INTRODUCTION
In lexical decision tasks (LDTs) participants are required to discriminate between real words and foils. The difficulty of the discrimination varies according to the characteristics of the foils: as orthographic and phonological overlap between words and foils increases, the LDT becomes progressively more difficult. Participants are faster and more accurate at rejecting unpronounceable illegal non-words (i.e., letter strings such as GLDT) than pronounceable pseudowords (i.e., nonsense strings of letters that respect the orthographic rules of a given language but have no semantic content such as RINAFO; e.g., Holcomb and Neville, 1990; Forster et al., 2003; Ratcliff et al., 2004). Evans et al. (2012) reported that, as foils become increasingly word-like, non-word (“no”) responses became significantly slower and less accurate: reaction times (RTs) were shorter, and accuracy was higher, for consonant strings compared to pseudowords. Moreover, also real word (“yes”) responses in the context of increasingly word-like foils were slower and less accurate. Passing from non-word to pseudoword foils there is a progressive increase in pronounceability as well as in orthographic and phonological similarity to real words. Moreover, as foils become more word-like and orthographic and phonological overlap increases, foils produce more activation of similar words (Harm and Seidenberg, 2004) and the discrimination becomes more difficult, indicating that a higher level of activation is required for a real word (“yes”) decision to avoid false alarms.
Some authors investigated whether pronounceability influences the depth of processing required for lexical decision. James (1975) found that this task involves retrieval of semantic information (as highlighted by the concreteness effect) only in the presence of pronounceable distractors, while, decreasing the similarity between words and foils, the use of unpronounceable distractors makes the semantic retrieval unnecessary. Similarly, Evans et al. (2012) reported smaller effects of imageability and semantic priming as decision difficulty and RTs decreased from pseudo-homophone, to pseudoword, and non-word foil contexts, with semantic effects minimized with unpronounceable foils. Moreover, semantic effects increased significantly as decisions became harder (and slower) with more word-like foils. Dimitropoulou et al. (2010) examined the masked onset priming effect (MOPE) by manipulating the primes’ lexicality, frequency, and pronounceability. The MOPE indicates faster naming latencies when a target word (e.g., BREAK) is preceded by a briefly presented masked prime that shares its initial sound with the target (e.g., belly) compared to when it does not (e.g., merry) or when it rhymes with it (e.g., stake; Forster and Davis, 1991). This effect has been interpreted as an advantage in speech planning of the response or as evidence of prime processing by the non-lexical route. Dimitropoulou et al. (2010) found the MOPE for all types of stimuli but unpronounceable non-words, a result favoring the speech planning hypothesis. Finally, pronounceability has been examined also in terms of the facilitation present for repeated stimuli. In a LDT, the repetition priming was larger in experiments with pseudowords than in experiments with non-words (Ratcliff et al., 2004).
The effect of pronounceability has been evaluated not only in LDTs, but also with other experimental paradigms. Seidenberg et al. (submitted) examined whether pronounceabililty influences the reading aloud process. Participants had greater difficulty in naming non-words containing grapho-tactically illegal sequences of letters (e.g., JULBZ) as compared to grapho-tactically legal non-words containing digraphs (i.e., multi-letter graphemes that map onto a single phoneme, e.g., the “ee” in NEESH). These findings indicate that pronounceability is a key factor in determining naming latencies, with pronounceable pseudowords being responded to faster than unpronounceable non-words. Differences are also observed when participants have only to identify a single letter in the stimulus in a post-cued letter-in-string identification task. Thus, letter recognition is more accurate in the context of a pronounceable pseudoword than in the context of a consonant string, the so-called pseudoword superiority effect (PSE; e.g., Baron and Thurston, 1973; Spoehr and Smith, 1975; Grainger and Jacobs, 1994, 2005).
It is worth noting that when also reading proficiency is taken into account the framework becomes more complex. In fact, if two groups (e.g., dyslexic and proficient readers) vary in general speed of processing (hereafter referred to as the global factor), group differences in latencies would depend on both the difficulty of a given task and the general group differences in processing speed (Faust et al., 1999). Then, for groups showing global differences in performance, one should expect to find over-additivity effects; i.e., the absolute group differences in performance would tend to grow as a function of task difficulty over and above the characteristics of the specific experimental manipulations (Faust et al., 1999). The presence of over-additivity may induce overestimation or underestimation of the contribution of specific variables modulating reading performance. Therefore, it may not be easy to identify the presence of a deficit in processing unpronounceable stimuli because of differences in task difficulty across experimental conditions and their interaction with basic group differences in rate of information processing. Models such as the rate and amount model (RAM; Faust et al., 1999) reveal the presence and characteristics of the global factor in information processing by distinguishing between the performance of dyslexic and proficient readers and isolating the conditions in which children with dyslexia show specific deficits not ascribable to over-additivity (Zoccolotti et al., 2008).
In previous studies, we found that a single global factor accounted for a very large proportion of the impaired performance of children with dyslexia in making lexical decisions and reading words and pseudowords (Di Filippo et al., 2006; Zoccolotti et al., 2008; Marinelli et al., 2011). Other studies showed that the global factor was present for orthographic but not pictorial stimuli (Zoccolotti et al., 2008) and in the visual, but not the auditory, modality (Marinelli et al., 2011). The global factor did not emerge in a variety of letter and bigram tasks even though their general difficulty was made similar to that of letter strings (i.e., both words or pseudowords; De Luca et al., 2010). In fact, tasks mapping letter (and bigram) recognition loaded on a separate factor other than that accounting for words and non-words.
These studies indicate that children with dyslexia are selectively impaired in processing visually presented strings of letters with or without lexical value. We have proposed that this deficit has a pre-lexical graphemic locus (e.g., De Luca et al., 2010), i.e., marks an impairment in forming a graphemic description of the letter string (Zoccolotti et al., 2008). This idea is in keeping with other proposals based on imaging and lesional studies of the so-called “visual word form area” (VWFA; Cohen et al., 2000, 2002). The local combination detector (LCD) model (Dehaene et al., 2005) posits that written words are encoded by a hierarchy of detectors tuned to increasingly larger and more complex word fragments (visual features, single letters, bigrams, quadrigrams and, possibly, words). At the neural level, information from letter features and single letter converges on the VWFA; here, a posterior-to-anterior gradient is present with a progression in selectivity to increasingly word-like stimuli (e.g., Dehaene et al., 2004; Vinckier et al., 2007). Over years of practice, frequent combinations of letters are selected to be represented by dedicated neurons (Cohen et al., 2008), and the VWFA becomes attuned to the regularities of the writing system, yielding fast parallel processing in reading (Vinckier et al., 2007; Cohen et al., 2008). Importantly, several studies found that dyslexic individuals show selective hypo-activation of the VWFA (for a review see Richlan et al., 2009). In a similar vein, Marsh and Hillis (2005) proposed that this area is involved in the computation of a prelexical “grapheme description” independent of case, font, location, or orientation. Notably, such graphemic description does not require stored knowledge of spelling or spelling-sound correspondences. In this perspective, the reading impairment of children with dyslexia might be ascribed to a deficit at the level of graphemic analysis.
Another interpretation of the dyslexic deficit is that the impairment is related to the inability to bind orthographic and phonological information (Ziegler et al., 2010; van den Broeck and Geudens, 2012). Evidence in this direction comes from imaging studies indicating a close association between letter and speech sounds early in development (for a review see Blomert, 2011). Accordingly, effective letter–speech sound integration is an emergent property of learning to read supported by an interrelated network of visual, auditory, and heteromodal brain areas. There is evidence that dyslexic individuals are impaired in letter–speech sound integration. For example, in a functional magnetic resonance imaging (fMRI) study, Blau et al. (2009) reported that adult dyslexic readers showed underactivation of the superior temporal cortex for the integration of letters and speech sounds. In a further study, Blau et al. (2010) reported that, unlike control readers, cortical responses to speech sounds of dyslexic individuals were not modulated by letter–speech sound congruency. In a complementary line of research, it has been reported that proficient readers activate orthographic representations in phonological LDT while children with dyslexia do not (van der Mark et al., 2011). Thus, dyslexic individuals fail to activate orthographic representations during spoken language processing. Finally, also previously described results on the VWFA are not necessarily incompatible with an orthographic–phonological binding perspective. The presence of interactions between orthographic and phonological processing is suggested by evidence indicating connections between the VWFA and language areas (Cai et al., 2008; Greve et al., 2013). First, it has been noted that the VWFA shows a clear lateralization with only the left, linguistic, hemisphere that becomes specialized for reading (Cai et al., 2008); furthermore, it has been reported that asymmetries in the VWFA are correlated with the ear advantage in a dichotic listening task (Greve et al., 2013). Overall, it has been proposed that a deficit in orthographic–phonological binding may represent a proximal cause of the reading slowness in dyslexia and may also help understanding the deficit in reading fluency of these individuals (Blomert, 2011).
In the present research, we tested these alternative hypotheses by evaluating the role of stimulus pronounceability on accuracy and latency in two different tasks: a LDT (Experiment 1), and a two-alternative forced-choice task (the so-called Reicher–Wheeler paradigm: see Reicher, 1969; Wheeler, 1970; Experiment 2).
In the LDT we are interested in assessing whether the impairment shown by children with dyslexia in processing letter strings is present only for pronounceable words and pseudowords or it is also detectable (and of a similar size) with unpronounceable letter strings. The first outcome would favor an orthographic–phonological binding interpretation while the latter a pre-lexical graphemic locus of the reading deficit. In fact, strings of consonants are not pronounceable and, as such, would not activate orthographic–phonological binding to the same extent as words or pseudowords. Therefore, based on the orthographic–phonological binding hypothesis we would expect a smaller deficit for non-words as compared to words and pseudowords (once the effect of over-additivity is controlled for). Based on the hypothesis of a letter string graphemic deficit, no interaction between groups and item pronounceability would be expected. All condition means in the LDT (for words, pseudowords, and non-words) are supposed to fit, in the RAM, with the same letter string factor.
The Reicher–Wheeler paradigm proposes the same types of stimuli used in the LDT, i.e., words, pseudowords and non-words, but does not require the simultaneous processing of several letters. In fact, the decision is to be made on the discrimination between a target letter and a competitor in the context of a previously displayed letter string. Assuming that the main impairment of children with dyslexia has to do with simultaneous letter string processing and not with single letters, the lack of a global factor in the RAM model might be expected. However, examining the context effects (i.e., the lexicality and the pronounciability of the prime) it is possible to test whether children with dyslexia can gain advantage from lexical activation and pronounceability in graphemic processing as much as control children.
In particular, testing for the PSE (i.e., letter identification is more accurate in the context of a pseudoword than in the context of a unpronounceable non-word) would allow evaluating whether children can take advantage from a pronounceable letter string which forms a typical orthographic context in Italian. Furthermore, testing for the word superiority effect (WSE; i.e., letter identification is more accurate in the context of a word than in the context of a pronounceable pseudoword) would allow evaluating whether children can take advantage of the lexical activation triggered by a word context in the successive letter recognition. According to the orthographic–phonological binding hypothesis we would expect, for children with dyslexia, a lack of both PSE and WSE, while for skilled readers a role of lexicality and pronounceability of the context is expected. On the other hand, the letter string graphemic deficit hypothesis would predict no differences in the effect size of the WSE and PSE in relation to reading proficiency, as the task does not require any decision on the lexicality of a specific letter combination, but only the discrimination between a single target letter and a competitor one. Furthermore, as no selective deficit in orthographic–phonological interaction is envisaged, one would expect children with dyslexia to be able to take advantage of the ortho-phonotactic regularities of the language (i.e., they are expected to show a PSE).
