A dual-route perspective on eye movements of dyslexic readers
Introduction
A substantial number of studies have documented that dyslexic readers in orthographies more regular than English suffer from a pervasive and persistent reading speed deficit, but much less from the reading accuracy problem which is characteristic for English dyslexic readers (e.g., Dutch: Van den Bos, 1998, Yap and Van der Leij, 1993; German: Wimmer, 1993; Italian: Zoccolotti et al., 1999; Spanish: Gonzalez & Valle, 2000; Norwegian: Lundberg & Hoien, 1990; Greek: Porpodas, 1999). The reasons for the rather accurate reading performance simply follow from the nature of these orthographies: first, there are rather few irregular words which would require knowledge of how specific letter strings are pronounced. Second, grapheme–phoneme rules correspond to the correct phonological forms in the majority of cases. These features may raise doubts on the usefulness of the well-known dual-route model of visual word processing (DRC, Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001) for regular orthographies (Share, 2008). Specifically, one may question the need for the lexical route to word phonology (via orthographic word recognition units) as this route was originally introduced to provide correct readings of the irregular words of English (e.g., Coltheart, Curtis, Atkins, & Haller, 1993). However, as pointed out by Share (2008), the more important distinction captured by the dual-route architecture is the one between familiar and unfamiliar visual words and this distinction – different from the regular–irregular distinction – applies to both regular and irregular orthographies. In dual-route theorizing, the remarkable speed and effortlessness with which familiar visual words are read, is taken to reflect lexical route processes, whereas the well-documented word length effect in the case of unfamiliar visual words is taken to reflect serial sublexical grapheme–phoneme conversion in order to access phonology and meaning.
From this perspective, one source of the mentioned reading speed problem of dyslexic readers in regular orthographies may be an impoverished orthographic lexicon (e.g., Reitsma, 1983). Support for this and additional dual-route explanations of the dyslexic reading speed problem was indeed provided by a recent study from our group with German dyslexic readers (Bergmann & Wimmer, 2008). This study used a phonological lexical decision task (i.e., Does xxx sound like a real word?) and presented familiar (e.g., Taxi) and unfamiliar letter strings (e.g., Taksi) of existing phonological forms and of nonwords (e.g., Tazi) and an orthographic lexical decision task (i.e., Is xxx correctly written?) assessed whether the supposedly familiar orthographic forms were indeed familiar. From the dual-route perspective, familiarity amounts to orthographic word representations and availability of such representations was inferred from the correct response patterns in the orthographic task (e.g., from YES to Taxi and NO to Taksi). Dyslexic readers exhibited a marked orthographic deficit on this task with only about 60% correct decisions (controls: close to 90%). With respect to speed impairments, a critical finding was that dyslexic readers exhibited markedly slower phonological lexical decisions for orthographically unfamiliar words compared to orthographically familiar words suggesting an important role of the orthographic lexicon for reading speed. However, frequent unavailability of orthographic word recognition units was not the only source of the dyslexic speed deficit in the phonological task. Dyslexic readers exhibited slow phonological lexical decisions (compared to controls) even for orthographically known words. This pointed to inefficiency of the lexical route and was interpreted as speed impaired access from orthographic to phonological word representations. Furthermore, our dyslexic readers exhibited slow phonological lexical decision on pseudohomophones and nonwords which must have been unfamiliar for both groups. This was interpreted as inefficient functioning of the sublexical route and was assumed to arise from slow access from graphemes to phonemes.
Obviously, phonological lexical decisions are rather distant from visual word processing in natural situations so that one may question, whether the mentioned dual-route account of the dyslexic speed deficits in the phonological lexical decision task does apply to silent reading of sentences. Of specific interest will be whether dyslexic readers will exhibit eye movement evidence for speed impaired processing even of high frequency words for which availability of whole-word recognition units in the orthographic lexicon can be assumed. One may reason that the slowed phonological lexical decisions in response to such words – observed by Bergmann and Wimmer – were due to the specific demands of the phonological lexical decision task which required access to word phonology and explicit phonological lexical decision. In a broader perspective, one could argue that the phonological lexical decision task is biased towards support of the dominant phonological deficit explanation of dyslexia (e.g., Snowling, 2000). No such bias is expected from eye movements during silent sentence reading.
The present dual-route perspective on dyslexic eye movements may also be of interest for models of eye movement control during reading. Models such as the E-Z Reader model (Pollatsek et al., 2006, Reichle et al., 2003) and the SWIFT model (Engbert et al., 2005, Kliegl and Engbert, 2003) include a word identification component, which – in rough analogy to the lexical route of the dual-route model – differentiate between orthographic whole-word recognition and access to the phonological word form and meaning. Different from dual-route models there is no explicit provision for the possibility – occurring quite frequently for dyslexic readers – that orthographic word recognition may fail.
