

Reading in the Brain by Stanislas Dehaene is a readable account of how the reading brain works, as well as how it doesn't.  Given the fact that printed text is a relatively recent human invention and, given the time scale of evolution, the brain could not have evolved specialized structures for reading per se but rather has adapted structures that evolved for more general visual purposes and applied them to this specialized task which combines seeing and recognizing objects and the reception of the abstract thoughts of another person. The form of letters have little to do with the meaning they ultimately communicate. As a result it has become the work of seven to ten of our earlier years to learn rote the relationship between our culture's symbols representing the units of sound (phonemes).  These are combined into (words) from which we generate continuous phrases and sentences which accomplish the transfer of information and point of view. This is done at a minimum with some coherence, if not also some beauty, and the composer of those same units of meaning doesn't even have to be around to explain himself. It might seem roundabout that printed text has to take a two step journey from sound to meaning rather than going straight to meaning, but this is what allows language to communicate abstract thought:
I suspect that any radical reform whose aim would be to ensure a clear, one-to-one transcription of English speech would be bound to fail, because the role of spelling is not just to provide a faithful transcription of speech sounds. Voltaire was mistaken when he stated, elegantly but erroneously, that "writing is the painting of the voice: the more it bears a resemblance, the better." A written text is not a high-fidelity recording. Its goal is not to reproduce speech as we pronounce it, but rather to code it at a level abstract enough to allow the reader to quickly retrieve its meaning.
The brain accomplishes this remarkable feat, Dehaene tells us (without even being there), via two pathways that operate simultaneously when reading is fluent. One path transfers the letter-string to it sound content (and the motor requirements of our making that sound with our vocal apparatus) prior to its meaning, and the other that goes for the identity of the word first and then the sound. A great number of pages in the book is spent on discussing the fruits of Dehaene and his colleague Laurent Cohen's labors identifying the left hemisphere's visual word-form area, a region of the brain whose purpose seems to be to be the visual analysis of the symbols that make up letters and words irrespective of their superficial differences. That is to say we can read word, WORD, or even WoRd equally easily and can tell the difference between ANGER and RANGE. You can see the visual word-form area in the picture representing the relative activity of regions of Dehaene's reading brain below, its the area right above his ear. There are other areas that accomplish the conversion of printed text into units of sound, still others that agree on the meaning of the assembly given not only its form but its context.
The typical right-handed person's brain has developed most of its key language processing areas in the left hemisphere (left handers are less reliable in this regard). This is true whether the personn reads from left to right or right to left and whether they read an alphabet whose symbols map to units of sound (as in these roman letters you are reading right now) or comprise pictures of whole words (as in logographic alphabets like Chinese). Dehaene, in fact, explores the evolution of different writing systems from pictoral markers in depth as he builds his case for how the human brain evolved the skill of reading, a section of the book I very much enjoyed.

Much of this case centers on the brain's ability to adapt cortex to multiple functions, something he calls neuronal recycling.
We would not be able to read if our visual system did not spontaneously implement operations close to those indispensable for word recognition, and if it were not endowed with a small dose of plasticity that allows it to learn new shapes. During schooling, a part of this system rewires itself into a reasonably good device for invariant letter and word recognition.

According to this view, our cortex is not a blank slate or a wax tablet that faithfully records any cultural invention, however arbitrary. Neither is it an inflexible organ that has somehow, over the course of evolution, dedicated a "module" to reading. A better metaphor would be to liken our visual cortex to a Lego construction set, with which a child can build the standard model shown on the box, but also tinker with a variety of other inventions.

My hypothesis disagrees with the "no constraints" approach so common in the social sciences, according to which the human brain is capable of absorbing any form of culture. The truth is that nature and culture entertain much more intricate relations. Our genome, which is the product of millions of years of evolutionary history, specifies a constrained, if partially modifiable cerebral architecture that imposes severe limits on what we can learn. New cultural inventions can only be acquired insofar as they fit the constraints of our brain architecture.
In the book's final chapter, Dehaene discusses cortical plasticity - a neuroscientific idea that is relatively recent and much in vogue.  It is the ability of brain's neuron's to adapt their function from one purpose to another - for example, when a blind person's visual cortex cells become able to decode sensation of the fingertips to braille letters.  Dehaene makes a case for the necessity of cortical plasticity in inventing cultural forms like number systems and the arts. It's one of those fun bits of reaching for the stars that a researcher has to save for when they write a book rather than a journal article. His voice comes off a little stuffy at times, but his theory is intricate - composed of many interleaving units - so his writing must be systematic in driving home each concept and then attaching is to its predecessor. His model for how the brain accomplishes reading, I must emphasize, is one of several, but he does acknowledge alternate viewpoints along the way. The lay reader may not find this book as accessible as Maryanne Wolf's Proust and the Squid, but it goes into more depth and synthesizes a lot of information into a coherent narrative arc. His diction is clear and the reading experience fluid and even entertaining. Dehaene's work is at the cutting edge of our understanding about the relationship between language and the brain so I found it a pleasure to get the story from one of its sources.

If you in or around NYC on Thursday March 13 at 6pm, join me for a film about dyslexia called THE BIG PICTURE: RETHINKING DYSLEXIA.  Click here for information.

No comments: