2.2. Apparatus and procedure

Eye movements were recorded (monocular for the right eye) with an EyeLink 1000 tower mount system (sampling rate: 1000 Hz; SR Research, Ontario, Canada). Participants sat at a viewing distance – held constant by a forehead and a chin rest – of 52 cm to the screen. The nine point calibration routine took about 3 min and was conducted before the presentation of 12 familiarization trials, was repeated after training and at the halftime of the experimental run (i.e., after the presentation of 72 sentences) after which the participants were offered a short break. Criterion for calibration was a tracking error of less than 0.5° of visual angle.

Sentences were presented typed in mono-spaced, bold font (Courier New, 14 pt.) presented on a 21 in. computer screen (1024 × 768 pixel resolution, 120 Hz frame rate). From the fixed viewing distance a single character had a width of 0.3°. Sentence length varied from 5 to 11 words (M = 7.9).

Prior to each sentence presentation a fixation point was presented near the middle of the left screen frame (position in pixels: x = 200, y = 384). After the system had detected a fixation (minimum duration: 100 ms) on the fixation point, sentences were presented in such a way that the first word was centrally fixated. Participants were instructed to silently read the sentence. Looking at an x – presented at the bottom left corner of the screen – terminated sentence presentation and triggered the reappearance of the fixation point (or the appearance of a question mark, see below). When the system did not detect a fixation at the fixation point within 5 s the system was re-calibrated.

To assure that participant fully processed the sentences, comprehension questions were orally presented by the experimenter after about every fourth sentence. The questions could mostly be answered by uttering a single word. The verbal response was not crucially hampered by the chin rest and did not influence the quality of the eye movement recordings. Importantly, dyslexic readers had no problems in comprehending the sentences as wrong answers rarely occurred (both groups: M < 1).

3.2. First fixation location

Fig. 1 shows the effect of word length on the first fixation location for the two groups. Our unimpaired readers exhibited a typical pattern with “overshooting” the center of short words (i.e., up to about 5 letters), hitting the center for words with 6 and 7 letters, and “undershooting” longer words. In contrast, the dyslexic readers hit the center of short words only, and massively “undershot” medium and long words. The group difference was reliable from 4-letter words onwards (ts > 3).

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Fig. 1

The location of the first fixation in relation to word length. Error bars indicate 1 standard error of the mean.

3.3. Effects of length and frequency

In a next step, we examined group differences in the effects of word length and frequency on number of fixations and gaze duration per word (see Fig. 2). Note that, for the purpose of presentation, the x-axis of word frequency was scaled from high to low so that the effects of length and frequency go in the same direction. The main observation from Fig. 2 is that both number of fixations (first section) and gaze duration (second section) of dyslexic readers were much more affected by length and frequency than those of typical readers. Furthermore, for each length and for each frequency level, dyslexic readers exhibited more fixations and longer reading times. This overall difference becomes most evident from the observation that dyslexic reading performance for the shortest (3- and 4-letter) words corresponded roughly to unimpaired performance for the longest (8- and 9-letter) words. The same relationship applies for word frequency with again dyslexic performance for the most frequent words corresponding to unimpaired performance for the most infrequent words.

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Fig. 2

The upper section shows how mean number of fixations and gaze durations related to word length and frequency. The lower section shows how word length and frequency affected the fixation duration of singly fixated words. The bar charts represents the linear fixed effects (and 1 standard error) of length and frequency on number of fixations, gaze duration and single fixation duration. Stars above the bars indicate the significance of the fixed effects and stars above brackets indicate the significance of the group differences with *p < .05, **p < .01, and ***p < .001.

The final section of Fig. 2 shows the effects of length and frequency for those words which received a single fixation only. Such single fixations are suggestive of lexical route processing via orthographic whole-word recognition, if there is little effect of word length. Indeed, as evident from Fig. 2, there was no systematic increase of fixation duration with length. Actually, both groups exhibited the shortest durations for 6- and 7-letter words and increases of fixation duration occurred only for 8- and 9-letter words. Also of interest is the observation that single fixation durations of both groups were not affected by frequency from high to medium and increases were only observed for the two bins of the most infrequent words.

The right section of Fig. 2 shows the results of regression analyses for each of the three dependent measures using linear-mixed effect models (LME; Pinheiro & Bates, 2000). Specifically, we regressed each dependent measure (number of fixations, gaze duration and single fixation duration) on length (fixed factor) and included frequency and predictability as random factors. In a corresponding set of regression analyses, each dependent measure was regressed on frequency with length and predictability as random factors. These analyses allow an estimation of the unique effects of length and frequency. Estimation of these effects was important, because length and frequency were substantially correlated (r =−.61) and there were also small correlations of length and frequency with predictability (−.19 and .30, respectively). The error bars in Fig. 2 represent 1 standard error of the fixed effects which were used to test the significance of the fixed effects against 0 by the means of one sample t-tests and 17 degrees of freedom (n − 1). Group comparisons (two-sided independent sample t-tests) were conducted with the fixed effect of each group, the associated standard errors and 34 degrees of freedom (n − 2).

For number of fixations, the fixed effects of length differed reliably from zero and the fixed effect of the dyslexic group was reliably higher than that of the control group. For the typical readers, the fixed effects of frequency did not differ reliably from 0. For the dyslexic readers, the fixed effect of frequency was reliably higher than 0 and reliably higher than that of the typical readers. For gaze duration, the fixed effects of both length and frequency in each group were substantial and the effects were larger for the dyslexic group. The main result of the regression analyses for single fixation duration is the absence of a systematic length effect for both the dyslexic and unimpaired readers.

4.1. Summary of eye movement findings

1. With respect to number of fixations during reading, our findings with adult dyslexic participants extend findings from previous eye movement studies with dyslexic children in regular orthographies (De Luca, Di Pace, Judica, Spinelli, & Zoccolotti, 1999; Hutzler & Wimmer, 2004; Zoccolotti et al., 1999). In the present task, requiring silent reading of isolated sentences, our dyslexic readers exhibited about twice the number of fixations compared to unimpaired readers. The frequency of word skipping was massively reduced, whereas the frequency of more than one fixation per word was increased. In relation to number of fixations, regressive eye movements between words were rather infrequent and did not differ from controls. The analyses of within-word eye movements revealed that the higher number of fixations per word of the dyslexic readers were primarily due to frequent progressive refixations. Furthermore, the first fixation location of the dyslexic readers was less affected by word length than that of the typical readers and tended to massively “undershoot” the center of longer words (De Luca et al., 2002; MacKeben et al., 2004).

2. The global eye movement measures (number of fixations and gaze duration) of dyslexic readers were much more affected by word length (number of letters) than those of unimpaired readers and this was also the case for word frequency. A substantial dyslexic abnormality was already evident for the shortest words and for the most frequent words. Specifically, dyslexic reading performance on the shortest (3- and 4-letter) words – in terms of number of fixations and gaze duration – corresponded roughly to unimpaired performance on the longest (8- and 9-letter) words. The same relationship applied to the effect of frequency. Again dyslexic performance on the most frequent words corresponded to unimpaired performance on the most infrequent words.

3. Of main importance is the finding of an abnormally strong length by frequency interaction shown by dyslexic readers for both number of fixations and gaze duration. Specifically, they exhibited a close to normal length effect for high frequency words and clearly abnormal length effects for words of medium and low frequency. Corresponding to their close to absent length effect for high frequency words, again similar to controls, there was no effect of word length on gaze duration for words which received a single fixation only. Actually, both groups exhibited shortest single fixations for 6- and 7-letter words.

4.2. Dual-route interpretation

In the following, we relate the eye movement findings to the dual-route model, because this model has served as a main framework for interpreting word processing difficulties of dyslexic readers (e.g., Castles & Coltheart, 1993; Ziegler et al., 2008). The left panel of Fig. 4 shows the main components of the model and also illustrates how a familiar (i.e., frequent) letter string and an unfamiliar (infrequent) string may be processed by the model. The first component of the model is a visual feature/letter processor which in parallel transforms the visual input from a word into an abstract letter string which is provided simultaneously to both the orthographic word lexicon (lexical route) and to serial grapheme–phoneme coding (sublexical route) for further processing. The familiar letter string is assumed to find a match in the orthographic word lexicon which provides access to the phonological word with its syntactic and semantic features. In contrast, the unfamiliar string is assumed to reach the phonological word entry via serial grapheme–phoneme conversion of the sublexical route. The right panel of Fig. 4 illustrates how the components of the lexical route may correspond to the processing stages of the word identification module of the E-Z Reader model.

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Fig. 4

The left panel is a schematic illustration of how the dual-route model of visual word processing may process the highly familiar word “Vater” (father; log frequency: 3.2) and the less familiar word “Spinat” (spinach; log frequency: 1.3). For the former, the model relies on the lexical route and gain access to word phonology via an instantiated orthographic whole-word recognition unit. For the latter, word phonology is assembled via serial grapheme–phoneme conversion. The right panel illustrates how the processes of the word identification system of E-Z Reader (Reichle et al., 2003; Pollatsek et al., 2006) may correspond to the lexical route of the dual-route model. See text for details.

In dyslexia research, the dual-route model gave rise to the distinction between surface dyslexia and phonological dyslexia (e.g., Castles & Coltheart, 1993). Surface dyslexia refers to a specific impairment of the lexical route for processing familiar words and phonological dyslexia refers to a specific impairment of the sublexical route which is assumed to be critically involved in reading unfamiliar words. In English-based dyslexia research, the diagnosis of surface dyslexia is mainly based on specific difficulties with accurate reading of irregular words and the diagnosis of phonological dyslexia is based on difficulties with accurate reading of nonwords (Castles & Coltheart, 1993). As noted in Section 1, in orthographies more regular than English, such a diagnostic approach, based on reading errors, is not possible, because there are no proper irregular words and dyslexic readers – at least from about Grade 3 onwards (Wimmer, 1993) – typically do not suffer from a marked accuracy problem even for nonwords. They typically suffer from a massive and persistent reading speed problem which in most cases affects both words and nonwords (e.g., Landerl & Wimmer, 2008). Bergmann and Wimmer attempted to distinguish between surface and phonological dyslexia on the basis of the speed of orthographic and phonological lexical decisions. This attempt also failed, because of a very high correlation between the two speed measures. However, this difficulty to find subjects with specific lexical and sublexical speed impairments does not imply that the dual-route model may not be useful for characterizing the dyslexic reading speed problem in terms of proximal mechanisms.

4.2.1. Orthographic lexicon deficit

In Section 1, we suggested that part of the reading speed problem of dyslexic readers stems from an orthographic lexicon deficit. This implies – in terms of the dual-route model of Fig. 4 – that the output of the visual feature/letter processor (i.e., the abstract letter string) finds no matching entry in the orthographic lexicon with the consequence of continuation of serial sublexical processing. As noted in Section 1, this proposal was backed by a recent study from our group (Bergmann & Wimmer, 2008) which used accuracy of orthographic lexical decisions to diagnose word-specific orthographic lexicon deficits and found these deficits to be accompanied by slowed phonological lexical decisions. One motivation for the present eye movement study was the concern that phonological lexical decision may pose specific problems for dyslexic readers with the consequence that slow latencies may not reflect reading speed problems in natural silent reading situations. This concern did not find support as the present eye movement findings – similar to latencies of phonological lexical decisions – do speak for an important role of the orthographic lexicon deficit for the reading speed problem of dyslexic readers. A first crude indication of an orthographic lexicon deficit affecting reading speed was the observation that dyslexic readers were much more affected by word length than unimpaired readers. This negative effect was present for number of fixations and even more so for gaze duration. From the dual-route perspective, these abnormal effects of word length indicate reliance on serial sublexical processing which has to take place when letter strings find no match in a deficient orthographic lexicon. In E-Z Reader terminology, this would translate into a failure of the familiarity check. Another indication for a deficient lexicon was the observation that dyslexic readers exhibited single fixation on a word or skipping of a word for 67% of the present word items, whereas unimpaired readers did so for 90%. Important support for the interpretation that single fixations reflect orthographic whole-word recognition is provided by the finding that the number of letters of singly fixated words had no reliable effect on the duration of single fixations. Such absence of a word length effect is expected when all the letters are processed in parallel by the visual feature/letter processor of the dual-route model and in parallel activate an orthographic lexicon entry. Obviously, the same argument can be made when a word receives no fixation at all, because in this case one can assume that letter string processing and orthographic word recognition occurred when the eyes (but not attention) were still on the word which preceded the skipped one.

Further support for an orthographic lexicon deficit comes from the finding of an abnormally strong length by frequency interaction on both number of fixations and gaze duration of dyslexic readers. Specifically, dyslexic readers, compared to unimpaired readers, exhibited a much increased length effect for words of medium and low frequency. Only for high frequency words did dyslexic readers exhibit small length effects which were similar to those of the typical readers. These findings suggest that dyslexic readers were able to rely on lexical route processes based on orthographic lexicon entries only for highly familiar words, but not for words of lower frequency. The present finding of a marked length by frequency interaction for dyslexic readers, stands in interesting contrast to the findings of two studies with dyslexic children which found the same large increase of fixations or reading onset time from short to long items for both words and nonwords (De Luca et al., 2002; Ziegler, Perry, Ma-Wyatt, Ladner, & Schulte-Körne, 2003). The similar length effect for words and nonwords – from the dual-route perspective – suggests that processing of the words was completely dependent on serial sublexical processing. The critical difference between the present study, which found a length by frequency interaction for dyslexic readers, and the studies of De Luca et al. and Ziegler et al. which did not find such an interaction, is the age of the dyslexic participants. The participants of De Luca et al. and Ziegler et al. were children (mean ages: 13 and 11 years, respectively), whereas the present participants were young adults (mean age: 18 years). Presumably, the present dyslexic readers were advanced compared to the dyslexic children in the mentioned studies in the sense that they were able to rely on the lexical route for a proportion of high frequency words.