Of interest for expectations on eye movement abnormalities of dyslexic readers is how the stages of word identification are linked to control processes which move visual attention and the eyes from word to word. To investigate the link between word recognition and the control of eye movements and attention, we relied on the E-Z Reader model, because – like dual-route models of visual word recognition – the model processes one word at a time (by strictly serial shifts of attention from one word to the next). The SWIFT model processes up to four words simultaneously. To illustrate relevant assumptions of the E-Z Reader model, for the present study, let us assume that both the eyes and visual attention are focused on word n and word identification processes (i.e., orthographic recognition and access to word phonology) are performed. The model assumes that programming of the saccade to word n + 1 begins with successful orthographic recognition (termed familiarity check; Pollatsek et al., 2006) and movement of attention (not necessarily of the eyes) to word n + 1 is triggered by accessing word phonology (and meaning; termed lexical completion). Let us now consider the case of a competent reader with quick orthographic recognition and fast access to word phonology. In this case, the attentional spotlight moves to word n + 1 soon after orthographic recognition and processing of word n + 1 begins. If saccade programming is still in an early (i.e., labile) stage, when the familiarity check of word n + 1 is completed, then saccade programming for word n + 1 will be canceled and replaced by programming a saccade to word n + 2. This feature of the model explains the frequent word skipping of competent readers. Let us now consider the case of a dyslexic reader which exhibits successful quick orthographic recognition of word n, but abnormally slow access to word phonology (i.e., slow lexical completion). This would result in late movement of attention to word n + 1 during fixating word n and, therefore, no skipping of word n + 1 will occur, because saccade programming has reached its non-labile stage. An even more consequential problem would arise when, due to absence of an entry in the orthographic word lexicon, even the first stage of word identification fails. In this case there will be no programming of a saccade to the next word. Instead, serial decoding of the unfamiliar visual word – according to dual-route models – may result in left to right movement of attention over the letter string or in execution of short saccades within the currently fixated word (i.e., frequent refixations). The manifestation of this would be a marked effect of word length (i.e., number of letters) on single fixation duration or on number of refixations per word.
For programming of saccades, E-Z Reader assumes a preferred saccade length of 7 letter spaces and a modification of this length to the center of the upcoming word, which constitutes the optimal viewing position. The often less than optimal adjustment of the preferred saccade length explains the systematic “overshoots” for short and the “undershoots” for long words (McConkie, Kerr, Reddix, & Zola, 1988). For the dyslexic readers one can consider two possibilities. One is that saccade programming for dyslexic readers is similar to that of typical readers, that is, conforms to a preferred saccade length with adjustments to the center of the upcoming word. In this case one would not expect a difference in the initial fixation location of the dyslexic and the typical readers, but frequent regressions back to the word beginning (i.e., within-word regressions) due to frequent failure of orthographic word recognition. Alternatively, frequent orthographic recognition failures may have resulted in a general tendency to target the beginnings of words. Studies with German and Italian dyslexic children provided evidence for the latter possibility (De Luca et al., 2002, MacKeben et al., 2004). Of specific interest is the study by MacKeben et al. which presented words of varying lengths right of a fixation point. Typically reading children linearly adjusted saccade length to the length of the word, whereas the dyslexic children largely failed to do so. It will be of interest whether the present, more advanced dyslexic readers in a silent reading task will also exhibit this pattern.
The present study examined the aforementioned expectations – derived from the dual-route model and the E-Z Reader model – for the eye movements of dyslexic German-speaking adolescents with a long history of a severe reading speed problem. They read silently the Potsdam Sentence Corpus (144 sentences with nearly 1000 words; see Kliegl, Grabner, Rolfs, & Engbert (2004) for details) for which the Potsdam group collected eye movement data from large samples of typical readers (e.g., Kliegl, Nuthmann, & Engbert, 2006). The availability of frequency and predictability information for each word allowed separation of potentially confounded influences of word length, frequency and predictability on eye movement measures.
Section snippets
Participants
The participants of the present study were 18 dyslexic and 18 typically reading German-speaking, young male adults who were recruited from two large longitudinal studies on reading development (e.g., Wimmer, Mayringer, & Landerl, 2000). Invitation for participation was based on previous reading assessments and final group assignment was based on a reading assessment 2 years before the current study. Criteria for dyslexia were a performance below percentile 10 on a reading speed test and an
Dyslexic eye movement abnormalities
Table 2 shows group differences for several measures which are commonly used to characterize dyslexic eye movement abnormalities. The dyslexic readers exhibited massively increased gaze durations (i.e., the sum of fixation durations during first pass reading) and a doubled number of fixations per word compared to the typical readers. The mean forward saccade length of the dyslexic readers was substantially shorter than that of the typical readers. Importantly, there were no group differences in
Discussion
Before interpreting the present dyslexic eye movement findings, a summary of these findings may be useful. As context, we note that our adult sample of poor readers exhibited the typical manifestation of dyslexia in a regular orthography (German in the present study). They suffered from slow effortful reading, but not from a reading accuracy problem. However, due to the asymmetric regularity of German (high in the reading, low in the writing direction), their high reading accuracy was
Acknowledgements
We are grateful for the helpful comments of Erik D. Reichle and two anonymous reviewers which greatly improved the manuscript. This research was supported by the Austrian Science Fund (FWF; I57-G14) as a part of a European Science Foundation EUROCORES Project (05_ECRP_FP006) coordinated by Reinhold Kliegl, University of Potsdam, who also made available the Potsdam Sentence Corpus.
References (50)
- et al.
Varieties of developmental dyslexia
Cognition
(1993) - et al.
Reading words and pseudowords: An eye movement study of developmental dyslexia
Brain and Language
(2002) - et al.
Eye movement patterns in linguistic a non-linguistic tasks in developmental surface dyslexia
Neuropsychologia
(1999) - et al.
Rapid “automatized” naming (RAN): Dyslexia differentiated from other learning disabilities
Neuropsychologia
(1976) - et al.
Impaired visual processing of letter and digit strings in adult dyslexic readers
Vision Research
(2006) - et al.
Visual target detection is not impaired in dyslexic readers [Letter to the Editor]
Vision Research
(2008) - et al.
Perhaps correlational causal: No effect of dyslexic readers’ magnocellular system on their eye movements during reading
Neuropsychologia
(2006) - et al.
Eye movements of dyslexic children when reading in a regular orthography
Brain and Language
(2004) - et al.
SWIFT explorations
- et al.
Eye movement control during reading: I. The location of initial eye fixations on words
Vision Research
(1988)