Is There a Visual Dyslexia? R G. Aaron Indiana State University Terre Haute, Indiana
Traditionally, it has been speculated that weaknesses in the visual processing of cognitive aspects of the written word could lead to reading problems, and if so, such a condition would constitute a subtype of developmental dyslexia. This putative subtype has been referred to as visual dyslexia. In this article, the role of cognitive deficits that are visual in nature as a potential etiological factor of developmental dyslexia is examined. Following a brief history of the study of dyslexia, a critique of studies of visual dyslexia is presented. Subsequently, the nature of the visual processes involved in word-recognition is examined. Finally, three research studies that assessed the contribution of visual memory to word-recognition are presented. It is concluded that, even though defects in the physiological aspects of visual processing can lead to reading difficulties, at present little convincing evidence is available to conclude that a subtype of dyslexia caused by cognitive deficits associated with visual processing of information exists.
Introduction
When developmental reading disability was recognized as a clinical syndrome almost one hundred years ago, a language related etiology was not suspected; instead, some defect in the visual memory for words was thought to be the cause of the disability. For instance Hinshelwood (1895), the pioneer, attributed developmental reading disAnnals of Dyslexia, Vol. 43, 1993. Copyright 9 1993by The Orton DyslexiaSociety ISSN 0736-9387 110
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ability to an underdevelopment of visual memory center and called it congenital word-blindness. Approaching the problem from yet another neuropsychological perspective, Orton (1937) considered dyslexia to be caused by a conflict of "engrams" located in the two cerebral hemispheres. Understandably, a vision-based etiology of reading disability is a reasonable proposition for, after all, reading involves visual language whereas speech involves aural language. Despite its intuitive appeal, the "visual etiology" of reading disability failed to gain universal acceptance even during these early days. For instance, educators such as Burr (1921) and Gates (1929) attributed reading disability to nonvisual factors such as poor word analysis skill and weak phonetic skills. This disagreement inevitably led to the consideration that there may be more than one causal factor of reading disability. Eventually, this culminated in a formal proposition by some researchers that developmental dyslexia is not a homogenous deficit but consists of subtypes. Reasoning along these lines, Johnson and Myklebust (1967) proposed that there are two subtypes: visual dyslexia and auditory dyslexia. Boder's (1973) dyseidetic and dysphonetic subtypes also roughly correspond to the visual and auditory subtypes, respectively. The belief that visual factors can play an etiological role in dyslexia, however, has been seriously challenged by Vellutino (1979), who, on the basis of findings of experimental studies, argued that disordered language processing, and not a visual-perceptual deficit, is the main cause of reading disability. The importance of phonology as a causal factor in dyslexia has been further buttressed by the numerous studies which show that young dyslexic children are deficient in phonological skills (Bradley and Bryant 1983, 1985; Liberman and Shankweiler 1985) and that training in phonological awareness improves the reading skills of these poor readers (Bradley and Bryant 1985; Lundberg, Frost, and Peterson 1988). In light of these research findings, it would appear that the issue of visual dyslexia had been settled once and for all. This, however, is not the case. Over a decade ago, Stein and Fowler (1980) reported that dyslexic children they had studied showed signs of impaired vergence control, unstable binocular fixation, and inaccurate visual direction sense. More recently, Gjessing and Karlsen (1989), on the basis of their longitudinal study of a large number of dyslexic children, claimed that they have identified a group of poor readers whom they classified as belonging to the visual subtype. Within the last few years, visionrelated problems such as scotopic sensitivity and abnormalities in the neurons of the parvo and magnocellular systems of the lateral geniculate nuclei of the visual pathways have also been implicated in developmental dyslexia. On the basis of their experimental studies, Lovegrove, Martin, and Slaghuis (1986) have argued that a slow transient visual
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system of the dyslexic reader impedes the efficient coordination of early visual input and the phonological processes. Evidence obtained from evoked potential data and anatomical studies led Livingstone et al. (1991) to suggest that a weak magnocellular system (which is part of the transient visual system), may account for a slow processing of visual information by dyslexic subjects under certain conditions. Thus, after a few years of hiatus, the issue of visual dyslexia has been recently resurrected. The issue whether developmental dyslexia is a homogenous entity or consists of two major subtypes is not a trivial one because the two subtypes may call for drastically different remedial approaches. Clearly, phonological awareness training, which has been remarkably successful in improving the reading skills of poor readers with phonological deficits, will not be appropriate for children whose reading disability is entirely visual in origin. The resolution of the issue of visual dyslexia has, therefore, important educational implications. In the present article, a critique of the major studies that have concluded that a visual dyslexia subtype exists is presented first; subsequently, the nature of the visual processes involved in word recognition is analyzed; finally, research studies carried out by the author to assess the contribution of visual memory to word recognition are presented. A Critique of Studies of Visual Dyslexia There are two important issues that need to be addressed and resolved by studies that claim that dyslexia can be caused by visual problems; one is definitional, the other is methodological. Developmental dyslexia is traditionally defined as a cognitive problem; definitions of developmental dyslexia, have, therefore, explicitly excluded deficits in sensory processes as causative agents of developmental dyslexia. Putative factors such as myopia, scotopic sensitivity, and abnormalities of the parvo and magnocellular systems are sensory phenomena; if these factors affect cognitive functions, such defects are secondary to abnormal sensory processes and are not cognitive dysfunctions per se. By definition, therefore, reading problems caused by sensory defects cannot be included in the category of dyslexia. It may be possible that sensory and cognitive processes interact to produce the condition of dyslexia; if so, developmental dyslexia has to be redefined to embrace such a possibility. The strength of this argument becomes all the more evident when it is recognized that dyslexia is a syndrome with problems that are manifest both in receptive and expressive aspects of the written language (Aaron 1989). For example, it is known that development dys-
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lexics, in addition to being slow and erratic readers, also have deficits in output functions such as spelling and the correct use of syntax in the written language. While sensory deficits may be able to explain away reading problems seen in the input stage, sensory processes cannot satisfactorily account for output deficits such as poor spelling and defective syntax. Furthermore, dyslexic subjects have difficulty not only in processing a sequence of stimuli (such as words in a sentence) that is to be acted u p o n in quick temporal succession, as a weak magnocellular transient system or lateral masking effects would posit, but also in processing isolated words (see Hulme 1988) whose constituent letters are processed in parallel. The methodological problem that has to be addressed pertains to the criteria and tests that have been used for identifying visual dyslexia. Investigators have traditionally used criteria such as reversals in writing (Gjessing and Karlsen 1989) and spelling patterns (Boder 1973) for identifying visual dyslexia; tests that have been frequently used for this purpose include tests of visual perception such as the Bender Gestalt (Johnson and Myklebust 1967) and tests of visual memory such as the Benton Visual Retention Test (ScheviU 1978). At present, there is a general consensus that criteria such as letter and word reversals in reading and writing are not reliable criteria of developmental dyslexia. In spite of having coined the term strephosymbolia, even Orton (1937) recognized that reversals in writing and reading are not invariant symptoms of dyslexia. This conclusion has also been experimentally documented by Liberman et al. (1982). Furthermore, reversals are hardly ever seen in the writings of mature dyslexic subjects such as college students (Aaron and Phillips 1986) which indicates that reversals are at best inconsistently associated with dyslexia. As noted earlier, Boder (1973) labelled individuals w h o committed phonologically acceptable spelling errors (girl --- gal) dyseidetics and attributed their spelling errors to poor visual memory. Individuals who committed phonologically unacceptable errors (girl = gril) were called dysphonetics and were thought to have phonological weakness. It turns out, however, that these two types of spelling errors do not represent two subtypes but two substages of grapheme-phoneme mastery, dysphonetic errors indicating a stage prior to the dyseidetic stage. This interpretation is substantiated by the observation that mature dyslexic students very seldom commit phonologically unacceptable spelling errors in any significant number. Additional evidence comes from a longitudinal study (Phillips, Taylor, and Aaron 1985) in which it was found that younger poor readers produced phonologically unacceptable spelling errors whereas older poor readers produced more phonologically acceptable errors than unacceptable ones. As far as tests are concerned, it is known that visual-perceptual tests such as the Bender Gestalt have a
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language component and that performance on these tests cannot be attributed solely to visual-perceptual problems (e.g., Silberberg and Feldt 1968). When these factors are taken into consideration, the case for the existence of visual dyslexia built on findings of visuo-perceptual tests and symptoms such as reversals and spelling error patterns is considereably weakened. In recent years, visual cognition has become a major topic of study by cognitive psychologists. Within the field of visual cognition, considerable attention has been given to object recognition. Findings of studies that have explored visual cognition can, therefore, shed light on visual word recognition directly and on "visual dyslexia" indirectly. In the next section, the nature of the cognitive processes associated with visual word-recognition is examined.
