from language to reading and dyslexia

From Language to Readingand Dyslexia1

Margaret J. Snowling*University of York, UK

This paper reviews evidence in support of the phonologicaldeficit hypothesis of dyslexia. Findings from two experimentalstudies suggest that the phonological deficits of dyslexic childrenand adults cannot be explained in terms of impairments inlow-level auditory mechanisms, but reflect higher-level languageweaknesses. A study of individual differences in the pattern ofreading skills in dyslexic children rejects the notion of‘sub-types’. Instead, the findings suggest that the variation seenin reading processes can be accounted for by differences in theseverity of individual children’s phonological deficits, modifiedby compensatory factors including visual memory, perceptualspeed and print exposure. Children at genetic risk who go on tobe dyslexic come to the task of reading with poorly specifiedphonological representations in the context of a more generaldelay in oral language development. Their prognosis (and thatof their unaffected siblings) depends upon the balance ofstrengths and difficulties they show, with better language skillsbeing a protective factor. Taken together, these findings suggestthat current challenges to the phonological deficit theory can bemet. Copyright © 2001 John Wiley & Sons, Ltd.

Keywords: dyslexia; reading; genetics; phonological deficits

One of the UK’s pioneers in the field of dyslexia is T.R. Miles. Workingwith colleagues at the Invalid Children’s Aid Association (ICAA) WordBlind Centre in the 1960s, he gained important insights into the

strengths and difficulties of dyslexic children, insights that have been thefoundation of his research and his clinical work (Miles, 1993) up until thepresent time. This paper takes as its starting point his understanding ofdyslexia, that it is an anomaly of development that can properly be called asyndrome, that it is characterized by a weakness in verbal processing and thatits symptoms are the consequence of phonological difficulties (Miles and Miles,1990, 1999).

* Correspondence to: Professor M.J. Snowling, Department of Psychology, University ofYork, Heslington, York, YO24 1AX UK. E-mail: [email protected] This paper was originally given as the T.R. Miles lecture for 2000.

DYSLEXIA 7: 37–46 (2001)DOI: 10.1002/dys.185

Copyright © 2001 John Wiley & Sons, Ltd.

M.J. Snowling38

For many years it has been known that dyslexia is a brain-based disorder thathas a hereditary component and is on the continuum of language disorder(Orton, 1925). Although there is not yet a consensus about the causes of dyslexia(Nicolson and Fawcett, 1990; Stein and Talcott, 1999), the most widely acceptedcognitive explanation of dyslexia is that it stems from an underlying phonolog-ical deficit (Stanovich and Siegel, 1994). The phonological system is part of thelanguage processing system. Dyslexia is an anomaly of development becausephonological processing is selectively impaired in dyslexic people while otheraspects of their language, for instance their vocabulary and grammatical skills,are normal (Goulandris, Snowling and Walker, 2000). To the extent that learningto read depends upon phonological skills, dyslexic individuals are impaired.However, aspects of reading that depend more upon semantic processing, suchas reading comprehension (Frith and Snowling, 1983) and exception wordreading (Metsala, Stanovich and Brown, 1998), can be relatively intact.

At a fundamental level, learning to read requires the child to set up a systemof mappings between the letter strings of printed words (orthography) and thephonemic sequences that comprise spoken words (phonology). This process iscaptured well by connectionist models that view reading as the transfer ofpatterns of activation across sets of simple processing elements in input andoutput systems comprising orthographic and phonological units (Seidenbergand McClelland, 1989). These models are computationally explicit metaphors forlearning to read (Snowling, Hulme and Nation, 1997). A critical feature is that,after training to associate many input patterns (written words) with manyoutput patterns (spoken words), these models are able to generalize to wordsthey have not been explicitly taught to read (Plaut et al., 1996). In similar vein,normally developing children abstract the relationships between spelling pat-terns and their pronunciations in the course of reading development, and theycan use this knowledge as a ‘self-teaching device’ (Share, 1995). Within thisframework, dyslexic children have difficulties both initially in establishingconnections between orthographic and phonological representations, and later,in generalizing their knowledge to the reading of novel words (Rack, Snowlingand Olson, 1992).