The present study did not directly assess a deficit of the orthographic word lexicon in dyslexic readers. However, the present participants are similar to those of Bergmann and Wimmer (2008) who used an orthographic lexical decision task to directly assess such a deficit. Bergmann and Wimmer found their dyslexic readers to exhibit correct distinctions between correct and incorrect homophonic spellings (e.g., YES to Taxi and NO to Taksi) for only about 60% of the word–pseudohomophone pairs, whereas unimpaired readers did so for 90% of the word–pseudohomophone pairs. Furthermore, there is direct evidence for an orthographic lexicon deficit of the present dyslexic sample from an assessment in primary school (9 years before the present assessment) which included a spelling test. The present dyslexic readers wrote correctly only 56% of dictated words, whereas the present unimpaired readers did so for 92%. This early measure of the size of the orthographic lexicon was highly associated (r = .83) with the combined number of skipped or singly fixated words in the present study. This supports the interpretation that single fixation and skipping of words reflects orthographic whole-word recognition.

4.2.2. Impaired speed of the lexical route

A rather direct reflection of impaired efficiency of the lexical route is the prolongation of fixation duration of about 60 ms shown by the dyslexic readers for words which received a single fixation only and for which dyslexic readers, similar to unimpaired readers, did not exhibit a length effect on fixation duration.

There are three potential sources of the speed impairment of the lexical route. One is that the lexical route impairment arises from slow functioning of the visual feature/letter processor of the dual-route model which in the E-Z Reader model may correspond to early visual processing. Further potential sources are slow activation of orthographic lexicon entries, corresponding to the familiarity check, or slow activation of phonological lexicon entries from instantiated orthographic lexicon entries which corresponds to lexical completion (see Fig. 4). A deficit of the visual feature/letter processor is implied by explanatory accounts of dyslexia which assume a low-level visual deficit (Stein & Walsh, 1997) or a visual attentional problem posed by letter string processing (Facoetti et al., 2006; Valdois, Bosse, & Tainturier, 2004). Contrary to these accounts, the present dyslexic readers did not differ from unimpaired readers on a string processing task which required fast detection of the presence of a target among strings of 5 letters or 5 pseudoletters. Actually, as shown in Table 1, our dyslexic readers tended to perform faster than the controls (for details see Hawelka and Wimmer (2008)). Several further studies from our group provided negative evidence for a speed impairment of dyslexic readers on purely visual processing tasks (Hutzler, Kronbichler, Jacobs, & Wimmer, 2006; Kronbichler, Hutzler, & Wimmer, 2002; Wimmer & Mayringer, 2001). Of specific interest are the findings from an eye movement study with dyslexic readers which found an enormous increase of number of fixations for reading of nonwords, but not when the task was to detect the presence of two adjacent identical letters in sequences of consonant strings (Hutzler et al., 2006). An extensive examination of potential eye movement control problems in Italian dyslexic children also arrived at a negative conclusion (De Luca et al., 1999). However, the largely negative evidence from group studies does not preclude that there may be dyslexic cases with a dysfunction of the visual feature/letter processor of the dual-route model (e.g., Ziegler et al., 2008). What we can conclude is that such cases must be quite rare among dyslexic readers who exhibit the characteristic reading speed problem in regular orthographies.

The present study offers little of direct relevance for evaluation of the two other potential sources of the speed impairment of the lexical route (i.e., slow access from letter strings to orthographic lexicon entries and slow access from orthographic to phonological lexicon entries). However, the slow performance of the present dyslexic readers on versions of the Rapid Automatized Naming (RAN) tests of Denckla and Rudel (1976) provides some tentative information. We first note that the processes required by the RAN tests correspond to lexical route processes, because the RAN tests require rapid naming of sequences of pictured objects or digits. In the present versions, we used pictured objects (for the assessment at the beginning of Grade 1) and pictured animals and single digits (at the assessment 3 years before the present one). Similar to orthographic whole-word recognition of the letter strings, the RAN pictures (or digits) must be recognized (i.e., instantiate a stored representations) and, similar to access from orthographic to phonological word representations in lexical reading, RAN requires access from instantiated visual representations to whole-word phonology. Since our dyslexic readers performed similarly to controls on the letter string processing task of Hawelka and Wimmer (2008), it seems unlikely that the poor performance on the RAN tasks is due to slow visual recognition. Furthermore, visual recognition is very simple when single digits have to be recognized. Therefore, the slow performance on the RAN tests speaks for slow access from instantiated visual recognition units to phonological lexicon entries. This interpretation is further supported by the finding that slow RAN performance of dyslexic readers result from prolonged pause times between name articulations, and not from slow articulation of names (Neuhaus, Foorman, Francis, & Carlson, 2001).