Visual Processes and Word Recognition Several models of word recognition have been proposed by cognitive psychologists, but visual long-term memory (LTM, hereafter) is a common denominator of all these models. For instance, the "template matching model" proposes that the retinal image of a word is transmitted to the brain and is compared with various stored patterns. The "feature analysis model" holds that the input stimuli are processed as a combination of features which are then compared to a stored list of features; in the "Fourier model," the stimulus is decomposed into components and then pattern recognition is carried out by matching these Fourier transforms against stored memory transforms. The "structural description model" holds that the stimuli are factored out for relevant cognitive features which are then matched to stored structures and propositions. In a recent review article, Treisman (1992) proposes that object identification is accomplished by matching the visual input to a set of stored descriptions about that object. Thus, longterm visual memory emerges as an important factor in visual recognition process. Several investigators (e.g., Anderson 1990; Farah 1988) have proposed that visual memory is made up of two components: memory for spatial representations and memory for visual details. In common parlance, these are referred to as the where system and the what systems, respectively (Farah 1988). Both animal studies and neuropsychological observation of human beings support this two-componential view of visual LTM. For instance, Ungerleider and Mishkin (1982) found that monkeys with lesions in parietal lobes were grossly impaired on tasks that assess spatial relations skill, but performed normally in visual discrimination tasks. In contrast, monkeys with lesions to inferior temporal cortex were unimpaired on the spatial tasks, but were impaired
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in learning to discriminate between different forms and patterns. In h u m a n beings, Potzl (1928) and Lange (1936) noted that patients with posterior cortical lesions displayed symptoms of visual agnosias whereas they were unimpaired in their spatial localization ability. Recently, Farah et al. (1988) have described a case of a brain-damaged college student w h o showed a similar pattern of performance. This 36year-old subject had sustained injury to the temporal lobe region some 18 years ago in an automobile accident. At the time of testing, his verbal IQ was 132 but his performance IQ was only 93. Tests revealed that he could not recognize pictures of many common objects and could not reliably identify his wife or children. He could, however, copy pictures of these objects and correctly locate states on the map, better than many college students. Because m e m o r y for spatial representations and memory for visual details are two components of the long-term visual memory, it is plausible that visual dyslexia, if it exists, could be caused by defects in either of these two components. For this reason, memory for spatial represenations and m e m o r y for details are examined to see if these two forms of memories can be potential etiological factors in dyslexia.
Visual Memory for Spatial Representations It appears that information regarding spatial location and right-left directionality is not stored in LTM in a picture-like format, but is retained most likely in the form of semantic structures and propositions. For example, many subjects fail to answer correctly questions such as "Which city is farther west, San Diego or Reno, Nevada"?; or "Which is farther north, Montreal or Seattle"? This suggests that our visual LTM of the U.S. map is not picture-like, off which one can read, but is in a form which is the product of an integration of visual information, semantic concepts and verbal propositions. In one study, Gernsbacher (1985) showed subjects pictures one at a time, and after a ten minute interval tested them with the original picture and a reversed form of the picture. The subjects had to tell which one of the two pictures they had seen earlier. Only 57 percent of the responses (which is barely above chance level) were correct indicating that information about pictures is stored in LTM without reference to the left-right orientation. Spatial information need not even be exclusively the propriety of visual modality. For instance, w h e n mental rotation tasks such as the ones designed by Shepard and Metzler (1971) were presented to congenitally blind subjects as a tactile task, it was found that blind subjects could perform the tasks successfully, and that their reaction time pattern was similar to that of sighted subjects (Carpenter and Eisenberg 1978). These studies indicate that spatial memory may not be in a picture-like format but may be present in an abstract representational
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form sustained by semantic, phonological, and tactile props. For this reason, in order to classify an individual as "visual dyslexic," it becomes essential to establish that his/her semantic and phonological systems are intact and that only the visual memory is defective. Studies that have controlled for semantic and phonological factors and have tested pure visual memory by using nonsense figures have failed to secure evidence for the presence of weak visual memory in subjects with reading disability (e.g., Liberman and Shankweiler 1978; Vellutino et al. 1975). Visual Memory for Details
Visual memory for details, similar to spatial memory, also appears not to be in a picture-like format, but is stored as analogs or abstract representations of objects. This lack of specificity for details may be the reason why we find it hard to spell words backwards even though we can spell the same words correctly and effortlessly forwards. Reed (1974) presented complex geometric forms such as the Star of David to subjects to test the nature of visual memory for details. Subsequent to the presentation of each form, the subject was shown parts of the original form and was asked if the part came from the original stimulus. While familiar parts such as the triangle were recognized most of the time, u n c o m m o n parts such as the parallelogram were correctly identified only 10 percent of the time. Mandler and Ritchey (1977) showed pictures such as a classroom scene to subjects and followed each picture by two test pictures. The subject had to tell if each test picture was the same as the one seen before. One of the two test pictures contained a "type distractor" such as having a tea-pot on the desk instead of the globe in the original picture. Not surprisingly, the pictures that contained the "type distractors" were correctly rejected most of the time (94 percent) whereas pictures that contained "token distractors," such as a change in the teacher's dress, were erroneously accepted as the original picture most of the time. The investigators concluded that we tend to remember the meaning of pictures rather than visual details. If these observations are extended to visual memory for words, it would appear that memory for orthography (spelling patterns) is not purely visual in nature but is supported by semantic an d phonological features of words. In fact, orthography is defined as the visual pattern of the written language as it relates to the graphemic, phonological, and semantic features of the language (Henderson 1984). Ehri and Wilce (1980) suggest that the orthographic representation may not be stored as rote-memorized visual images but as sequences bearing systematic relationships to acoustic and/or articulatory segments detected in the word's phonological identity. Furthermore, visual memory ca-
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pacity for orthographic representation appears to be limited to only two or three items (Zhang and Simon 1985) thus limiting the influence of visual memory on reading and spelling. In addition, the variance seen in reading ability associated with orthographic memory has been found to be primarily a product of reading experience whereas phonological skill has a sizeable heritability (Olson et al. 1989; Stanovich and West 1989). Reading failure that is attributable to a lack of experience is not generally considered reading disability. This brief review indicates that visual memory in isolation, as compared to phonology and semantics, plays a relatively limited role in word recognition. The following research study was conducted to assess more precisely the extent of the contribution made by visual memory toward word recognition.