One hypothesis that has been forwarded to explain the reading difficulties ofdyslexic children is that they come to the task of learning to read with poorlyspecified phonological representations (Hulme and Snowling, 1992; Swan andGoswami, 1997; Snowling, 2000). Put simply, the way in which their brain codesphonology is less efficient than that of normally developing children, despitetheir relative strengths in semantic processing. This problem at the level ofphonological representation causes a range of typical symptoms at the be-havioural level (Frith, 1997). These include problems of verbal short-termmemory, nonword repetition deficits, poor phonological learning of new verbalinformation, word retrieval and rapid naming problems (Snowling, 2000, for areview). A persistent feature of dyslexia in children learning to read in opaqueorthographies such as English, is a problem in the development of phonologicalawareness. However, this problem is much less marked in readers of transpar-ent languages (e.g. Italian; Cossu, 1999), in which the regular relationshipsbetween letters and sounds in the written language offer consistent feedbackabout how words are structured. In such languages, it appears that slownessin naming is one of the key behavioural markers of dyslexia (Landerl andWimmer, 2000).

Copyright © 2001 John Wiley & Sons, Ltd. Dyslexia 7: 37–46 (2001)

From Language to Reading and Dyslexia 39

Despite the appeal of the phonological deficit hypothesis of dyslexia, it is notwithout its critics. Challenges to the theory have been posed by proponents ofthe following theoretical positions:

� phonological deficits can be traced to lower-level deficits in auditoryprocessing (Tallal, 1980)

� phonological processing deficits cannot explain individual differences indyslexia (Castles and Coltheart, 1993)

� the reciprocal influence of reading on phonology is such that phonologi-cal deficits may be exacerbated in dyslexic readers as a result of limitedreading experience.

I shall take each of these challenges in turn and try to argue that thephonological deficit theory remains a parsimonious explanation of what T.R.Miles referred to as the ‘dyslexia syndrome’.

DYSLEXIA AS AN AUDITORY PROCESSING DEFICIT

According to an influential theory proposed by Tallal et al. (1997, for areview) language learning impaired children, including dyslexics, have diffi-culty with the processing of rapidly presented auditory stimuli. Rapidauditory processing (RAP) is critical to speech perception, particularly ofstop consonants that contain rapid formant transitions. According to thetheory, the RAP deficit causes speech perceptual deficits that compromisethe development of phonological representations, and literacy difficultiesensue.

Marshall (2000) from our laboratory set out to test various aspects ofTallal’s theory with dyslexic children, comparing their performance withthat of children of the same age and of the same level of reading skill. In thefirst experiment of this series, 12-year-old dyslexic children were comparedwith age-matched normal readers and younger reading age (RA)-matchedcontrols reading at the 7;09 level. The dyslexic children showed significantdeficits for their age on a rhyme oddity task, and they performed morepoorly than the younger RA-controls in nonword repetition and on aphoneme deletion task. The dyslexics were, therefore, typical of those de-scribed in the literature, showing significant phonological deficits.

In a direct replication of one of Tallal’s experiments, each child was taughtto associate two pure tones with two different responses, a high tone of 350Hz and a low tone of 100 Hz. When they had learned these associations, thechildren were trained to criterion to repeat sequences of tones presented at400 ms intervals (e.g. high-low, high-high, etc.). Next, the inter-stimulusinterval (ISI) between the tones was varied in the experimental trials withISIs ranging between long (150 ms) and short (10 ms).

The prediction from Tallal’s theory was that the dyslexic group wouldshow impairments for their age at short but not at long interstimulusintervals. Moreover, if Tallal’s theory is correct, children’s performance onthis auditory processing task should predict their phonological processing;those with RAP deficits should show phonological impairments. The find-ings of this study were not, however, in line with the predictions of the


M.J. Snowling40

auditory processing theory (Marshall, Snowling and Bailey, submitted).While the dyslexic children performed more poorly than age-matched con-trols on the RAP task, they did as well as RA-controls with similar readingexperience. Moreover, there was no interaction between group and ISIindicating that the dyslexics were not differentially impaired at fast rates ofpresentation. Perhaps more importantly, only a small sub-group of 4/16dyslexic children were impaired on the RAP task. However, these childrenwere not the most impaired in phonological processing; indeed, the onlyremarkable feature of these children’s performance was that they were ratedby their teachers as being more hyperactive than the remainder of the group.

