Magnocellular deficit in dyslexia: a preliminary analysis of possible patterns of visual field deficit tested with Frequency Doubling Illusion

Purpose: To identify distinguishing visual field (VF) deficiencies in dyslexic children by exploring responses to spatial frequency doubling illusions, characteristically involving the magnocellular visual pathway. Methods: Eighteen dyslexic subjects and nine controls [mean age 8.4 ± 1.4 years (range, 7-13 years)] were consecutively enrolled and underwent VF examination on a Frequency Doubling Technology (FDT) Visual Field Instrument. Among the 19 locations of the right eye (RE) and left eye (LE) VFs, 7 topographic patterns were identified: central (CE), superior-temporal (ST), and outer superior-nasal (OSN), inner superior-nasal (ISN), inner inferior-nasal (IIN), outer inferior-nasal (OIN), inferiortemporal (IT). The default settings included measurements of both eyes separately at all 19 VF locations, exploring the central 30-degree radius of eccentricity. For each pattern indicated the thresholds of contiguous 5 point clusters of the VF in both eyes were averaged: the 7 threshold averages obtained were compared for each eye between the control and dyslexic groups. Results: For RE and LE there was non-statistically significant difference between control and affected cases for all patterns except for CE, IIN, IT patterns in the left eye (P<0.05). The two groups were comparable for loss of fixation, false positive error, and false negative error. These data indicate that the dyslexic reader's left eye presents less sensitivity to FD illusions in almost all inferior retinal hemifield. Conclusion: Dyslexics analyzed using FD illusions may be less sensitive in specific sections of the VF. This topographic failure may help to confirm that a deficit in the magnocellular visual pathway is correlated to reading