Relative Contribution of Visual Memory towards Word Recognition Study 1 Procedure A series of four word-recognition tasks was administered to 124 children (66 gifts and 58 boys) from grades 2 through 5. These children were in regular classrooms and, therefore, are representative of the general population which includes a few children with reading disabilities. Each word-recognition task consisted of a list of 12 target words and the test list of 12 pairs of words of which one word was the target word. Each subject was asked to read the 12 target words silently and then select from the 12 pairs of test words, the one he/she had seen before. The tasks were individually administered in a predetermined random order. The target words in task I consisted of nonhomophonic content words and the test consisted of pairs of words of similar kind (e.g., target word = cat; test words = dog, cat). It is assumed that the target word can be correctly identified by relying on phonologic, semantic, as well as visual features of the target words and, consequently, presented an optimal condition for word recognition. Performance on this task was, therefore, taken as a measure of word-recognition skill of the subject and was used as the dependent variable for statistical analysis of data. The target stimuli in task 2 consisted of nonwords; the test words were homophonic nonword pairs (e.g., dore - doar, dore). It is assumed that this task cannot be performed successfully by relying on meaning because they are nonwords; the target cannot be correctly identified by relying on phonology because the test words are homophones. Successful performance on this task, therefore, depends almost entirely on visual memory for the target words. Stimuli in task 3 were "grammar" words and the test words
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were also pairs of grammar words (e.g., has - has, let). This task can be performed successfully by utilizing phonology as well as visual memory for words, but not by relying on word meaning. Task 4 consisted of content words and their homophones (e.g., sea - see, sea). This task can be successfully carried out by utilizing visual and semantic features of the word but not by relying on the phonological features of the word. These four lists of words therefore, can be used to partial out the relative contribution made by visual, phonological, and semantic processes towards word recognition. Results The data were analyzed in order to determine the degree of variance each one of the variables (viz., visual memory, phonology, and semantics) contributed to word recognition. A stepwise regression analysis was carried out with performance on task 1 as the dependent variable. Regression analysis showed that phonological feature of the word made the greatest contribution to word recognition with a multiple R of .23. The change it made in the prediction of word recognition skill was significant at .01 level. Word meaning (semantics) was the next highest contributor; the change in predicting word recognition was significant at the .04 level. Visual memory made little or no contribution to word recognition and the change it made in word recognition was not significant. On the basis of these data, it is concluded that amongst the three variables examined; visual memory for words accounts for the least amount of variance seen in word recognition skill; phonology accounts for the greatest amount of variance, and word meaning comes next. Study 2 Procedure Children from grades 3 and 4 in the above study who performed at or below chance level (i.e., a score of 6 or lower) on task 2 (visual memory) but had a score above the group mean on task 3 (phonology) were identified. Similarly, children who obtained a score of 6 or lower on task 3 (phonology) but a higher than mean score on task 2 (visual memory) were also identified. Thus, the first group of subjects had poor visual memory but adequate phonological memory; the second group had adequate visual memory but poor memory for phonology. Using these criteria, seven children (five boys and two girls) were identified as having poor visual memory but adequate phonological memory; two boys were identified as having poor phonological memory but adequate visual memory. The small number of subjects is due to the fact that many children with low word recognition scores performed poorly in both visual memory and phonological memory tasks. The reading scores of these children on the Indiana Statewide Testing for Educational Progress (ISTEP) were obtained from their school records. The ISTEP also provides measures of language ability and vocabulary.
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Table I Performance of Children with Low Visual/Normal Phonology Memory Scores and That of Children with Low Phonology/Normal Visual-memory Scores
Children with Low Visual Memory for Words Reading Student Scores on Number Task1 Task2 Task3 Task4 Vocab. Comp. Lang. 1. 2. 3. 4. 5. 6. 7.
