Thursday, April 14, 2011

Specific reading disability (dyslexia): What have we learned in the past four decades? (2004)

"Frank R. Vellutino, Jack M. Fletcher, Margaret J. Snowling, and Donna M. Scanlon

Abstract quote: "in most cases [of deficiencies in reading-related cognitive abilities], phonological skills deficiencies associated with phonological coding deficits are the probable causes of the disorder rather than visual, semantic, or syntactic deficits, although reading difficulties in some children may be associated with general language deficits. . . . general learning abilities (e.g., attention, association learning, cross-modal transfer) and low-level sensory deficits have weak validity as causal factors in specific reading disability (supported by research evaluating the biological foundations of dyslexia). Many poor readers are impaired because of inadequate instruction or other experiential factors. Implications: focus on the need for enhanced instruction."

Purposes of the review: 
Examines the literature for causal relationships in dyslexia; presents research evidence for distinguishing between biological/cognitive deficits and experiential/instructional deficits as possible causes.

Cognitive Model of knowledge, skills, and abilities assumed to underlie reading ability

Permanent Memory
Background knowledge: cultural, environmental, experiential

Linguistic Coding Processes            Lexical Knowledge              Sublexical Knowledge                   Visual Coding Processes
Phonological                                       Spoken words                      Phonological Awareness                  and knowledge
Semantic                                               Printed words                      Orthographic Awareness
Morphological                                                                                   Alphabetic Knowledge
Syntactic                                                                                            Orthographic conventions

Written Text                                       Word Identification Language Comprehension       
                                                            Reading Comprehension

Knowledge of Print                          Working Memory Processes                          Metalinguistic Processes
Concepts and Conventions                                                                                       and knowledge
                                                               Permanent Memory

Visual coding process: sensory and higher level visualization skills that link visual stimuli with symbols
Linguistic coding: processes that facilitate language development and its use for coding, memory storage, and retrieval. Includes:

  • phonological coding (speech codes), 
  • semantic and morphological coding (word and word part meanings), 
  • syntactic coding (word order rules - organizing ideas into coherent sentences), 
  • pragmatic coding (storing information about conventions of language use for communication, including pitch, volume, intensity, punctuation in written language)
  • phonological awareness: understanding that speech sounds and combinations of them create
Linguistic and visual coding processes help establish associations between spoken and written words and aid in the acquisition of sight words, i.e., words identified automatically as meaningful lexical units. This association is dependent upon knowledge of print concepts and print conventions - left to right, letters represent phonemes comprising oral language. An understanding of the alphabetic principle facilitates sight word learning, because words contain so many visually similar features. Phonological decoding reduces the load on visual memory, providing letter-sound information as the child constructs access to lexical knowledge.
This necessitates building sublexical knowledge, specifically phonological awareness and orthographic knowledge (reciprocally related), as well as skills in metalinguistic analysis. (p.5)

Causes of reading disability

Diagnosis of dyslexia is determined by inadequate facility in printed word identification and related skills of spelling and decoding. The primary deficity is in learning to decode print (Gough & Tumer, 1986; Perfetti, 1985; Snowling 2000; Stanovich, 1991; velutino, Scanlon, & Chen, 1995; Vellutino, et al., 1996). Reading comprehension problems are also identified by inadequate word identification skills due to decoding deficits.

Concepts of print

"Limitations in such knowledge are probably not basic causes of specific reading disability in the biological sense, and are, in most cases, caused by experiential and instructional deficits rather than by biologically based cognitive deficits" (p. 7).

Regression studies abound contributing to a causal link between reading ability, PA deficits and phonological decoding.

Cognitive Deficit theories of dyslexia (discredited)

The following hypotheses have been discredited by empirical studies
Specific learning disabilities are linked to deficiencies in visual, linguistic and low level sensory functions, and general learning abilities involved in all learning enterprises and not just learning to read.
SLD in reading attributed to deficiencies in selective attention, associative learning, cross-modal transfer, serial-order processing, pattern analysis and rule learning.
Reviews of this literature can be found in: Vellutino, 1979, 1989;' Vellutino & Scanlon, 1982; Katz, Shankweiler, & Lieberman, 1981; Katz, Healy, and Shankweiler, 1983). Where group differences were noted in studies, most could be attributed to a lack of control in verbal coding and working memory processes affected by verbal coding deficits. When these controls were implemented, group differences between normal readers and poor readers were eliminated for general learning abilities.

Cross modal transfer invalidated by Vellutino and associates in 1979, 1982 and 1987 and the theory was invalidated numerous times (Givson, 1969; Bryant, 1974; Meltzoff & Kuhn, 1994;  )

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