In order to apply the RAM model to the data from both the LDT and Reicher–Wheeler paradigms, we examined the speed of processing of children with dyslexia in responding to these tasks. There is evidence that, even though most studies on the Reicher–Wheeler paradigm focused on accuracy, parallel effects have also been reported with time measures (RTs and visual evoked potentials; Ziegler et al., 1997; Martin et al., 2006). To test the global factor it is important to have a sizeable spread of performances across conditions (Faust et al., 1999). To this aim, in LDT we presented high-frequency words, low-frequency words, pseudowords and non-words, varying for length within each category (from 4 to 6 letters), for a total of 12 different conditions. Previous data on Italian children indicate that children with dyslexia show frequency (e.g., Barca et al., 2006) and lexicality (e.g., Zoccolotti et al., 2008) effects both in reading and LDT (Paizi et al., 2013). These effects tend to be greater in children with dyslexia than in typically developing children in raw data analyses but, typically, this group interaction disappears when over-additivity is controlled for (Di Filippo et al., 2006; Zoccolotti et al., 2008; Paizi et al., 2013). In the Reicher–Wheeler paradigm, primes were 4-letter high-frequency words, pseudowords and non-words, with the target letter in first, second, or third position, for a total of nine different conditions. Note that no data are yet available on this paradigm on Italian children.
GENERAL DISCUSSION
The results from the LDT in Experiment 1 indicated that a single global factor explained the performance with orthographic strings, independent from stimulus pronounceability, as well as frequency and lexicality. The children with dyslexia’s impairment was evident (and of a comparable size) when they had to process strings, not only of pronounceable stimuli (such as words and pseudowords) as already reported in previous studies (e.g., Di Filippo et al., 2006; Marinelli et al., 2011; Paizi et al., 2013), but also of unpronounceable stimuli, a deficit well accounted for by the same global factor. Thus, the present study adds a new piece of information to the understanding of the nature of the global component affecting performance of children with dyslexia. Previous studies indicated that a single global factor explains the deficit of children with dyslexia in making lexical decisions and reading words and pseudowords, i.e., independent of word frequency and lexicality (Di Filippo et al., 2006; Zoccolotti et al., 2008; Marinelli et al., 2011), but not in dealing with pictorial stimuli (Di Filippo et al., 2006; Zoccolotti et al., 2008) or stimuli in the auditory modality (Marinelli et al., 2011). The present study adds to this picture that the global factor is independent not only from the lexical status of the stimulus, but also from the pronounceability of the letter string: when the over-additivity effect was controlled for, the deficit of children with dyslexia in the LDT was detectable, and of a comparable size, when rejecting pronounceable pseudowords or unpronounceable non-words. Therefore, the present findings are consistent with the proposal that an impairment in pre-lexical graphemic analysis (i.e., in forming a graphemic description of the letter string) is a core deficit in developmental dyslexia (Zoccolotti et al., 2008); by contrast, they do not support the idea that the deficit in dyslexia is due to an inability to bind orthographic and phonological information (Ziegler et al., 2010; van den Broeck and Geudens, 2012), not even in Italian, a language with very consistent orthography.
The RT data from the Reicher–Wheeler paradigm indicated that group differences in this task did not generate global differences between children with dyslexia and control readers. These data are generally in keeping with previous observations by De Luca et al. (2010) indicating that children with dyslexia were only mildly affected in tasks requiring the naming or matching of individual letters, bigrams or two-letter syllables and no over-additivity effect was present for these tasks. Therefore, it appears that the global factor accounting for the impairment of children with dyslexia is present when the child processes a (relatively long) string of letters in parallel, not when the task concerns isolated letters. The present results add to this picture that, even if the processing of a letter string is slowed down in these children, they can take advantage from the ortho-phono-tactic information deriving from such processing in discriminating a subsequent isolated target letter from a competitor; i.e., they showed a clear PSE (at least in the case of accuracy) in the Reicher–Wheeler paradigm. This differentiation can be appreciated most clearly by comparing the performance in making a lexical decision on pseudowords with that of recognizing a target letter in the presence of a pseudoword context. In the first condition, children with dyslexia were severely impaired in both accuracy and speed; in the second, they were more accurate than in the case of a four-letter non-word context (i.e., they had a PSE) and the group difference with typically developing children was quantitatively quite small. Therefore, when cognitive tasks (e.g., lexical decision, naming, semantic categorization, etc.) are to be applied to letter strings as a whole, children with dyslexia are in difficulty. On the contrary, when tasks involve isolated letter processing, also these children can make use of the ortho-phono-tactic information derived from a previously seen letter string. This spared ability appears inconsistent with the idea that children with dyslexia suffer from a selective deficit in orthographic–phonological binding. By contrast, it is consistent with a pre-lexical graphemic interpretation; according to this view, online simultaneous processing of multi-letter elements is generally impaired. However, if sufficient time is given for processing a letter string, children with dyslexia may effectively use its ortho-phono-tactic information to modulate orthographic processing of isolated letters.