6.0 5.0 4.0 5.0 5.0 7.0 5.0
9.0 9.0 10.0 9.0 10.0 11.0 9.0
9.0 9.0 6.0 4.0 7.0 12.0 11.0
11.0 9.0 11.0 8.0 8.0 10.0 8.0
CSI (IQ)
45% 40% 15% 26% 10% 18% 46%
28% 28% 10% 20% 40% 49% 28%
40% 14% 27% 11% 36% 46% 39%
83 89 87 80 82 91 --
Children with Low PhonologicalMemory for Words 1. 11.0 5.0 9.0 11.0 33% 2. 8.0 6.0 9.0 9.0 42%
56% 37%
55% 53%
117 105
Their scores on the Cognitive Skills Test, also referred to as Cognitive Skills Index (CSI), was also secured from this source. The CSI is often regarded as equivalent to an IQ score. Results Data presented in Table 1 show that children with poor visual memory for words are poor readers. However, they also have low vocabulary and language scores in addition to below-average IQ. The two children with adequate visual memory but poor phonological skills are also deficient in reading skills. However, unlike the seven children in the first group, their language skill as well as their IQ are within the normal range. These two children, because of their normal IQ, conform to the traditional definition of dyslexia. These results suggest that poor visual memory may be associated with poor reading skill, but it does not affect the reading process as an independent agent. Poor visual memory appears to be one aspect of a generalized cognitive weakness. Study 3
Procedure This study utilized two groups of children to collect data. The first group consisted of 31 children (17 girls and 14 boys) from grade 2 taken from the larger group of 124 children described in study 1. These 31 children provided the normative data against which the performance of children in the second group was compared. The sec-
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ond group consisted of eight boys w h o had been referred to the Psychology clinic and had been diagnosed as dyslexic. At the time of testing seven of them were attending the second grade; the eighth subject was in the third grade. These two groups of children were administered five different tasks which constituted a locally developed modified Stroop task. The intent of this testing was to determine how fast dyslexic children process verbal and nonverbal visual information and to see if they differ from an unselected group of second graders in this respect. Task I required each child to name the color of 20 patches as quickly as possible. Task 2 and 3 required each child to read aloud, as quickly as possible, a list of 20 high frequency grammar words and a list of 20 common content words. The words in these two lists were printed in different colors and were matched for frequency and syllable length. In tasks 4 and 5, the child was presented with the same lists of words in tasks 2 and 3 but was instructed to tell the name of the color of each printed word. Before administering these tasks, each child was given practice items. All these tasks were administered individually and children's responses were taped. The time it took to perform each task was the dependent variable in all these tasks. The data obtained on these tasks are shown in Table 2, along with other pertinent information. Results The performance of these two groups of children on these five tasks leads to the following conclusions. Dyslexic children are as fast as children from grade 2 in naming the colors of patches, the difference between these two groups being nonsignificant (t = .38, p < .70). Dyslexic children are, however, slower than children in the control group in reading both content words and function words and the differences are significant (t = 2.20, p < .03; t = 2.24, p < .03). These data indicate that dyslexic children are slow in processing only written verbal stimuli and are not slow in processing nonverbal visual stimuli. Children from the normative group, as well as the dyslexic group, take longer time to read grammar words compared to content words even though this difference is more pronounced for the dyslexic group. Dyslexic children as well as children in the control group show interference due to the Stroop effect. However, in the case of the control group, the Stroop interference effect was equal for both content word list and grammar word list (t = .39, p < .70). In contrast, dyslexic subjects show greater interference effect when they named the colors of content words than when they named the colors of grammar words, the difference being significant (t = 3.18, p > .01). This can be interpreted to mean that in the case of dyslexic children, word meaning interferes more than word pronunciation with naming colors. Such a differential effect is not seen in the control group.
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Conclusions
The results of these studies suggest that compared to the phonological and semantic features of the written word, visual memory for words plays a relatively insignificant role in word-recognition. Investigations described here also show that dyslexic children can process nonverbal visual information as fast as normal readers do. It appears, therefore, that some non-visual feature associated with the written language is responsible for the slow word-recognition of a majority of dyslexic readers. The finding that dyslexic children are much slower in processing grammar words than content words indicates that word meaning plays an important role in their word-processing behavior. The observation that dyslexic children take longer time to name the colors of content words than grammar words supports this view. If visual memory plays a contributory role, one would expect the RT for both grammar and content words to be similar. This, however, is not the case for the dyslexic children, as a group. Taken in conjunction with the findings of experimental studies from cognitive psychology, the results of the present investigation suggest that, at present, there is no good reason to conclude that a subtype of developmental dyslexia, caused by a weakness in visual memory, exists. The results presented in this paper, of course, do not prove that there is no visual dyslexia subtype. To say so would be tantamount to proving a null hypothesis. Rather, the burden of proof is shifted to those who advocate the existence of a visual dyslexia subtype and to those who implicate visual processes in reading disability. Such proof would include evidence of weak visual processes in the presence of intact phonological and semantic skills in dyslexic readers. References Aaron, P. G. 1989. Dyslexia and Hyperlexia. Boston, MA: Kluwer Academics. Aaron, P. G. and Phillips, S. 1986. A decade of research with dyslexic college students. Annals of Dyslexia 36:44-66. Anderson, J. R. 1990. Cognitive Psychology and its Implications. New York: Freeman and Co. Boder, E. 1973. A diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15:663-687. Bradley, L. and Bryant, E 1983. Categorizing sounds and learning to r e a d - - A causal connection. Nature 301:419-421. Bradley, L. and Bryant, P. 1985. Rhyme and Reason in Reading and Spelling. Ann Arbor, MI.: Univ. of Michigan Press. Burt, C. 1921. Mental and Scholastic Tests. London: P. S. King and Son. Carpenter, P., and Eisenberg, P. 1978. Mental rotation and the frame of reference in blind and sighted individuals. Perception and Psychophysics 23:117-124.
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Shepard, R. N., and Metzler, J. 1971. Mental rotation of three-dimensional objects. Science 171:701-703. Silberberg, N., and Feldt, L. 1968. Intellectual and perceptual correlates of reading difficulties. Journal of School Psychology 6:237-245. Stanovich, K. and West, R. 1989. Exposure to print and orthographic processing. Reading Research Quarterly 24:402-433. Stein, J. E, and Fowler, M. 1980. Visual dyslexia. British Orthoptic Journal 37:11-17. Treisman, A. 1992. Perceiving and re-perceiving objects. American Psychologist 47: 862-875. Ungerleider, L., and Mishkin, M. 1982. Two cortical visual systems. In D. J. Ingle, M. A. Goodale, and R. W. Mansfield (eds.). Analysis of Visual Behavior. Cambridge, MA: The MIT Press. VeUutino, F. R. 1979. Dyslexia: Theory and Research. Cambridge, MA.: The MIT Press. Vellutino, F. R., Steager, J., DeSetto, L., and Phillips, E 1975. Immediate and delayed recognition of visual stimuli in poor and normal readers. Journal of Experimental Child Psychology 19:223-232. Zhang, G. and Simon, H. 1985. STM capacity for Chinese words and idioms: Perception and Psychophysics 36:277-284.
Traditionally, it has been speculated that weaknesses in the visual processing of cognitive aspects of the written word could lead to reading problems, and if so, such a condition would constitute a subtype of developmental dyslexia. This putative subtype has been referred to as visual dyslexia. In this article, the role of cognitive deficits that are visual in nature as a potential etiological factor of developmental dyslexia is examined. Following a brief history of the study of dyslexia, a critique of studies of visual dyslexia is presented. Subsequently, the nature of the visual processes involved in word-recognition is examined. Finally, three research studies that assessed the contribution of visual memory to word-recognition are presented. It is concluded that, even though defects in the physiological aspects of visual processing can lead to reading difficulties, at present little convincing evidence is available to conclude that a subtype of dyslexia caused by cognitive deficits associated with visual processing of information exists.
