Isolating global and specific factors in developmental dyslexia: a study based on the rate and amount model (RAM).

Using the reading-age match approach, research on developmental dyslexia focuses on specific (e.g., phonological) deficits and disregards the possible role of global influences in determining the disturbance. In the present study, we set out to investigate the role of both global and specific factors in Italian developmental dyslexics using the rate-amount model (RAM; Faust et al. in Psychol Bull 125:777-799, 1999). Vocal reaction times (RT) in naming pictures, words and non-words of varying length were measured in a group of 26 sixth- to eighth-grade dyslexics and 81 age-matched control readers. Dyslexics' raw RTs showed greater lexicality (longer RTs to non-words than words) and length (longer RTs to long stimuli than short ones) effects than controls'. We found that one global factor predicted most individual variation in naming words and non-words, but not pictures. When data transformations, effective in controlling for the global factor, were applied to the data, the greater lexicality effect in dyslexics vanished, due to the influence of the global factor and not a specific failure in the non-lexical reading procedure. Conversely, the greater length effect in dyslexics persisted. Overall, dyslexics' reading performance was best explained as due to the influence of both a global factor for processing orthographic material prelexically and to the specific influence of stimulus length. This conceptualisation appears more promising for bridging the gap between behavioural and functional imaging studies than traditional approaches, which focus on the detection of specific reading deficits. It is concluded that RAM is a useful tool for disentangling the components that are impaired in reading and for defining the characteristics of the global factor, because the paradigm is more powerful for studying developmental dyslexia than the reading-age match method.