Dyslexia is a specific reading difficulty in a person with adequate vision and at least average skills in academic areas other than reading. Like the terms backache or headache, the term dyslexia refers to symptoms, not causes. The causes are varied and certainly not yet well understood; most investigators attribute dyslexia to minor brain abnormalities, but others attribute it to learned habits of visual perception.
Here is a demonstration of another process that is relevant to dyslexia. Focus your eyes on the centrl dot in each display below and, without moving your eyes back and forth, try to read the letters to the left and right of the dot:
If you are like most people, you found that you had no trouble reading the letters in the first lines, which were close to the fixation point (.). As the spacing increased on successive lines, you found it harder to identify each letter. Depending on many details of procedure, people sometimes show a slightly greater accuracy for letters to the right of the fixation point than for those to the left. The tendency to read better to the right is a function of our long experience in reading from left to right; it is absent in first through third graders and reversed in people who read Hebrew, which is printed right to left.
Now try the following display. The instrcutions are the same as for the previous display, except that, instead of trying to read one letter, you will try to read the middle letter of each three-letter combination:
The task became more difficult becuase of lateral masking, the interference generated by adjacent letters. One can also demonstrate lateral masking by briefly flashing a central fixation point and a letter to its left or right. If the fixation point is literally just a dot (.), it does not interfere with reading the letter. But if the fixation point is another letter, it does interfere.
For most people a letter at the fixation point generates little inteference with nearby letters but major interference with remotely placed letters. For example, the H does not maske the W in this display:
HW
but it does mask the N in this display:
H N
(Remember, we are talking about letters flashed briefly on a screen. This interference is less impressive when you have a chance to stare at letters on a page.)
Now, it turns out that the results I just described apply to most people, but not to all. For a certain minority of people, a letter at the fixation point produces noticeable interference for an immediately adjacent letter (like the W in the HW display), but produces less than the normal amount of interference for a remote letter (like the N in the H N display). People showing this pattern of results from dyslexia! That is, when people with certain kinds of dyslexia focus on one letter, that letter produces lateral masking of the immediately adjacent letters, but not much masking of letters farther to the right. So when such a person focusses on a word, he or she is worse than average at reading that word, but better than average at reading the next word over.
What causes…(Please skip the following 2 paragraphs if you are not interested in the medical terms associated to dyslexia)
According to one extensive review of the literature, dyslexic people are more likely than other people to have a bilaterally symmetrical cerebral cortex, instead of having their planum temporale and certain other areas larger in the left hemisphere than the right. In some dyslexic people, certain language-related areas are actually larger in the right hemisphere than they are in the left. Also, many dyslexic people have small anatomical abnormalities in their brains, especially on the left side and especially in the frontal and temporal cortex. All these results support Geshwind and Galaburda’s theory linking anamalous lateralization of function with a predisposition to dyslexia and other problems.
Going beyond the general issue of lateralization, what can we say about impairments of specifc neuronal or behaviour processes? The parvocellular pathway in the visual system deals with details, especially of stationary objects. The magnocellular pathway deals with overall patterns and moving objects. Many people with dyslexia show indications of a relatively unresponsive magnocellular system. Consequently, they are impaired on detecting overall patterns (such as words) and have trouble with rapidly changing stimuli (such as happens when one mov one’s eyes across a page). People with dyslexia generally show impairments on reading words and sentences, although they are as quick and accurate as anyone else in reading a single isolated letter.