Children with dyslexia have trouble recognizing the sounds that make up words, but it is not clear why. One hypothesis is that dyslexic readers are less able to perceive visual cues that are processed by the magnocellular, or M, pathway of the visual system. However, new research suggests that the parvocellular, or P, pathway also plays a role.
The M pathway processes differences in brightness and signals that change rapidly in time, whereas the P pathway handles signals that have a high spatial frequency, such as narrow stripes. Anne Sperling, a postdoctoral fellow in neurology at Georgetown University Medical Center, and colleagues realized that the past experiments testing children’s ability to process M signals had used visual patterns displayed on a “noisy” background, or displays where there was more going on than changes in the M signal. Sperling thought the problem might not be the M pathway per se, but rather an inability to distinguish the signal from the noise.
The researchers found that if they asked children to look for a pattern on a screen with a background resembling television static, the children with dyslexia required more contrast between the signal and background before they could detect an image. They had trouble seeing a pattern regardless of whether the visual cues were processed by the M pathway or the P pathways, according to the study, published in the July Nature Neuroscience.
The team concludes that the trouble differentiating signals from noise affects more than one pathway, and may even involve other sensory systems. “Anything we can do to jack up the volume” on the important signals may help children with dyslexia, Sperling concludes.