In a second series of experiments, my colleagues and I have examinedanother aspect of basic auditory processing—the phase locking hypothesisof McAnally and Stein (1996). Following on from Tallal’s position, McAnallyand Stein (1996) reported that dyslexic listeners have problems with fre-quency discrimination at low but not at high frequencies. On the basis ofthese findings, they proposed that dyslexics have a selective impairment ofphase locking, a mechanism used in the analysis of low-frequency sounds.Hill et al. (1999) asked a group of 12 compensated dyslexic students to listento sets of four tones and to detect the one that was different in frequencyfrom the others. The odd one was always either in the second or thirdposition of the set. The tones were presented at 1 kHz, the low frequencylevel at which an impairment was predicted, and at 6 kHz, a level that wasnot expected to cause difficulty because discrimination in this high-fre-quency range does not invoke the phase-locking mechanism.

To assess the threshold for frequency discrimination, a staircase procedurewas used such that, after each correct response the frequency discriminationwas made harder, and after each incorrect response, the discrimination wasmade easier. The mean thresholds for the dyslexic group were larger thanfor controls, as predicted by the auditory deficit hypothesis. However,contrary to the predictions of the phase-locking hypothesis, the dyslexics’impairments were observed at high as well as at low frequencies. Moreimportantly, as in the previous experiment, we found that only a minority ofdyslexic listeners were impaired. In fact, the group difference was largelydue to a sub-set of four dyslexic listeners (out of 12) who showed elevatedthresholds at both 1 and 6 kHz. The majority of the dyslexic listeners in thisexperiment had impaired phonological processing deficits and, as a group,they scored one standard deviation below the mean of controls on tests ofphonological awareness, phonological processing and short-term memory.

Taken together, the results of these experiments suggest that only aminority of dyslexic readers perform poorly on low-level auditory tasks.Furthermore, these auditory deficits appear to be unrelated to their phono-logical processing problems. We propose that there are many reasons whydyslexic listeners may show poor performance in auditory psychophysicalparadigms. A major contender is the poor levels of attention control charac-terizing some dyslexic readers. However, we have found to date no strongevidence to argue in favour of a low-level processing impairment. Rather,the deficits of dyslexic children and adults appear to be with higher-levelphonological processes.



INDIVIDUAL DIFFERENCES IN DYSLEXIA

An important advantages of the phonological deficit definition of dyslexia isthat it makes sense in terms of what is known about the normal acquisitionof reading. Since the pioneering study of Bradley and Bryant (1983), it hasbeen known that phonological awareness measured in pre-school is a goodpredictor of subsequent reading achievement, even when the substantialeffects of intelligence quotient (IQ) are controlled. Indeed the ability to reflectupon the sound structure of words at the phonemic level is critical to thedevelopment of the alphabetic principle which allows children to decodewords they have not seen before (Byrne and Fielding-Barnsley, 1989). How-ever, because of their phonological deficits, dyslexic children typically fail tomake the transition from logographic reading to the alphabetic phase (Frith,1985).

Notwithstanding this, some dyslexic children appear to have masteredalphabetic skills. These children have been described as developmental‘surface’ or ‘morphemic’ dyslexics (Coltheart et al., 1983; Seymour, 1986). Theclassic characteristic of their single-word reading is that they rely heavilyupon a phonological strategy. Thus, they tend to pronounce irregular wordsas though they were regular (e.g. glove�gloave; island� izland), they haveparticular difficulty distinguishing between homophones like pear–pair andleek– leak, and their spelling is usually phonetic.

While few theorists argue strongly for distinct subtypes of dyslexia inchildhood, most systematic studies have revealed individual differences inthe reading skills of dyslexic children (Castles and Coltheart, 1993). Manis etal. (1996) and also Stanovich, Siegel and Gottardo (1997) found that dyslexicchildren who had relatively more difficulty in reading nonwords thanexception words (phonological dyslexia) performed significantly less wellthan younger RA-matched controls on tests of phonological awareness. Incontrast, dyslexic children who had more difficulty with exception wordsthan nonwords (classified as surface dyslexic) performed at a similar level tocontrols on these tests.

