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Inducing Letter-By-Letter Dyslexia in Normal Readers

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Inducing Letter-By-Letter Dyslexia in Normal Readers Inducing Letter-by-letter Dyslexia in Normal Readers Daniel Fiset1, Fre´de´ric Gosselin1, Caroline Blais1, and Martin Arguin1,2 Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/18/9/1466/1756331/jocn.2006.18.9.1466.pdf by guest on 18 May 2021 Abstract & Letter-by-letter (LBL) dyslexia is an acquired reading dis- word recognition. We claim that, instead, they rely on lower order characterized by very slow reading and a large linear spatial frequencies for parallel processing, that these lower word length effect. This suggests the use of a sequential LBL spatial frequencies produce confusions between visually simi- strategy, in sharp contrast with the parallel letter processing lar letters, and that the LBL compensatory strategy allows them used by normal subjects. Recently, we have proposed that to extract higher spatial frequencies. The LBL strategy would the reading difficulty of LBL dyslexics is due to a deficit in thus increase the spatial resolution of the visual system, ef- discriminating visually similar letters based on parallel letter fectively resolving the issue pertaining to between-letter simi- processing [Arguin, M., Fiset, S., & Bub, D. Sequential and larity. In Experiments 1 and 2, we succeeded in replicating parallel letter processing in letter-by-letter dyslexia. Cogni- the main features characterizing LBL dyslexia by having nor- tive Neuropsychology, 19, 535–555, 2002]. The visual mecha- mal individuals read low-contrast, high-pass-filtered words. nisms underlying this deficit and the LBL strategy, however, Experiment 3, conducted in LBL dyslexic L.H., shows that, are still unknown. In this article, we propose that LBL dys- indeed, the letter confusability effect is based on low spatial lexic patients have lost the ability to use, for parallel letter frequencies, whereas this effect was not supported by high processing, the optimal spatial frequency band for letter and spatial frequencies. & INTRODUCTION Several authors thus assume LBL dyslexia to be the Letter-by-letter (LBL) dyslexia is a reading disorder that result of damage to the mechanisms responsible for the can appear following a brain lesion in previously literate parallel processing of letters and, consequently, to re- adults. The brain regions critically concerned in this flect serial encoding of the component letters in words pathology are the left fusiform gyrus (Cohen et al., (Rayner & Johnson, 2005; Cohen et al., 2003; Behrmann, 2003) or the white matter fibers conducting visual infor- Shomstein, Black, & Barton, 2001). This neuropsycho- mation to this region (Binder & Mohr, 1992; Damasio logical disorder could result from a number of functional & Damasio, 1983). Given the posterior localization of deficits, as is suggested by the individual differences the lesion, a homonymous hemianopia is also frequently between LBL dyslexics (e.g., Price & Humphreys, 1992, found, constraining the patient to process the ortho- 1995). Alternatively, LBL dyslexics might have a common graphic stimuli with his or her right visual cortex. The functional impairment, with relatively superficial differ- behavioral diagnostic criteria for this pathology are very ences (Behrmann, Plaut, & Nelson, 1998). This last slow reading and an abnormally large word length ef- position is the one that we adopt throughout the article. fect (i.e., reading time is proportional to the number The research conducted in our laboratory on the of letters in the word). Depending on the patient, the functional cause of LBL dyslexia has highlighted the fact time needed to read a word aloud can increase from that these patients confound visually similar letters (see 300 msec to several seconds for each additional letter in also Perri, Bartolomeo, & Silveri, 1996; Patterson & Kay, the stimulus (see Lambon Ralph, Hesketh, & Sage, 1982; Levine & Calvanio, 1978) and this especially when 2004). This performance pattern suggests an LBL strat- they try to recognize several letters simultaneously egy that contrasts with normal reading, where the (Arguin & Bub, 2005; Fiset, Arguin, Bub, Humphreys, number of letters has no significant impact on reading & Riddoch, 2005). In fact, all the patients examined so latencies, at least with relatively short (six letters or less) far (n = 7) showed a letter confusability effect (letter words (Weekes, 1997). This suggests that normal sub- confusability is defined as the visual similarity between a jects process letters in parallel for word recognition. particular letter and the remaining letters of the alpha- bet; see Arguin, Fiset, & Bub, 20021) in word recogni- tion, but no such effect in single-letter identification. 