Introduction
When developmental reading disability was recognized as a clinical syndrome almost one hundred years ago, a language related etiology was not suspected; instead, some defect in the visual memory for words was thought to be the cause of the disability. For instance Hinshelwood (1895), the pioneer, attributed developmental reading disAnnals of Dyslexia, Vol. 43, 1993. Copyright 9 1993by The Orton DyslexiaSociety ISSN 0736-9387 110
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ability to an underdevelopment of visual memory center and called it congenital word-blindness. Approaching the problem from yet another neuropsychological perspective, Orton (1937) considered dyslexia to be caused by a conflict of "engrams" located in the two cerebral hemispheres. Understandably, a vision-based etiology of reading disability is a reasonable proposition for, after all, reading involves visual language whereas speech involves aural language. Despite its intuitive appeal, the "visual etiology" of reading disability failed to gain universal acceptance even during these early days. For instance, educators such as Burr (1921) and Gates (1929) attributed reading disability to nonvisual factors such as poor word analysis skill and weak phonetic skills. This disagreement inevitably led to the consideration that there may be more than one causal factor of reading disability. Eventually, this culminated in a formal proposition by some researchers that developmental dyslexia is not a homogenous deficit but consists of subtypes. Reasoning along these lines, Johnson and Myklebust (1967) proposed that there are two subtypes: visual dyslexia and auditory dyslexia. Boder's (1973) dyseidetic and dysphonetic subtypes also roughly correspond to the visual and auditory subtypes, respectively. The belief that visual factors can play an etiological role in dyslexia, however, has been seriously challenged by Vellutino (1979), who, on the basis of findings of experimental studies, argued that disordered language processing, and not a visual-perceptual deficit, is the main cause of reading disability. The importance of phonology as a causal factor in dyslexia has been further buttressed by the numerous studies which show that young dyslexic children are deficient in phonological skills (Bradley and Bryant 1983, 1985; Liberman and Shankweiler 1985) and that training in phonological awareness improves the reading skills of these poor readers (Bradley and Bryant 1985; Lundberg, Frost, and Peterson 1988). In light of these research findings, it would appear that the issue of visual dyslexia had been settled once and for all. This, however, is not the case. Over a decade ago, Stein and Fowler (1980) reported that dyslexic children they had studied showed signs of impaired vergence control, unstable binocular fixation, and inaccurate visual direction sense. More recently, Gjessing and Karlsen (1989), on the basis of their longitudinal study of a large number of dyslexic children, claimed that they have identified a group of poor readers whom they classified as belonging to the visual subtype. Within the last few years, visionrelated problems such as scotopic sensitivity and abnormalities in the neurons of the parvo and magnocellular systems of the lateral geniculate nuclei of the visual pathways have also been implicated in developmental dyslexia. On the basis of their experimental studies, Lovegrove, Martin, and Slaghuis (1986) have argued that a slow transient visual
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system of the dyslexic reader impedes the efficient coordination of early visual input and the phonological processes. Evidence obtained from evoked potential data and anatomical studies led Livingstone et al. (1991) to suggest that a weak magnocellular system (which is part of the transient visual system), may account for a slow processing of visual information by dyslexic subjects under certain conditions. Thus, after a few years of hiatus, the issue of visual dyslexia has been recently resurrected. The issue whether developmental dyslexia is a homogenous entity or consists of two major subtypes is not a trivial one because the two subtypes may call for drastically different remedial approaches. Clearly, phonological awareness training, which has been remarkably successful in improving the reading skills of poor readers with phonological deficits, will not be appropriate for children whose reading disability is entirely visual in origin. The resolution of the issue of visual dyslexia has, therefore, important educational implications. In the present article, a critique of the major studies that have concluded that a visual dyslexia subtype exists is presented first; subsequently, the nature of the visual processes involved in word recognition is analyzed; finally, research studies carried out by the author to assess the contribution of visual memory to word recognition are presented. A Critique of Studies of Visual Dyslexia There are two important issues that need to be addressed and resolved by studies that claim that dyslexia can be caused by visual problems; one is definitional, the other is methodological. Developmental dyslexia is traditionally defined as a cognitive problem; definitions of developmental dyslexia, have, therefore, explicitly excluded deficits in sensory processes as causative agents of developmental dyslexia. Putative factors such as myopia, scotopic sensitivity, and abnormalities of the parvo and magnocellular systems are sensory phenomena; if these factors affect cognitive functions, such defects are secondary to abnormal sensory processes and are not cognitive dysfunctions per se. By definition, therefore, reading problems caused by sensory defects cannot be included in the category of dyslexia. It may be possible that sensory and cognitive processes interact to produce the condition of dyslexia; if so, developmental dyslexia has to be redefined to embrace such a possibility. The strength of this argument becomes all the more evident when it is recognized that dyslexia is a syndrome with problems that are manifest both in receptive and expressive aspects of the written language (Aaron 1989). For example, it is known that development dys-
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lexics, in addition to being slow and erratic readers, also have deficits in output functions such as spelling and the correct use of syntax in the written language. While sensory deficits may be able to explain away reading problems seen in the input stage, sensory processes cannot satisfactorily account for output deficits such as poor spelling and defective syntax. Furthermore, dyslexic subjects have difficulty not only in processing a sequence of stimuli (such as words in a sentence) that is to be acted u p o n in quick temporal succession, as a weak magnocellular transient system or lateral masking effects would posit, but also in processing isolated words (see Hulme 1988) whose constituent letters are processed in parallel. The methodological problem that has to be addressed pertains to the criteria and tests that have been used for identifying visual dyslexia. Investigators have traditionally used criteria such as reversals in writing (Gjessing and Karlsen 1989) and spelling patterns (Boder 1973) for identifying visual dyslexia; tests that have been frequently used for this purpose include tests of visual perception such as the Bender Gestalt (Johnson and Myklebust 1967) and tests of visual memory such as the Benton Visual Retention Test (ScheviU 1978). At present, there is a general consensus that criteria such as letter and word reversals in reading and writing are not reliable criteria of developmental dyslexia. In spite of having coined the term strephosymbolia, even Orton (1937) recognized that reversals in writing and reading are not invariant symptoms of dyslexia. This conclusion has also been experimentally documented by Liberman et al. (1982). Furthermore, reversals are hardly ever seen in the writings of mature dyslexic subjects such as college students (Aaron and Phillips 1986) which indicates that reversals are at best inconsistently associated with dyslexia. As noted earlier, Boder (1973) labelled individuals w h o committed phonologically acceptable spelling errors (girl --- gal) dyseidetics and attributed their spelling errors to poor visual memory. Individuals who committed phonologically unacceptable errors (girl = gril) were called dysphonetics and were thought to have phonological weakness. It turns out, however, that these two types of spelling errors do not represent two subtypes but two substages of grapheme-phoneme mastery, dysphonetic errors indicating a stage prior to the dyseidetic stage. This interpretation is substantiated by the observation that mature dyslexic students very seldom commit phonologically unacceptable spelling errors in any significant number. Additional evidence comes from a longitudinal study (Phillips, Taylor, and Aaron 1985) in which it was found that younger poor readers produced phonologically unacceptable spelling errors whereas older poor readers produced more phonologically acceptable errors than unacceptable ones. As far as tests are concerned, it is known that visual-perceptual tests such as the Bender Gestalt have a
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language component and that performance on these tests cannot be attributed solely to visual-perceptual problems (e.g., Silberberg and Feldt 1968). When these factors are taken into consideration, the case for the existence of visual dyslexia built on findings of visuo-perceptual tests and symptoms such as reversals and spelling error patterns is considereably weakened. In recent years, visual cognition has become a major topic of study by cognitive psychologists. Within the field of visual cognition, considerable attention has been given to object recognition. Findings of studies that have explored visual cognition can, therefore, shed light on visual word recognition directly and on "visual dyslexia" indirectly. In the next section, the nature of the cognitive processes associated with visual word-recognition is examined.
