Bridging the gap between different measures of the reading speed deficit in developmental dyslexia.

The study assessed how decoding and pronunciation times contribute to total reading time in reading aloud and how these measures change in the presence of developmental dyslexia. Vocal reaction times (RTs), pronunciation times, and total reading times were measured while 25 children with dyslexia and 43 age-matched typically developing readers read singly presented words and non-words that varied for length. Group differences were large for vocal RTs; children with dyslexia were increasingly slower as a function of condition difficulty (over-additivity effect); lexicality and length influenced RTs even when over-additivity was controlled for by z-score transformation. The group differences were also large for vocal total reading times, but the effect of over-additivity was smaller than that of vocal RTs and no selective influence of lexicality and length was detected. Pronunciation times showed very small individual differences and no over-additivity effect; children with dyslexia were more sensitive to the effect of lexicality and length than controls. To assess the contribution of the cognitive and sensory-motor compartments in determining group differences, we applied the difference engine model. As for RTs, the relationship between means and standard deviations closely supported the prediction of a general cognitive delay in the slow group, with no group difference in the sensory-motor compartment. The variance in total reading times was predicted by combining the model results for RTs with the linear relationship between pronunciation times and task difficulty. The results help clarify the internal structure of reading times, a measure largely used in clinical testing to assess reading rate.