The double-deficit hypothesis for the developmental dyslexias.

TLDR

The authors propose the double‑deficit hypothesis, positing phonological deficits and naming‑speed processes as two separable sources of reading dysfunction, and examine two hypotheses about how naming‑speed relates to reading. They explain how naming‑speed and phonological‑awareness variables uniquely contribute to reading, illustrate this with a visual‑letter‑naming model that shows naming‑speed’s multicomponential nature and argues it should not be subsumed under phonology, and discuss the implications of processing speed as a second core deficit for diagnosis and intervention. Cross‑sectional, longitudinal, and cross‑linguistic data reveal two single‑deficit subtypes with limited impairments and a double‑deficit subtype with pervasive, severe impairments, supporting the distinct contributions of naming‑speed and phonological‑awareness to reading.

Abstract

The authors propose an alternative conceptualization of the developmental dyslexias, the double-deficit hypothesis (i.e., phonological deficits and processes underlying naming-speed deficits represent 2 separable sources of reading dysfunction). Data from cross-sectional, longitudinal, and cross-linguistic studies are reviewed supporting the presence of 2 single-deficit subtypes with more limited reading impairments and 1 double-deficit subtype with more pervasive and severe impairments. Naming-speed and phonological-awareness variables contribute uniquely to different aspects of reading according to this conception, with a model of visual letter naming illustrating both the multicomponential nature of naming speed and why naming speed Should not be subsumed under phonological processes. Two hypotheses concerning relationships between naming-speed processes and reading are considered. The implications of processing speed as a second core deficit in dyslexia are described for diagnosis and intervention.