Visual Processes and Word Recognition Several models of word recognition have been proposed by cognitive psychologists, but visual long-term memory (LTM, hereafter) is a common denominator of all these models. For instance, the "template matching model" proposes that the retinal image of a word is transmitted to the brain and is compared with various stored patterns. The "feature analysis model" holds that the input stimuli are processed as a combination of features which are then compared to a stored list of features; in the "Fourier model," the stimulus is decomposed into components and then pattern recognition is carried out by matching these Fourier transforms against stored memory transforms. The "structural description model" holds that the stimuli are factored out for relevant cognitive features which are then matched to stored structures and propositions. In a recent review article, Treisman (1992) proposes that object identification is accomplished by matching the visual input to a set of stored descriptions about that object. Thus, longterm visual memory emerges as an important factor in visual recognition process. Several investigators (e.g., Anderson 1990; Farah 1988) have proposed that visual memory is made up of two components: memory for spatial representations and memory for visual details. In common parlance, these are referred to as the where system and the what systems, respectively (Farah 1988). Both animal studies and neuropsychological observation of human beings support this two-componential view of visual LTM. For instance, Ungerleider and Mishkin (1982) found that monkeys with lesions in parietal lobes were grossly impaired on tasks that assess spatial relations skill, but performed normally in visual discrimination tasks. In contrast, monkeys with lesions to inferior temporal cortex were unimpaired on the spatial tasks, but were impaired
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in learning to discriminate between different forms and patterns. In h u m a n beings, Potzl (1928) and Lange (1936) noted that patients with posterior cortical lesions displayed symptoms of visual agnosias whereas they were unimpaired in their spatial localization ability. Recently, Farah et al. (1988) have described a case of a brain-damaged college student w h o showed a similar pattern of performance. This 36year-old subject had sustained injury to the temporal lobe region some 18 years ago in an automobile accident. At the time of testing, his verbal IQ was 132 but his performance IQ was only 93. Tests revealed that he could not recognize pictures of many common objects and could not reliably identify his wife or children. He could, however, copy pictures of these objects and correctly locate states on the map, better than many college students. Because m e m o r y for spatial representations and memory for visual details are two components of the long-term visual memory, it is plausible that visual dyslexia, if it exists, could be caused by defects in either of these two components. For this reason, memory for spatial represenations and m e m o r y for details are examined to see if these two forms of memories can be potential etiological factors in dyslexia.
Visual Memory for Spatial Representations It appears that information regarding spatial location and right-left directionality is not stored in LTM in a picture-like format, but is retained most likely in the form of semantic structures and propositions. For example, many subjects fail to answer correctly questions such as "Which city is farther west, San Diego or Reno, Nevada"?; or "Which is farther north, Montreal or Seattle"? This suggests that our visual LTM of the U.S. map is not picture-like, off which one can read, but is in a form which is the product of an integration of visual information, semantic concepts and verbal propositions. In one study, Gernsbacher (1985) showed subjects pictures one at a time, and after a ten minute interval tested them with the original picture and a reversed form of the picture. The subjects had to tell which one of the two pictures they had seen earlier. Only 57 percent of the responses (which is barely above chance level) were correct indicating that information about pictures is stored in LTM without reference to the left-right orientation. Spatial information need not even be exclusively the propriety of visual modality. For instance, w h e n mental rotation tasks such as the ones designed by Shepard and Metzler (1971) were presented to congenitally blind subjects as a tactile task, it was found that blind subjects could perform the tasks successfully, and that their reaction time pattern was similar to that of sighted subjects (Carpenter and Eisenberg 1978). These studies indicate that spatial memory may not be in a picture-like format but may be present in an abstract representational
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form sustained by semantic, phonological, and tactile props. For this reason, in order to classify an individual as "visual dyslexic," it becomes essential to establish that his/her semantic and phonological systems are intact and that only the visual memory is defective. Studies that have controlled for semantic and phonological factors and have tested pure visual memory by using nonsense figures have failed to secure evidence for the presence of weak visual memory in subjects with reading disability (e.g., Liberman and Shankweiler 1978; Vellutino et al. 1975). Visual Memory for Details
Visual memory for details, similar to spatial memory, also appears not to be in a picture-like format, but is stored as analogs or abstract representations of objects. This lack of specificity for details may be the reason why we find it hard to spell words backwards even though we can spell the same words correctly and effortlessly forwards. Reed (1974) presented complex geometric forms such as the Star of David to subjects to test the nature of visual memory for details. Subsequent to the presentation of each form, the subject was shown parts of the original form and was asked if the part came from the original stimulus. While familiar parts such as the triangle were recognized most of the time, u n c o m m o n parts such as the parallelogram were correctly identified only 10 percent of the time. Mandler and Ritchey (1977) showed pictures such as a classroom scene to subjects and followed each picture by two test pictures. The subject had to tell if each test picture was the same as the one seen before. One of the two test pictures contained a "type distractor" such as having a tea-pot on the desk instead of the globe in the original picture. Not surprisingly, the pictures that contained the "type distractors" were correctly rejected most of the time (94 percent) whereas pictures that contained "token distractors," such as a change in the teacher's dress, were erroneously accepted as the original picture most of the time. The investigators concluded that we tend to remember the meaning of pictures rather than visual details. If these observations are extended to visual memory for words, it would appear that memory for orthography (spelling patterns) is not purely visual in nature but is supported by semantic an d phonological features of words. In fact, orthography is defined as the visual pattern of the written language as it relates to the graphemic, phonological, and semantic features of the language (Henderson 1984). Ehri and Wilce (1980) suggest that the orthographic representation may not be stored as rote-memorized visual images but as sequences bearing systematic relationships to acoustic and/or articulatory segments detected in the word's phonological identity. Furthermore, visual memory ca-
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pacity for orthographic representation appears to be limited to only two or three items (Zhang and Simon 1985) thus limiting the influence of visual memory on reading and spelling. In addition, the variance seen in reading ability associated with orthographic memory has been found to be primarily a product of reading experience whereas phonological skill has a sizeable heritability (Olson et al. 1989; Stanovich and West 1989). Reading failure that is attributable to a lack of experience is not generally considered reading disability. This brief review indicates that visual memory in isolation, as compared to phonology and semantics, plays a relatively limited role in word recognition. The following research study was conducted to assess more precisely the extent of the contribution made by visual memory toward word recognition.
