Abstract
The ‘Cerebellar Deficit Theory’ of developmental dyslexia proposes that a subtle developmental cerebellar dysfunction leads to deficits in attaining ‘automatic’ procedures and therefore manifests as subtle motor impairments (e.g., balance control, motor skill learning) in addition to the reading and phonological difficulties. A more recent version of the theory suggests a core deficit in motor skill acquisition. This study was undertaken to compare the time-course and the nature of practice-related changes in volitional (manual) and non-volitional (posture) motor performance in dyslexic and typical readers while learning a new movement sequence. Seventeen dyslexic and 26 skilled young adult readers underwent a three-session training program in which they practiced a novel sequence of manual movements while standing in a quiet stance position. Both groups exhibited robust and well-retained gains in speed, with no loss of accuracy, on the volitional, manual, aspects of the task, with a time-course characteristic of procedural learning. However, the dyslexic readers exhibited a pervasive slowness in the initiation of volitional performance. In addition, while typical readers showed clear and well-retained task-related adaptation of the balance and posture control system, the dyslexic readers had significantly larger sway and variance of sway throughout the three sessions and were less efficient in adapting the posture control system to support the acquisition of the novel movement sequence. These results support the notion of a non-language-related deficit in developmental dyslexia, one related to the recruitment of motor systems for effective task performance rather than to a general motor learning disability.
Highlights
In a series of studies and theoretical papers during the last two decades, Nicolson and Fawcett developed the ‘Cerebellar Deficit’ theory to explain evidence for non-verbal, sensory-motor, impairments among dyslexic readers [1,2,3,4,5]
Experimental measurements Training on the given set of hand movements led to significant within- and between-session improvements in both the volitional touch-sequence task performance (Figure 4) and the nonvolitional, posture control measures (Figure 5), but these measures were differentially affected by training
The current results suggest that the dyslexic readers were as effective as skilled readers in their abilities to acquire and robustly retain the trained movement sequence, they were impaired in the recruitment and integration of some aspects of the task compared to skilled readers
Summary
In a series of studies and theoretical papers during the last two decades, Nicolson and Fawcett developed the ‘Cerebellar Deficit’ theory to explain evidence for non-verbal, sensory-motor, impairments among dyslexic readers [1,2,3,4,5]. Two specific impairments were suggested: posture and balance deficits ascribed to an unspecified cerebellar dysfunction [1,11] and deficits in ‘‘automaticity’’ or skill (procedural) learning [12]. These deficits have not been verified in several studies[6,7,13] and it is not clear whether dyslexic reader individuals suffer from deficits in their posture control system or in their ability to acquire a novel (volitional motor) skill.
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