In a series of single-case studies, we have also compared the performanceof children who differ in reading profile on tests of phonological processing.Generally, children with more severe phonological processing impairmentsshow more significant impairments in their decoding skills and in theirability to spell phonetically (Snowling, Goulandris and Stackhouse, 1994)than children who have milder phonological impairments and show more ofa surface dyslexic profile (Snowling, Goulandris and Defty, 1997). However,it is perhaps important to note that pure sub-types of dyslexia are in factrare; Griffiths (1999) from our laboratory conducted a large scale comparisonof 59 dyslexic readers, aged 9–15 years, comparing them with 59 RA-controls, aged 6–10 years. In this sample she was able to classify 16 childrenas showing a distinct phonological dyslexic profile and only one as showingthe surface dyslexic profile. As a group, the dyslexic children showed thetypical deficits in phoneme awareness and nonword reading, although theyread exception words as well as the younger controls. However, the majorityshowed a pattern of reading that was normal for the reading level they hadreached.


M.J. Snowling42

An important issue therefore is, not what underpins different subtypes ofdyslexia, but how can variation in reading sub-skills be explained? In thisstudy, each dyslexic child was administered tasks tapping phonologicalawareness (rhyme production and phoneme deletion), phonological process-ing (nonword repetition, memory span and speech rate), visual memory (forabstract shapes) and perceptual speed (Wechsler Intelligence Scale for Chil-dren (WISC) Coding and Symbol Search). Performance on each of thesemeasures was then used to predict nonword reading in a series of multipleregressions. At the first step in these regressions, age, IQ and reading agewere entered as control variables. After the influence of these variables wastaken into account, phonological processing, perceptual speed and visualmemory all accounted for unique variance in the prediction of nonwordreading, but phonological awareness did not (its influence was subsumed bythat of phonological processing in the model).

These findings are in line with the predictions of the phonological deficithypothesis. The dyslexic children were impaired for their age on phonologi-cal processing tasks, but the extent of their difficulty varied in severity.Consistent with the ‘severity’ hypothesis (Snowling, Goulandris and Stack-house, 1994), those with better phonological skills had developed betterdecoding abilities than those with more severe phonological processingimpairments. These findings also highlight two possible sources of compen-sation for children; visual memory and perceptual speed. Children withbetter visual memory generally did better in nonword reading, as didchildren with slower speed of processing.

At first pass, these findings seem counterintuitive. However, it turns outthat children who gained the highest scores on the visual memory test werethose who could more readily assign ‘fictitious labels’ to the abstract stimuli.In the same way, we proposed these children were likely to have usedgeneral processing resources more readily in learning to read in the face ofa phonological deficit. The children with slow speed of processing weremore likely to do well in nonword reading (because they were apt to take apainstaking approach), but this did not stand them in good stead whenpresented with exception words, which they also tended to decode labori-ously and therefore to ‘regularize’.

Thus, it does not seem useful to classify dyslexic children into subtypesbecause all taxonomies leave a substantial number of children unclassified(cf. Castles and Coltheart, 1993). Rather, individual differences in phonolog-ical processing predict individual differences in nonword reading and, themore severe a child’s phonological deficit, the greater their impairment innonword reading. In contrast, Griffiths (1999) found that variation in excep-tion word reading is tied to reading experience, reflecting the fact that printexposure is required to learn about the inconsistencies of the English ortho-graphic system. While exception word reading builds on a foundation ofmappings between orthography and phonology, it is supported by seman-tics. Semantics is a relative strength, at least in some dyslexic children. Tothis extent, exception word reading may develop independently of decodingskill, forging a pattern of ‘phonological dyslexia’ at the behavioural level.



THE RECIPROCAL INFLUENCE OF READING ON PHONOLOGICALPROCESSING

We have seen that the phonological deficit view of dyslexia provides agood account of the range of behavioural symptoms of the disorder, and iscompatible with theories of normal literacy development. However, it hasbeen argued that literacy experience is crucial for the development ofphonological awareness (Morais et al., 1979) and there is some evidence thatexposure to alphabetic literacy may affect phonological processing moregenerally (Castro-Caldas et al., 1998). It follows that the phonologicaldeficits that are marked among dyslexic readers of English may be aconsequence as much as a cause of their failure to learn to read.