Relative Contribution of Visual Memory towards Word Recognition Study 1 Procedure A series of four word-recognition tasks was administered to 124 children (66 gifts and 58 boys) from grades 2 through 5. These children were in regular classrooms and, therefore, are representative of the general population which includes a few children with reading disabilities. Each word-recognition task consisted of a list of 12 target words and the test list of 12 pairs of words of which one word was the target word. Each subject was asked to read the 12 target words silently and then select from the 12 pairs of test words, the one he/she had seen before. The tasks were individually administered in a predetermined random order. The target words in task I consisted of nonhomophonic content words and the test consisted of pairs of words of similar kind (e.g., target word = cat; test words = dog, cat). It is assumed that the target word can be correctly identified by relying on phonologic, semantic, as well as visual features of the target words and, consequently, presented an optimal condition for word recognition. Performance on this task was, therefore, taken as a measure of word-recognition skill of the subject and was used as the dependent variable for statistical analysis of data. The target stimuli in task 2 consisted of nonwords; the test words were homophonic nonword pairs (e.g., dore - doar, dore). It is assumed that this task cannot be performed successfully by relying on meaning because they are nonwords; the target cannot be correctly identified by relying on phonology because the test words are homophones. Successful performance on this task, therefore, depends almost entirely on visual memory for the target words. Stimuli in task 3 were "grammar" words and the test words
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were also pairs of grammar words (e.g., has - has, let). This task can be performed successfully by utilizing phonology as well as visual memory for words, but not by relying on word meaning. Task 4 consisted of content words and their homophones (e.g., sea - see, sea). This task can be successfully carried out by utilizing visual and semantic features of the word but not by relying on the phonological features of the word. These four lists of words therefore, can be used to partial out the relative contribution made by visual, phonological, and semantic processes towards word recognition. Results The data were analyzed in order to determine the degree of variance each one of the variables (viz., visual memory, phonology, and semantics) contributed to word recognition. A stepwise regression analysis was carried out with performance on task 1 as the dependent variable. Regression analysis showed that phonological feature of the word made the greatest contribution to word recognition with a multiple R of .23. The change it made in the prediction of word recognition skill was significant at .01 level. Word meaning (semantics) was the next highest contributor; the change in predicting word recognition was significant at the .04 level. Visual memory made little or no contribution to word recognition and the change it made in word recognition was not significant. On the basis of these data, it is concluded that amongst the three variables examined; visual memory for words accounts for the least amount of variance seen in word recognition skill; phonology accounts for the greatest amount of variance, and word meaning comes next. Study 2 Procedure Children from grades 3 and 4 in the above study who performed at or below chance level (i.e., a score of 6 or lower) on task 2 (visual memory) but had a score above the group mean on task 3 (phonology) were identified. Similarly, children who obtained a score of 6 or lower on task 3 (phonology) but a higher than mean score on task 2 (visual memory) were also identified. Thus, the first group of subjects had poor visual memory but adequate phonological memory; the second group had adequate visual memory but poor memory for phonology. Using these criteria, seven children (five boys and two girls) were identified as having poor visual memory but adequate phonological memory; two boys were identified as having poor phonological memory but adequate visual memory. The small number of subjects is due to the fact that many children with low word recognition scores performed poorly in both visual memory and phonological memory tasks. The reading scores of these children on the Indiana Statewide Testing for Educational Progress (ISTEP) were obtained from their school records. The ISTEP also provides measures of language ability and vocabulary.
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Table I Performance of Children with Low Visual/Normal Phonology Memory Scores and That of Children with Low Phonology/Normal Visual-memory Scores
Children with Low Visual Memory for Words Reading Student Scores on Number Task1 Task2 Task3 Task4 Vocab. Comp. Lang. 1. 2. 3. 4. 5. 6. 7.
6.0 5.0 4.0 5.0 5.0 7.0 5.0
9.0 9.0 10.0 9.0 10.0 11.0 9.0
9.0 9.0 6.0 4.0 7.0 12.0 11.0
11.0 9.0 11.0 8.0 8.0 10.0 8.0
CSI (IQ)
45% 40% 15% 26% 10% 18% 46%
28% 28% 10% 20% 40% 49% 28%
40% 14% 27% 11% 36% 46% 39%
83 89 87 80 82 91 --
Children with Low PhonologicalMemory for Words 1. 11.0 5.0 9.0 11.0 33% 2. 8.0 6.0 9.0 9.0 42%
56% 37%
55% 53%
117 105
Their scores on the Cognitive Skills Test, also referred to as Cognitive Skills Index (CSI), was also secured from this source. The CSI is often regarded as equivalent to an IQ score. Results Data presented in Table 1 show that children with poor visual memory for words are poor readers. However, they also have low vocabulary and language scores in addition to below-average IQ. The two children with adequate visual memory but poor phonological skills are also deficient in reading skills. However, unlike the seven children in the first group, their language skill as well as their IQ are within the normal range. These two children, because of their normal IQ, conform to the traditional definition of dyslexia. These results suggest that poor visual memory may be associated with poor reading skill, but it does not affect the reading process as an independent agent. Poor visual memory appears to be one aspect of a generalized cognitive weakness. Study 3
Procedure This study utilized two groups of children to collect data. The first group consisted of 31 children (17 girls and 14 boys) from grade 2 taken from the larger group of 124 children described in study 1. These 31 children provided the normative data against which the performance of children in the second group was compared. The sec-
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ond group consisted of eight boys w h o had been referred to the Psychology clinic and had been diagnosed as dyslexic. At the time of testing seven of them were attending the second grade; the eighth subject was in the third grade. These two groups of children were administered five different tasks which constituted a locally developed modified Stroop task. The intent of this testing was to determine how fast dyslexic children process verbal and nonverbal visual information and to see if they differ from an unselected group of second graders in this respect. Task I required each child to name the color of 20 patches as quickly as possible. Task 2 and 3 required each child to read aloud, as quickly as possible, a list of 20 high frequency grammar words and a list of 20 common content words. The words in these two lists were printed in different colors and were matched for frequency and syllable length. In tasks 4 and 5, the child was presented with the same lists of words in tasks 2 and 3 but was instructed to tell the name of the color of each printed word. Before administering these tasks, each child was given practice items. All these tasks were administered individually and children's responses were taped. The time it took to perform each task was the dependent variable in all these tasks. The data obtained on these tasks are shown in Table 2, along with other pertinent information. Results The performance of these two groups of children on these five tasks leads to the following conclusions. Dyslexic children are as fast as children from grade 2 in naming the colors of patches, the difference between these two groups being nonsignificant (t = .38, p < .70). Dyslexic children are, however, slower than children in the control group in reading both content words and function words and the differences are significant (t = 2.20, p < .03; t = 2.24, p < .03). These data indicate that dyslexic children are slow in processing only written verbal stimuli and are not slow in processing nonverbal visual stimuli. Children from the normative group, as well as the dyslexic group, take longer time to read grammar words compared to content words even though this difference is more pronounced for the dyslexic group. Dyslexic children as well as children in the control group show interference due to the Stroop effect. However, in the case of the control group, the Stroop interference effect was equal for both content word list and grammar word list (t = .39, p < .70). In contrast, dyslexic subjects show greater interference effect when they named the colors of content words than when they named the colors of grammar words, the difference being significant (t = 3.18, p > .01). This can be interpreted to mean that in the case of dyslexic children, word meaning interferes more than word pronunciation with naming colors. Such a differential effect is not seen in the control group.
