Visual-Based Deficits (page 3)
Neurological factors that influence reading disabilities must have their immediate effect on cognitive-perceptual abilities that are not specific to reading because reading is an acquired skill. There is no aspect of cognition or a specific region of the brain that could fail to develop and just cause a reading disability (Ellis, 1985). If a reading disability is instrinsically motivated, it must be caused by differences in perceptual, cognitive, or linguistic abilities that have evolved to serve more primary human functions. We believe that the primary deficit underlying many reading disabilities is linguistic in nature. Later in this chapter, we will review the extensive body of research supporting the language basis of reading disabilities. First, however, we will consider the evidence that deficits in visual, auditory, or attentional processes play a causal role in reading disabilities.
Because the visual system is an important sensory system for reading, it should not be surprising that visual-based explanations of reading disabilities have a long history in the field (Bronner, 1917; Fildes, 1922; Frostig, 1968). Many early reported cases of reading disabilities were seen by ophthalmologists, who explained these problems in terms of visual difficulties. The term "word blindness" was frequently used to refer to reading disabilities. Several early clinics for reading difficulties also bore the name "Word Blind" in their title. Since these early accounts, there have been numerous attempts to uncover the visual deficits that might cause reading disabilities. These attempts have considered reversal errors, problems in visual memory, erratic eye movements, light sensitivity, and visual timing deficits.
Over the years, much attention has been focused on the reversal errors made by children with RD. These errors, which involve, for example, the reading/writing of b for d or was for saw, have traditionally been linked closely with dyslexia. Even today, most people still think of dyslexia as a problem reading letters or words backwards. Despite this view, there is surprisingly little research that has systematically investigated reversal errors. The few studies that have examined reversal errors have found that these errors do not actually occur that often in children with RD (Fischer, Liberman, & Shankweiler, 1978; Liberman, Shankweiler, Orlando, Harris, & Berti, 1971). Furthermore, when considered in terms of percentage of overall errors, reversal errors may be no more prevalent in young poor readers than they are in young good readers (Holmes & Peper, 1977). In other words, all beginning readers occasionally make reversal errors, just as all children learning to talk make errors involving grammatical morphemes (e.g., past tense -ed, third person -s). Just as children with language delays continue to have difficulty with grammatical morphemes beyond the developmental period, children with RD often continue to make reversal errors in later grades.
When reversal errors do occur, they generally are not the result of perceptual problems. Children who write saw as was or girl as gril typically do not have trouble perceiving letter sequences. Vellutino and his colleagues (Vellutino, Pruzek, Steger, & Meshoulam, 1973; Vellutino, Steger, DeSetto, & Phillips, 1975) found that children with RD could accurately copy what they sometimes failed to read correctly. Rather than having problems perceiving letter sequences, poor readers more likely have difficulties remembering the order of letters in words. Because of the spatial orientation of words, a primary way a word can be misspelled! misread is to fail to remember the correct order of its letters.
Apparent problems in the memory for the letters in words led some early investigators to propose that poor readers had generalized deficits in visual memory (Fildes, 1922). Vellutino (1979), however, maintained that most of the early work showing deficits in visual memory was confounded by the use of stimuli that could be verbally labeled. Consequently, children with RD might have performed poorly because of verbal memory deficits rather than visual deficits. In support of this possibility, Vellutino and his colleagues (Vellutino et al., 1975) showed that poor readers scored comparably to good readers on a visual memory task involving stimuli that could not be easily labeled (but see Willows, Kruk, & Corcos, 1993).
Rather than focusing on a generalized problem in visual memory, some researchers have investigated the possibility that poor readers have specific problems in orthographic processing. Orthographic knowledge involves the knowledge of letter sequences or spelling patterns. This knowledge allows the reader to directly access semantic memory without going through the intermediate step of phonological decoding. Orthographic processing has often been tested by tasks that ask subjects to choose which of two letter sequences (goat, gote) is a real word. Because the foil in each word pair (gote) can be pronounced like a real word, the subject must rely on orthographic knowledge to answer correctly. Research using this task has shown that orthographic processing ability is related to reading achievement in that children with good orthographic knowledge read better than those with limited orthographic knowledge (Conners & Olson, 1990; Stanovich & West, 1989). Researchers have been quick to point out, however, that orthographic processing skills may be heavily influenced by phonological processing abilities (Share & Stanovich, 1995). Children who have mastered the use of sound-letter correspondence rules should develop richer orthographic knowledge by virtue of many successful trials reading words. Nevertheless, some studies show that orthographic processing may make an independent contribution to reading ability (Barker, Torgesen, & Wagner, 1992; Conners & Olson, 1990; Stanovich & West, 1989). Such findings suggest the possibility that some children with RD may have specific deficits in remembering the letters in words.
Erratic Eye Movements
When reading, we get the impression that our eyes are moving smoothly and continuously across the printed page. Actually, eye movements for reading (and many other visual activities) involve a series of rapid jerks, called saccades, that move from left to right, and occasionally from right to left (i.e., regressions). Each of these saccades is followed by a short fixation period averaging 200 to 250 milliseconds. It is during these fixations that information is obtained for the purpose of recognizing words.
Could problems in eye movements be a cause of reading disabilities? Poor readers have been noted to have more fixations per line, longer fixations, shorter saccades, and more regressions than good readers (Rayner, 1978). Rayner (1985) and others point out, however, that these differences in eye movements may actually be a reflection of cognitive processing difficulties during reading rather than problems in oculomotor control. For example, because poor readers take longer to recognize words and often need to go back to refresh their memory, they may show longer fixations and more regressions. In opposition to such a conclusion, Pavlidis (1981, 1985) has reported that dyslexics demonstrated abnormal eye movements in non-reading tasks (also see Eden, Stein, Wood, & Wood, 1994, 1995). Olson, Conners, and Rack (1991), however, have argued that even such findings could be a consequence of a reading problem and not a cause. They demonstrated that when poor readers were matched for reading skill with younger normal readers, no differences were observed in eye movements during non-reading tasks (but see Eden et al., 1994).
The belief that erratic eye movements are a cause of reading disabilities has often led to the popularity of visually oriented treatment approaches that involve "eye movement training" devices (Metzer & Werner, 1984). The assumption is that if poor readers could learn to move their eyes in a smoother, less erratic fashion, reading would improve. But as we pointed out earlier, the basic premise that skilled reading involves smooth eye movements is false. Not surprisingly, these training programs have not proven to be effective. Today, most professionals agree that oculomotor exercises, and behavioral optometry in general, have little to offer in the treatment of reading disabilities (Clark & Uhry, 1995; Keogh & Pelland, 1985; Silver, 1995).
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