Threshold of coherent motion and contour integration in children with development dyslexia
Révész G., Séra L., Járai R., Berényi I.
First author's affiliation: Institute of Psychology, University of Pécs, Pécs, Hungary
Previous empirical research suggests that visual perceptual impairment may occur in up to 75% of developmental dyslexics (e.g., Boden & Giaschi, 2007; Samar & Parasnis, 2005). These deficits, especially the magnocellular and the dorsal stream of visual pathways deficit are well documented in dyslexia (e.g., Skottun, 2000; Skottun & Skoyles, 2005). On the other hand, another explanation proposes that magnocellular and parvocellular channels are intact but that there is an abnormal interaction between them: a failure of mutual inhibition (e.g., Slaghuis & Pinkus, 1993) or the transient but delayed magnocellular processing inhibits (masks) the sustained parvocellular processes (Keen & Lovegrove, 2000). Other possibilities are that the visual disorder in dyslexia is located in higher visual areas which may explain the disordered perception of coherent global motion in dyslexia. In this research we examined the threshold of coherent motion with our own developed method (CoMot & Geier, 2008) and the threshold of contour integration (Kovács & Julesz, 1993; Kovács, Kozma, Fehér, & Benedek, 1999; Kovács, 2000) in two age and IQ matched dyslexic and typically developed groups. There was no difference between the two groups in the task of contour integration, while the threshold of coherent motion was higher in the group with dyslexia. This result suggests that the deficit of visual processing may not be in the lower – local - pathways, but instead in the higher - integrative - visual areas (MT).