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Conclusions
The results of these studies suggest that compared to the phonological and semantic features of the written word, visual memory for words plays a relatively insignificant role in word-recognition. Investigations described here also show that dyslexic children can process nonverbal visual information as fast as normal readers do. It appears, therefore, that some non-visual feature associated with the written language is responsible for the slow word-recognition of a majority of dyslexic readers. The finding that dyslexic children are much slower in processing grammar words than content words indicates that word meaning plays an important role in their word-processing behavior. The observation that dyslexic children take longer time to name the colors of content words than grammar words supports this view. If visual memory plays a contributory role, one would expect the RT for both grammar and content words to be similar. This, however, is not the case for the dyslexic children, as a group. Taken in conjunction with the findings of experimental studies from cognitive psychology, the results of the present investigation suggest that, at present, there is no good reason to conclude that a subtype of developmental dyslexia, caused by a weakness in visual memory, exists. The results presented in this paper, of course, do not prove that there is no visual dyslexia subtype. To say so would be tantamount to proving a null hypothesis. Rather, the burden of proof is shifted to those who advocate the existence of a visual dyslexia subtype and to those who implicate visual processes in reading disability. Such proof would include evidence of weak visual processes in the presence of intact phonological and semantic skills in dyslexic readers. References Aaron, P. G. 1989. Dyslexia and Hyperlexia. Boston, MA: Kluwer Academics. Aaron, P. G. and Phillips, S. 1986. A decade of research with dyslexic college students. Annals of Dyslexia 36:44-66. Anderson, J. R. 1990. Cognitive Psychology and its Implications. New York: Freeman and Co. Boder, E. 1973. A diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15:663-687. Bradley, L. and Bryant, E 1983. Categorizing sounds and learning to r e a d - - A causal connection. Nature 301:419-421. Bradley, L. and Bryant, P. 1985. Rhyme and Reason in Reading and Spelling. Ann Arbor, MI.: Univ. of Michigan Press. Burt, C. 1921. Mental and Scholastic Tests. London: P. S. King and Son. Carpenter, P., and Eisenberg, P. 1978. Mental rotation and the frame of reference in blind and sighted individuals. Perception and Psychophysics 23:117-124.
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Ehri, L. C. and Wilce, L. S. 1980. The influence of orthography on readers' conceptualization of the phonemic structure of words. Applied Psycholinguistics 1:371-385. Farah, M. J. 1988. Is visual imagery really visual? Overlooked evidence from neuropsychology. PsychologicalReview 95:307-317. Farah, M. J., Hammond, K. M., Levine, D. N., and Calvanio, R. 1988. Visual and spatial mental imagery: Dissociable systems of representation. Cognitive Psychology 20:439-462. Gates, A. C. 1929. The Improvement of Reading. New York: MacMillan. Gernsbacher, M. A. 1985. Surface information loss in comprehension. Cognitive Psychologys 17:324-363. Gjessing, H., and Karlsen, B. 1989. A Longitudinal Study of Dyslexia. New York: SpringerVerlag. Henderson, L. 1984. Introduction. In L. Henderson (ed.). Orthographies and Readings. Hillsdale, NJ.: Lawrence Erlbaum. Hinshelwood, J. 1895. Word-blindness and visual memory. The Lancet 21:1564-1570. Hulme, C. 1988. The implausibility of low-level visual deficits as a cause of children's reading difficulties. Cognitive Neuropsychology 5:369-374. Johnson, D., and Myklebust, H. 1967. Learning Disabilities. New York: Grune and Stratton. Lange, J. 1936. Agrosien und Apraxien. In O. Bunke and O. Foerster (eds.). Handbuchder Neurologie. Berlin: Springer. Liberman, I., and Shankweiler, D., 1978. Speech, the alphabet and teaching to read. In L. Resnick and P. Weaver (eds.). Theory and Practice of Early Reading. New York: John Wiley. Liberman, I., and Shankweiler, D., 1985. Phonology and the problem of learning to read and write. Remedial and Special Education 6:41-55. Liberman, I., Mann, V,, Shankweiler, D., and Werfelman, M. 1982. Children's memory for recurring linguistic and nonlinguistic material in relation to reading ability. Cortex 18:367-375. Livingstone, M., Rosen, G. D., Drislane, E W., and Galaburda, A. M. 1991. Physiological and anatomical evidence for a magnocellular deficit in developmental dyslexia. Proceedings of the National Academy of Sciences, USA. 88:7943-7947. Lovegrove, W., Martin, F., and Slaghuis, W. 1986. A theoretical and experimental case for a visual deficit in specific reading disability. Cognitive Neuropsychology 3:225-267. Lundberg, I., Frost, J., and Peterson, O. 1988. Effects of an extensive program for stimulating phonological awareness in preschool children. Reading Research Quarterly 23:263-284. Mandler, J. M., and Ritchey, G. 1977. Long-term memory for pictures. Journal of Experimental Psychology: Human Learning and Memory 3:386-396. Orton, S. 1937. Reading, Writing, and Speech Problems in Children. New York: Norton. Olson, R., Wise, B., Conners, E, Rack, J., and Fulker, D. 1989. Specific deficits in component reading and language skills: Genetic and environmental influences. Journal of Learning Disabilities 22:339-348. Phillips, S., Taylor, B., and Aaron, P. G. 1985. Developmental dyslexia: Subtypes or substages? Paper presented at the Annual convention, Indiana Psychological Association, Indianapolis, IN. Potzl, O. 1928. Di Aphasielehre vom Standpunkte der Kliniscen Psychiatrie. Leipzig: Franz Deudicte. Reed, S. K. 1974. Structural descriptions and the limitations of visual images. Memory and Cognition 2:329-336. Schevill, H. S. 1978. Tactile learning and reading failure. In H. R. Myklebust (ed.). Progress in Learning Disabilities, Vol. IV, New York: Grune and Stratton.
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