I must admit that my only experience with dyslexia is through family members. My youngest daughter has just started secondary school and she is struggling with all the reading related to three languages, a problem that continues in all other subjects. On tests, she scores high on reading comprehension, but very low on reading speed. My oldest son reads and reads and reads… but can’t write a shit. Both have difficulty internalizing spelling rules and multiplication tables.
These are all standard symptoms of dyslexia. Dyslexia is a spectrum disorder, a condition that encompasses many aspects of reading and writing, so when people start touting single causes my skepticism goes into overdrive. But it turns out that new research into the causes is pretty solid, and it raises some interesting questions.
Your brain in the mirror
When the brain creates an image, it has a problem. The two eyes report two images that are extremely similar, but offset from each other. The displacement is great, as it gives us better depth perception. However, in the absence of a large amount of alcohol, the brain still has to decide on a single coherent image so that it has something to offer to our consciousness. To do that, the two images are merged into one, which is fine for relocation. But for mirror images, the brain has to choose a single image.
One idea for how this is handled is that during the first eight years of development, one eye becomes dominant and determines the dexterity of images.
This has led to the idea that the lack of dominance may cause perceptual problems in reading, which seems to be an obvious line of research. The problem, according to the paper, is that the idea of dominance is not fully accepted and tests for dominance are not as reliable as we would like. The underlying problem is that the tests rely on spatial perception, so the reported dominance may change depending on how the test is set up.
This is where the main contribution of the researchers comes into play. In their test, you stare at a bright light in a dark room for a while, then close your eyes and cover them with your hands. The image of the light seems to be burned in and the afterimage persists for quite some time (six seconds or so). The researchers then have the subjects remove one hand or the other. This allows light to leak through the eyelid, which stimulates the neurons to keep reporting the image.
If one eye is dominant and light leaks through the eyelid of the dominant eye, the afterimage lingers much longer compared to when light only leaks through the eyelid of the non-dominant eye. The test does not rely on spatial perception at all and is still relatively easy to perform.
When tested in subjects who did not have dyslexia, the standard tests and the new test for dominance were virtually the same. That is, all subjects seemed to have a dominant eye, and all tests agreed on which eye was dominant. There were two exceptions. In both, the standard tests failed to identify a dominant eye, while the new test found one.
Unfortunately, this just tells us that the tests agree; they can still be consistently wrong. However, tests on dyslexic subjects showed that the new test actually reported something different. For these subjects, the standard dominance test was quite random in choosing a dominant eye or not finding one. The afterimage test showed that almost all dyslexic subjects did not have a dominant eye. That’s a pretty exciting correlation.
What’s in your eye
What came next was even cooler. The researchers imaged the subjects’ retinas. Humans have three color sensors in the eye: red, green, and blue. But the blue sensors are not in the crowded center of the retina. The researchers took advantage of the lack of blue light sensors in the center to map the shape of the center spot.
As it turns out, it’s not really just one place. For the dominant eye, the spot is nearly round; in the non-dominant eye it is very elliptical. This establishes a link between eye dominance and the physical layout of the eye, suggesting that the brain uses this physical difference as a signal for image processing.
If the brain really experiences a perceptual conflict while reading, then this should be able to be tested. The researchers performed a reading test with their test subjects. They took advantage of the fact that even though the brain presents us with one image, there is a delay between the first eye forming an image and the second one doing so. So if you’re in a dark room and a page of letters is illuminated by a flashing light – the flashes of light are too fast to see – only one eye at a time can form an image. In this case, the brain should not receive contradictory images.
The researchers state that there was no change in the reading performance of control subjects and, more importantly, the reading performance of dyslexic subjects was comparable to the controls. That’s a pretty amazing result.
That said, I’m still a bit hesitant. That last part of the experiment was reported almost as an afterthought. There was no data on the actual reading tests, and it was not clear that the experiment was completely blinded (meaning that neither the researchers nor the subject are allowed to know whether the light is blinking or not).
It’s also unclear to me why this would work, because it’s not that people can’t read with one eye closed. Putting an eye patch on my daughter doesn’t make her read any easier, but this research suggests she should. What am I missing?
My last comment is a reminder that dyslexia is a spectrum of symptoms. What excites me about this research is the physical link between eye physiology, eye dominance and a traditional dyslexia diagnosis. However, it’s unclear why the differences in eye physiology should lead to problems with writing and, even more puzzling, to automating certain processes. While this is interesting, it does not appear to be the cause of dyslexia.
Procedures of the Royal Society B2017, DOI: 10.1098/rspb.2017.1380