For direct comparison between the dyslexic deficit on lexical route processes in reading and RAN performance, we converted the items per minute measure of the digit RAN test into mean processing time per digit and found this measure to be 70 ms prolonged for the dyslexic compared to the unimpaired readers (M = 395 ms and 325 ms, respectively). This prolongation of access to phonology in the digit RAN test is close to the 60 ms prolongation of their single fixation durations in the present silent sentence reading task. Furthermore, the individual mean single fixation durations were reliably associated with the processing time in the digit RAN test, r = .67. However, this support from RAN test for slow access from orthographic to phonological word entries does not necessarily preclude a dyslexic speed deficit in accessing orthographic word entries from letter string information. From the central tendency measures in Table 1 it is obvious, that only about half of the dyslexic sample may have exhibited a serious RAN deficit with scores roughly about 1 standard deviation below the mean of the controls.

4.2.3. Inefficiency of the sublexical route

The present study – different from the eye movement studies with Italian and German dyslexic readers (De Luca et al., 2002; Hutzler & Wimmer, 2004) – did not present nonwords to examine directly inefficient functioning of the sublexical route of the dual-route model. However, consistent with a specific speed impairment of the sublexical route are the prolonged fixations on words which receive more than one fixation. Such multiple fixations can be taken as rough indication of serial sublexical processing (but see below). Compared to unimpaired readers, the first of these multiple fixations were prolonged by more than 60 ms on average and the successive fixations were prolonged by more than 50 ms. The finding of prolonged initial fixation durations compared to shorter successive fixations of multiply fixated words is important, because it speaks against the plausible possibility that the first of multiple fixations are primarily “misplaced” fixations. The reason is that misplacements of fixations due to oculomotor aiming errors are detected early which results in short fixation durations (e.g., O’Regan & Lèvy-Schoen, 1987; Vitu, McConkie, Kerr, & O’Regan, 2001). Thus, the longer initial fixation durations compared to shorter successive fixations reflect frequent failure of orthographic word recognition, and not frequent occurrence of misplaced fixations.

The dual-route model of Fig. 4 offers two plausible interpretations of these prolonged fixation durations. One is slow access from graphemes to phonemes and the other is slow activation of phonological word forms from the sequence of phonemes supplied by grapheme–phoneme conversion. Slow activation of phonemes is suggested by a study from our group which found that dyslexic readers required substantially longer presentation times for reliable report of a cued element of strings of consonant letters (Hawelka, Huber, & Wimmer, 2006). This task requires only activation of phonemes and no further activation of phonological word entries. Similar dyslexic speed deficits were found in a study which measured naming of singly presented letters (Castel, Pech-Georgel, George, & Ziegler, 2008; Jones, Branigan, & Kelly, 2009). This support for slow access from graphemes to phonemes does not rule out that slow access from phonemes (or assembled phonology) to word phonology may not also contribute to the speed impairment of the sublexical route.

A further aspect of inefficiency of sublexical reading may have to do with the grain size of sublexical orthographic and phonological segments (Ziegler & Goswami, 2006). Dyslexic readers may rely on small units such as graphemes and phonemes whereas competent readers may rely on lager orthographic segments such as letter clusters for consonant onsets or rimes. Consistent with this interpretation, Thaler et al. (2009) found that dyslexic children exhibited a specifically increased proportion of fixations on words which contained consonant clusters. A failure to rely on larger sublexical orthographic patterns may contribute to the disproportionately increased number of fixations for low frequency words shown by dyslexic readers in the present study.

We note that inefficient functioning of the sublexical route is expected from the dominant version of the phonological deficit explanation of developmental dyslexia which assumes a specific difficulty of dyslexic readers to access phonological segments below the lexical level (e.g., Snowling, 2000). In English-based dyslexia research, this difficulty is assumed to affect the acquisition of the grapheme–phoneme correspondences (e.g., Rack, Snowling, & Olson, 1992). In more regular orthographies with easy grapheme–phoneme associations, one may expect that a phonological deficit affects the speed with which sublexical phonological segments get activated. One of us introduced “phonological speed dyslexia” to distinguish this version of the phonological deficit account from the standard English-based account (Wimmer, 1993).

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.