title: Psychophysical Evidence for Impaired Magno, Parvo, and Konio-cellular Pathways in Dyslexic Children.
abstract: OBJECTIVE
  Dyslexia is one of the most common learning disabilities affecting millions of people worldwide. Although exact causes of dyslexia are not well-known, a deficit in the magnocellular pathway may play a role. We examined possible deficiency of magnocellular, as compared to parvocellular and koniocellular pathway function by measuring luminance and color perception.
  
  
  METHODS
  Visual stimuli consisted of a series of natural images, divided into layers of luminance, red-green and blue-yellow, which probed magnocellular, parvocellular, and koniocellular pathways, respectively. Thirteen children with dyslexia and 13 sex- and age- matched controls performed three psychophysical tasks. In the first task, subjects were instructed to match the contrast of luminance (magno) and red-green (parvo) images to that of the blue-yellow (konio) images. In the second task, subjects detected the isoluminant point of red-green images to probe parvocellular pathway. In the third task, temporal processing was assessed by measuring reaction time and percentage of correct responses in an identification task using four categories of images, activating all three pathways.
  
  
  RESULTS
  The dyslexic group had significantly elevated luminance and color contrast thresholds and higher isoluminant point ratio in comparison to the control group. Furthermore, they had significantly less correct responses than the control group for the blue-yellow images.
  
  
  CONCLUSIONS
  We may suggest that dyslexic subjects might suffer from both magnocellular and parvocellular deficits. Moreover, our results show partial impairment of the koniocellular pathway. Thus, dyslexia might be associated with deficits in all three visual pathways.
authors.affiliation: Department of Psychology, Shahid Beheshti University, Tehran, Iran.
authors.forename: Khazar
authors.initials: K
authors.lastname: Ahmadi
authors.affiliation: Department of Psychology, Shahid Beheshti University, Tehran, Iran; Tehran-Oxford Neurodevelopmental Center, Institute for Cognitive and Brain Sciences, Shahid Beheshti University, Tehran, Iran.
authors.forename: Hamid Reza
authors.initials: HR
authors.lastname: Pouretemad
authors.affiliation: Neuropsychology Lab, Institute for Cognitive Science Studies, Tehran, Iran.
authors.forename: Jahangir
authors.initials: J
authors.lastname: Esfandiari
authors.affiliation: Depatment of Ophthalmology, McGill Vision Research, McGill University, Montreal, QC, Canada.
authors.forename: Ahmad
authors.initials: A
authors.lastname: Yoonessi
authors.affiliation: Neuropsychology Lab, Institute for Cognitive Science Studies, Tehran, Iran; Department of Neuroscience, School of Advanced Technologies in Medicine, Tehran University of Medical Sciences, Tehran, Iran.
authors.forename: Ali
authors.initials: A
authors.lastname: Yoonessi
keywords: Dyslexia; Koniocellular; Magnocellular; Parvocellular