1Universite´ de Montre´al, Canada, 2Institut Universitaire de Ge´riatrie Note also that the diagnostic word length effect can de Montre´al, Canada be eliminated if words of different lengths are matched D 2006 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 18:9, pp. 1466–1476 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.2006.18.9.1466 by guest on 02 October 2021 on the sum of the confusability of their constituent let- tively) was observed, but its size was far less than what is ters, suggesting a fundamental role of letter confusabil- usually encountered in LBL dyslexics. Moreover, the ity in the abolition of parallel letter processing (Fiset significant word length effect was not statistically signif- et al., 2005). We have argued that the purpose of the icant in the majority of normal subjects in single-case LBL strategy is to avoid confusion between visually analyses. Task difficulty was not the cause of these weak similar letters by increasing the signal-to-noise ratio slopes, however, because mean reaction times were (see Arguin & Bub, 2005; Fiset et al., 2005; Arguin similar to those of dyslexics (mean RT = 2250 msec) et al., 2002). However, the visual mechanisms subtend- and error rates were greater than those usually observed ing this sequential strategy remain unknown. In this in patients (between 10% and 35% errors compared to Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/18/9/1466/1756331/jocn.2006.18.9.1466.pdf by guest on 18 May 2021 article, we propose a theory of the visual impairment 5% and 10% in most dyslexic patients). in LBL reading. These previous studies lacked a firm theoretical foun- By contrast with LBL dyslexic patients, normal readers dation. With hindsight, it seems obvious that presenting show no letter confusability effect with normal, broad- low-contrast or low-pass-filtered words, two manipula- band words (Fiset, Arguin & McCabe, 2006; Arguin tions exacerbating low spatial frequencies in words, et al., 2002). That a letter confusability effect is only would not be optimal for inducing LBL reading: Low present in LBL dyslexics emphasizes the abnormal sensi- spatial frequencies are better processed in periphery tivity of these patients to the visual similarity between (De Valois & De Valois, 1988), whereas LBL dyslexic letters and suggests that a low-level visual deficit inter- patients tend to fixate letters centrally one after the feres with letter encoding (see Behrmann, Plaut, et al., other (Rayner & Johnson, 2005; Behrmann et al., 2001). 1998; Farah & Wallace, 1991; Friedman & Alexander, Here, we propose that dyslexic patients have lost the 1984; Levine & Calvanio, 1978; Kinsbourne & Warrington, ability to use the spatial frequencies most useful for let- 1962, for similar suggestions). We therefore argue that ter and word recognition. We shall develop this idea in a coherent theory of LBL dyslexia requires a clear un- the General Discussion. We claim instead that they rely derstanding of the determinants of their letter confus- on a lower range of spatial frequencies for their residual ability effect. parallel letter processing ability, and that these lower One powerful approach to test a hypothesis regarding spatial frequencies produce confusions between visually a neuropsychological disorder consists in attempting to similar letters. This point is well supported by the study induce the critical pathological behavior in normal sub- of Fiset et al. (in press), which shows that low-pass jects (McLeod, Shallice, & Plaut, 2000; Vecera & Gilds, filtering causes a letter confusability effect to emerge in 1998). Thus, the hypothesis that a visual deficit is at the the word-reading performance of normal subjects. More origin of LBL dyslexia may be assessed by asking normal crucially here, we also argue that the compensatory subjects to read visually deteriorated words. If the LBL strategy allows dyslexics to extract low-energy, reading pattern characterizing LBL dyslexia occurs when high spatial frequencies by focusing attention on each normal subjects process degraded stimuli, it may be letter, thereby increasing the spatial resolution of the concluded that both patient and normal data reflect visual system and minimizing the between-letter similar- the malfunctioning of the same (or similar) mechanisms ity (e.g., Yeshurun & Carrasco, 1998). If the LBL strategy (McLeod et al., 2000). is what patients require to extract high spatial frequen- Nelson, Behrmann, and Plaut (1999) succeeded in cies not readily available for parallel processing, then producing a substantial word length effect—the diag- a word length effect with a magnitude close to that nostic criterion of LBL dyslexia—in normal readers using observed in LBL dyslexics should be obtained in nor- stimuli with a reduced luminance contrast. Fiset, Arguin, mal readers when
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