Magno, Parvo, and Konio. Sounds like a tripartite answer that Carnac the Magnificent might have divined.
But seriously … which visual pathways have now been implicated in dyslexia? A tip of the turban to Geoff Shayler, our British colleague, for the answer to this question through a significant article that appeared last year in the Journal of Ophthalmic & Vision Research: Psychophysical evidence for impaired Magno, Parvo, and Konio-cellular pathways in dyslexic children.
Here is an illustration of one of the experiments done in this study. Possible deficiencies in the three pathways in dyslexic children were measured using a contrast matching task. In each trial, three different versions of one image were shown.
(a) is the original image. (b) contains the koniocellular-activating (blue-yellow) image at the left; the magnocellular activating (luminance) at the right-bottom; and the parvocellular activating (red-green) at the right-top. The subjects were instructed to adjust the perceived contrast of the two images located on the right side of the screen (red-green and luminance) to match the one on the left side (blue-yellow).
The authors conclude that their results confirm the presence of magnocellular and parvocellular deficits, and indicate partial impairment of koniocellular pathway in dyslexia and suggest that all three pathways might be involved in reading. Their findings also suggest that natural scenes can be an ideal form of stimulus to probe changes in dyslexia.
We tend to think of the advantages of using natural scenes in optometric vision therapy when guiding patients toward opening up their periphery and in the use of visual imagery. We have been gradually employing enticing graphics in therapy procedures such as OSMO, and there has been interest and enthusiasm in immersive environmental scenes as experienced in the Oculus Rift for binocular stereoscopic stimuli. Perhaps this is another reason why incorporating visual discrimination and matching activities involving natural scenes into our therapeutic activities has transfer effects to reading readiness and efficiency beyond what is apparent at face value.