Binocular coordination during smooth pursuit in dyslexia: a multiple case study

Dyslexia is a selective impairment of reading and spelling abilities without a deficit in general intelligence. Dyslexia is commonly attributed to a phonological deficits (e.g., Ramus, Rosen, Dakin, Day, Castellote, White & Frith, 2003). However, a mild cerebellum dysfunction (Nicolson, Fawcett & Dean, 2001) and specific visual problems, such as a defective magnocellular system, have also been discussed as possible causal factors (Stein, 2001). The latter theory has been supported by many studies showing abnormal eye movements during reading in dyslexics, such as longer fixations, smaller saccade amplitudes and a higher percentage of regression, i.e. backward saccades (Pavlidis, 1981). Also abnormalities of other types of eye movements are reported: inaccurate saccades (Biscaldi, Gezeck & Stuhr, 1998), longer latency for saccades or vergence eye movements (Bucci, Bremond-Gignac & Kapoula, 2008a), higher rates of express latencies for saccades (Biscaldi, Fischer & Aiple, 1994) and for divergence (Bucci et al., 2008a), more frequent saccadic intrusions during smooth pursuit (Eden, Stein, Wood & Wood, 1994), binocular instability (Stein, Richardson & Fowler, 2000) and disordered vergence control (Stein, Riddell & Fowler, 1988). Since the magnocellular system dominates the visual guidance of eye movements by the posterior parietal cortex – PPC (Stein, 2001), cerebellum (Rae, Karmiloff-Smith, Lee, Dixon, Grant, Blamire, Thompson, Styles & Radda, 1998) and superior colliculi SC(Sparks & Jay, 1986), slight dysfunction may affect the most vulnerable oculomotor control system, e.g. the binocular control system for any type of eye movements. The unstable binocular control of dyslexics probably explains the unstable visual perceptions that they experience. Cornelissen, Hansen, Hutton, Evangelinou and Stein (1998) reported that letters seem to move around, merge, flip and jump over each other. Such subjective reports describe the kind of fluctuating diplopia that might result from unstable binocular motor control and fixation. Children with such unsteady eyes tend to confuse and disarrange letters when attempting to read, so that they often misread real words as nonsense words (Cornelissen, Bradley, Fowler & Stein, 1991). Recently, Bucci, BremondGignac and Kapoula (2008b) quantified binocular coordination during saccades to single word or target and reported decreased coordination in dyslexics relative to age-matched non-dyslexic children; similar observation was made by Kapoula, Bucci, Ganem, Poncet, Daunys and Bremond-Gignac (2008) in dyslexics for saccades and fixations during free exploration of paintings. The latter results indicate an intrinsic ocular motor deficiency Binocular coordination during smooth pursuit in dyslexia: a multiple case study