The evidence for a temporal processing deficit linked to dyslexia: A review

MARY E. FARMER 0 RAYMOND M. KLEIN 0 0 Dalhousie University , Halifax, Nova Scotia, Canada The existence of a phonemic deficit that is predictive of, and probably causal to, many cases of reading difficulty is well established. Tallal (1984) has suggested that this phonemic deficit is in fact a symptom of an underlying auditory temporal processing deficit. Our purpose in this paper is to evaluate the plausibility of this hypothesis. The various components that might constitute sequential (or temporal) processing are described. Our review of the literature reveals considerable evidence for a deficit in dyslexics in stimulus individuation tasks (e.g., gap detection) and temporal order judgments in both the auditory and visual modalities. The possibility that a general temporal processing deficit is associated with dyslexia, as suggested by Tallal (1984), is explored, and possible etiologies for such a deficit are discussed. The possibility of a causal link between temporal processing deficits and some reading disabilities is demonstrated, and converging evidence from morphological studies is reviewed. It is concluded that a temporal processing deficit does appear to be present in many developmental dyslexics, and strategies are suggested for further research aimed at evaluating the hypothesis that this deficit may be the root cause of a number of cases of dyslexia itself. - Portions of this work were included in a doctoral dissertation presented by the first author. The work was supported by a Natural Science and En gineering Research Council (NSERC) grant to the second author, and by an NSERC Graduate Scholarship and a Killam Memorial Scholarship awarded to the first author. The authors gratefully acknowledge the ideas and discussions contributed by Susan Bryson early in this paper's con ception, as well as further helpful contributions by Dennis Phillips. The helpful and critical comments on earlier versions of this manuscript by Randi Martin, Richard Olsen, Paula Tallal, and several anonymous re viewers have been very much appreciated. Finally, the careful and con structive suggestions of Jim Neely were crucial in refining this paper and are acknowledged with gratitude. Requests for reprints should be ad dressed to R. M. Klein, Department of Psychology, Dalhousie Univer sity, Halifax, NS, Canada B3H 411 (e-mail: or klein@ ac.dal.ca). quencing tasks are also discussed. We explore and reject the idea that temporal processing deficits simply reflect an attentional disorder; we consider the implications of the heterogenous nature of dyslexia and the possible develop mental course of the temporal processing deficit for stud ies in this field. Converging evidence from morphological and electrophysiological studies is reviewed. We conclude that there is sufficient evidence from a variety of para digms for an association between dyslexia and temporal processing deficits to warrant further investigation of the claim that a general temporal processing deficit may under lie some cases of reading disability. Finally, we consider the plausibility of the temporal processing deficit as a cause of some reading disabilities and outline several ways to evaluate this hypothesis. A Note on Sampling Heterogeneity Before beginning ourreview ofthe evidence, we should emphasize that it is clear that a temporal processing defi cit (or any other specific deficit, for that matter) will not be found to account for all cases of developmental dys lexia. Learning to read calls upon many cognitive pro cesses and involves many areas ofthe brain. A breakdown in any ofthe contributing processes or areas may thus lead to an inability to learn to read in the normal way. A diffi culty in learning to read, or dyslexia, should not be viewed as a condition in itself, but as a symptom of a breakdown in one or more of the various processes involved. To what degree, if any, a temporal processing deficit may con tribute to reading disabilities remains to be demonstrated. Considerable research has been conducted over the past few years in an effort to identify specific subtypes of dys lexic children, to see whether some skills deficits are evi dent in all reading-disabled children, or whether different dyslexics display different patterns of impairments. In 1971, Boder identified three groups ofdyslexics: dyspho netic dyslexics (the largest group), who had great diffi culty in applying grapheme-phoneme correspondence rules, and thus in reading nonwords; dyseidetic dyslexics, who could sound out words phonemically, but had great difficulty reading irregular words correctly; and a mixed dysphonetic--dyseidetic group. Some investigators have identified groups that are similar to one or more ofBoder's three groups (e.g., Freebody & Byrne, 1988; Johnson & Myklebust, 1967; Lyon, 1985; Satz & Morris, 1981). More recently, Castles and Coltheart (1993) have suggested that at least two varieties of developmental dyslexia can be identified and that these correspond approximately to the phonological and surface varieties identified in acquired dyslexia. The phonological dyslexics have difficulty in reading unfamiliar words and nonwords, but relatively bet ter ability at reading irregular words. Surface dyslexics, on the other hand, are relatively worse at reading irregular words. A majority of subjects studied by Castles and Colt heart (1993) appeared to have mixed deficits; neverthe less, fully one third had a relatively pure deficit. In a repli cation and extension of this study, Seidenberg and Manis (1994) also found some relatively pure phonological and surface dyslexics. Moreover, because the surface devel opmental dyslexics' performance on tasks used to tap or thographic and phonological knowledge was similar to that of reading-matched controls, this pattern was charac terized as a developmental delay. In contrast, the perfor mance of phonological dyslexics often differed from that of both control groups on these tasks, suggesting that a de viant pattern characterizes many subjects in this group. Other investigators have identified profiles that they have classified differently, such as Bakker's (1979) L- and P-type dyslexics; the 0, A, and S types of Doehring, Trites, Patel, and Fiedorowicz (1981); and Lovett's (1984) accu racy- and rate-disabled groups. As Fletcher (1985) has pointed out, there is considerable work to be done before reading-disabled children can be classified as belonging to a particular subtype. This work will be complicated by the evidence that reading disability subtypes evolve as time progresses (Hynd, 1992). In this respect, the knowledge contributed by single case studies may be invaluable. It is evident that there are many different expressions ofa read ing disability, and no one underlying cause will be found to account for such variety. It is also evident, however, that some types of impairment are shared by many dyslexics. The question of heterogeneity among reading-disabled childre (...truncated)