Processing of Derived Forms in High-Functioning Dyslexics

We report on an experiment designed to evaluate processing of derived forms in high-functioning dyslexics, defined as university students with a history of reading difficulties who have age-appropriate reading comprehension skills. We compared high-functioning dyslexics with a group of normal adult readers in their performance on a lexical decision task with derived items (such as cloudy and ably) and pseudo-derived items (such as belly and gravy). Some items contained an orthographic change (such as able-ably and gravy) and others did not (such as cloud - cloudy). The results indicated that although control participants' response times varied systematically as a function of morphological complexity, those of high-functioning dyslexics did not. Further, there was some evidence of a relationship between derivational processing and reading. It seems that high-functioning dyslexics have persistent difficulties in processing one particular aspect of morphology; that of derived forms.

Key Words: Derivational processing, high-functioning dyslexics, morphology, orthographic complexity, reading difficulties

A long line of research has demonstrated that phonological awareness is a key and causal factor in literacy development (Bradley & Bryant, 1983; National Reading Panel, 2000). Certainly, children and adults with reading difficulties have marked problems with the manipulation of sounds, or phonemes, within words (for reviews, see Morais, Carey, Alegria, & Bertelson, 1979; Snowling, 2000). These problems persist in adults with a history of reading difficulties long after they have managed to achieve some level of competence in their reading (e.g., Pennington, Van Orden, Smith, Green, & Haith, 1990; Snowling, Nation, Moxham, Gallagher, & Frith, 1997). It is estimated that 22% to 25% of children with a diagnosis of dyslexia will recover to the point that their adult reading is well within the normal range (Lefly & Pennington, 1991). Of course, there is wide variation in the degree of such compensation. The study reported here investigates "high-functioning dyslexies," individuals with a history of reading difficulties who have achieved a level of reading comprehension that permits their participation in postsecondary education. A key question lies in how these individuals have been able to achieve this relatively high degree of competence in reading comprehension despite well-established difficulties in phonological skills known to be so critical to reading outcomes. One possibility is that high-functioning dyslexies have relatively strong morphological processing skills that permit the compensation for poor phonological processing skills (as suggested by Elbro & Arnbak, 1996). The evidence to date on this question can be described, at best, as mixed. The present study is designed to investigate the morphological processing of derived forms in high-functioning dyslexies.

Morphemes are the smallest unit of meaning in language. The word uncovered, for example, is built out of the prefix unstem cover, and past tense suffix -ed. Several psycholinguistic models of the lexicon have proposed that affixes are "stripped off" prior to lexical access (e.g., Taft & Forster, 1975) resulting in faster response times to real than to pseudo-derived words such as unlike versus uncle, respectively (Rubin, Becker, & Freeman, 1979; Taft, 1981). As a reaction against this proposal, Butterworth (1983) proposed a full-listing model in which morphologically complex words are represented in their entirety in the lexicon (e.g., black, board, and blackboard each have separate representations). The majority of current models take a compromise position, suggesting that there is a "race" between the two processing (e.g., Marslen-Wilson, Tyler, Waksler, & Older, 1994; Taft, 1994). While the precise architecture of morphological organization remains under investigation, there is good evidence that morphemes play a role in lexical access in the typical adult lexicon. Morphological effects in word recognition cannot be accounted for by orthographic or phonological similarities between the items (e.g., Fowler, Napps, & Feldman, 1985; Napps & Fowler, 1987; Stoltz & Feldman, 1995) or by semantic overlap on its own (e.g., Bentin & Feldman, 1990; Feldman, 1992). We will build on this history of lexical access tasks in the investigation of the consistency of derivational processing between high-functioning dyslexies and normal adult readers. To place such a study within its broader context, we will review morphological processing as indexed by performance on a wide range of tasks by poor readers of all ages.

MORPHOLOGICAL PROCESSING IN YOUNG POOR READERS

We turn first to research that has contrasted the morphological skills of good and poor reading abilities. Giraudo (2001) compared priming effects from derived words (e.g., pottery - pot) to those of pseudo-derived words (e.g., potato - pot). Although 8-year-old normally developing children were significantly faster at recognizing words primed by morphological relatives, no such difference emerged in the dyslexic children. Similar evidence of morphological difficulties emerges in a study of oral and written production tasks. Rubin, Patterson, and Kantor (1991) found that learning disabled 7-year-olds lagged behind their peers in both oral and written morphological tasks. Parallel results emerged in the assessment of the use of morphology in word reading. Carlisle, Stone, and Katz (2001) found that 10- to 15-year-old readers were less accurate in their naming and lexical decision responses for words with than without changes in the phonology of the base form (e.g., natural and cultural, respectively), and these transformations were particularly problematic for the poor readers. Further evidence of impairments comes from Leong's (1989b) finding that poor readers in Grades 4 to 6 were slower than their peers in reading words presented according to morphological rather than phonological divisions (e.g., ACTor versus ACtor, respectively) (Taft, 1979), and in speed of production of base and derived forms to complete sentences (based on Carlisle, 1988; see Leong & Parkinson, 1995 for similar results). Leong (1989a) further noted that out of a large battery of measures, reaction times in the morphological production task were the best predictors of reading and spelling ability within the poor reader group. The generation of base forms with orthographic changes (e.g., generate begin from beginner) was particularly discriminative. Leong and Parkinson (1995) reported that similar delays in processing of derived forms occur in repetition priming tasks. These results suggest that the morphological processing of poor readers, as assessed across a wide range of tasks with both accuracy and response time measures, lags at least behind that of their same age peers.