By Daniel Willingham, Ph.D.
Is there a critical period of brain plasticity for literacy? We know that brain development progresses with age. If a child does not learn to read at the right age, has the brain lost its plasticity such that learning to read will be more difficult?
For at least one aspect of brain plasticity, we now have data indicating that the answer is “no.”
That aspect of plasticity that bears on reading is the loss of mirror invariance in visual perception.
Mirror invariance means recognizing a mirror image as the same object. It makes good sense for visual recognition to be set up this way. If I recognize a dog facing to my left, I ought to recognize the same dog facing to my right as the same object.
But mirror invariance is a problem when children are learning to read, because for that task one must NOT treat mirror-reversed objects as identical: b and d must be treated differently. Anyone who has observed children learning to read and write cannot help but notice that they initially make a lot of mirror reversal errors. The errors disappear with practice.
In a recent study Felipe Pegado and colleagues (2013) set out to investigate whether literacy changes mirror invariance not just for letters of the alphabet, but for other visual stimuli as well. Does the process of learning to read actually change this aspect of vision?
The researchers simply showed subjects pairs of stimuli, to which they were to respond “same” or “different” via button press. Mirror images were to be judged “same.”
The experimenters used three different types of stimuli: pictures, letter strings, and false fonts (that is, letter-like stimuli that were not actually letters.
Of most interest, they tested three groups of subjects: illiterate adults (“il” in the graph below), ex-illiterates (i.e, those who learned to read as adults; “ex” in the graph below) and literate adults (that is, those who learned to read at a typical age; “li” in the graph below).
The solid line shows reaction times to mirror reversed images, and the dashed line is reaction time to the non-reversed images. Look first at the data for the illiterate subjects for the pictures, strings, and false fonts: they respond equally quickly for mirror reversed and same stimuli.
But the ex-illiterate (ex) and the literate (li) subjects have trouble with the mirror-reversed images. They have BIG trouble with the letter strings, but they are slower even with the pictures.
What does this result mean?
A straightforward interpretation is that lots of practice with visual stimuli that are not mirror-reversible—that is, learning to read—changes the visual system. The natural state of the visual system is that mirror-reversed objects are treated as equivalent. Literate people can treat them as equivalent, but it takes a little extra time.
I find two aspects of these findings interesting: that that visual system changes in this way and that the “ex-illiterate” subjects show the same phenomenon. Thus (at least for this process) it’s not that case that, once brain development is finished the visual system is no longer open to change.
This visual process is not the only one that is changed by learning to read, but this one at least is not subject to a critical period of development.
Pegado, F., Nakamura, K., Braga, L. W., Ventura, P., Filho, G. N., Pallier, C., Jobert, A., Morais, J., Cohen, L., Kolinsky, R., & Dehaene, S. (2013, June 17). Literacy Breaks Mirror Invariance for Visual Stimuli: A Behavioral Study With Adult Illiterates. Journal of Experimental Psychology: General. doi: 10.1037/a0033198
Reprinted with permission of the author. Original publication date 11/18/13.