The best way to address this issue is to investigate the development ofdyslexic children before they fail to read. The only way of doing this is tofollow the progress of children at high risk of dyslexia before they go toschool, and then to conduct retrospective analyses to assess the validity ofthe phonological deficit hypothesis. In a pioneering study of this type,Scarborough (1990) followed the development of children of 2–7 years whowere ‘at risk’ of dyslexia by virtue of having one dyslexic parent. When thechildren were 7 and their reading skills could be assessed, it was possible tocompare retrospectively the pre-school data of children who went on tobecome dyslexic with children who did not develop reading difficulties. Animportant difference between the groups was in their early language skills.Although the dyslexic children used as large a range of vocabulary as theirnon-dyslexic counterparts at 2.5 years, they made more speech errors andtheir use of syntax was more limited. At 3 years the dyslexic children hadmore difficulty with object naming and at 5, their difficulties extended toproblems with phonological awareness. Their emerging literacy skills werealso poorer; they were less familiar with the letters of the alphabet andworse at matching pictures with print. Scarborough’s data are compatiblewith the phonological deficit hypothesis, but also suggest that the phono-logical problems, at least of familial dyslexics, may be less specific than isusually supposed, since language skills outside the phonological domainwere also affected.

In a study following Scarborough’s approach, we have been exploring theearly language precursors of dyslexia in a longitudinal study from 4 to 8years, of children at genetic risk by virtue of having a first degree affectedrelative (Gallagher, Frith and Snowling, 2000). Our preliminary findingsindicate that more than 60% of the high-risk group develop literacy prob-lems. Children with significant reading impairments at 8 years showed apattern of oral speech and language delay in the pre-school years andpoorly developed phonological awareness shortly after school entry. Inter-estingly, at an early stage in literacy development at the age of 6 years, bothgroups of high-risk children showed difficulty when compared to controlson tasks requiring the use of letter-sound relationships, namely in nonwordreading and in phonetic spelling processes. This difficulty applied equallyto those who did not go on to develop reading problems as well as to thosewho did.


M.J. Snowling44

Whilst the pattern of poorly developed decoding skills was to be ex-pected for the dyslexic children, it is intriguing that the unaffected childrenwere also impaired in their use of the ‘phonological’ pathway. These find-ings suggest that the inherited tendency in dyslexia may be a slowness inestablishing mappings between orthography and phonology. Since thisgroup was indistinguishable from the control group in early language andmetaphonological development, it is interesting to speculate that they wenton to avoid reading problems by drawing on their strong general languageresources—an early form of compensation. It remains an issue whetherthese unaffected children will develop literacy problems at a later stage intheir development, for instance, in developing reading fluency or in show-ing spelling problems. According to Miles and Miles (1990, p. ix) ‘there isno contradiction in saying that a person is dyslexic while nevertheless beinga competent reader’. Arguably, these at risk readers provide a good testcase.

CONCLUSIONS

This paper has considered three different challenges to the view that thecause of dyslexia, at the cognitive level, is an impairment at the level ofphonological representations. This theory provides a parsimonious accountof a wide range of symptoms of dyslexia in childhood and through to theadult years (Snowling et al., 1997). The paper rejects the view that thephonological deficits in dyslexia stem from more basic auditory processingproblems and argues instead, that the phonological deficits in dyslexia are amanifestation of a language weakness. There is still a considerable amountof work to be done both to specify how phonological skills relate to orallanguage skills (Carroll and Snowling, in press) and to specify the nature ofphonological representations (Griffiths and Snowling, in press). Impor-tantly, the severity of a child’s phonological processing deficit and theintegrity of their other language processes predicts how well they will learnto read (Nation and Snowling, 1998), and individual differences in theirprofile of reading and spelling.

More generally, the findings of the at risk study reported here are in linewith a biological explanation of dyslexia. In fact, the severity of the readingimpairment experienced by the high-risk children was directly proportionalto the severity (and persistence) of their parents’ literacy problems. The firstbehavioural manifestation of dyslexia among 4-year-olds was poor letterknowledge. In turn, letter knowledge and speech development drovephonological awareness at 6, and this was an important predictor of laterword-level reading and decoding skills. These findings are compatible withthe phonological deficit hypothesis and confirm that children who come tothe task of reading with poorly specified phonological representations goon to have reading problems. However, the study also shows that childrenwith a smaller ‘dose’ of dyslexia may be able to compensate for impair-ments of grapheme–phoneme knowledge observed early in their literacydevelopment, by virtue of stronger general language resources.



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