The present findings are in keeping with the available information on the characteristics of the VWFA. Thus, neuroimaging studies indicate that the VWFA is activated not only by orthographically legal stimuli, such as words and pronounceable pseudowords, but also by illegal letter strings (e.g., Cohen et al., 2002). There is clear evidence that event-related potentials (ERPs) recorded at posterior sites within the 150–250 ms time window at fronto-central, central, and temporo-parietal sites (typically in the form of the N200) are modulated by orthographic information. This finding is consistent with the hypothesis that the word form system analyzes visual linguistic stimuli at a pre-lexical level while information concerning lexical status and meaning is processed through additional neural systems (Bentin et al., 1999). In ERP studies, the VWFA does not generally differentiate between pseudowords and words (Hagoort et al., 1999) and no difference in N200 amplitude for these two stimuli is found (e.g., Tagamets et al., 2000 for a fMRI study). With regard to non-words, in some reports, the N200 was larger for non-words than for words (Compton et al., 1991; McCandliss et al., 1997), whereas, in others, the opposite pattern was reported (Cohen et al., 2000; Dehaene et al., 2002; Grossi and Coch, 2005) or no difference between legal and illegal orthographic letter strings was detected (e.g., Bentin et al., 1999). Some inconsistencies between studies may depend from differences in the experimental task. In fact, studies used several experimental paradigms: a letter search task (Ziegler et al., 1997), a letter-in-string identification task (Coch and Mitra, 2010), a LDT (Rosazza et al., 2009; Massol et al., 2011), and a rhyme judgment task (Bentin et al., 1999). In general, research comparing the processing of consonant strings and pronounceable pseudowords reports a divergence in the ERP waveforms as a function of target type only starting at around 200–250 ms post-target onset (Ziegler et al., 1997; Rosazza et al., 2009; Massol et al., 2012). This is in line with Grainger and Holcomb’s (2009) proposal that processing up to around 200 ms post-target onset is largely identical for these two types of stimuli mostly involving parallel independent letter processing. Overall, it seems that the VWFA is activated by orthographic stimuli, independent from the lexical status or the pronounceability of the stimuli. In pinpointing a parallel between the present results and the characteristics of the VWFA it is important to observe that several studies reported a marked underactivation of this area in dyslexic individuals (for a review see Richlan et al., 2009).
A recent proposal which helps in placing the letter string deficit shown by children with dyslexia is the dual-route approach to orthographic processing proposed by Grainger and Ziegler (2011). According to this model, the initial mapping of visual features onto abstract letter identities operates in parallel and simultaneously for all the letters in the stimulus (e.g., see also Grainger and van Heuven, 2003; Adelman et al., 2010): “…the alphabetic array codes for the presence of a given letter at a given location relative to eye fixation along the horizontal meridian. It does not say where a given letter is relative to other letters in the stimulus…. Thus, processing at the level of the alphabetic array is insensitive to orthographic regularity of letter string” (Grainger and Ziegler, 2011, p. 2). The distinction between non-words, pseudowords, and words would turn out only later in the pathway, when the letter identity is referred to a specific position within the word (defined as a string of letters separated by spaces). Two different types of sublexical word-centerd orthographic representations develop in the reading acquisition process, according to the frequency of occurrence of given combinations of letters: (a) coarse-grained representations (open-bigram representations) that code for the presence of “ordered pairs of letters independently of their contiguity” (e.g., for the string WORD open-bigram representations are WO, WR, WD, OR, OD, RD); (b) fine-grained representations, that code for clusters of frequently co-occurring letter combination (e.g., multi-letter graphemes, syllables, morphemes, rhymes, etc.). The coarse-grained code offers diagnostic features for a rapid bottom-up activation of whole-word representations. However, for the correct identification of the target word is necessary the top-down activation from whole-word orthography level to coarse-grained orthography level. Only real words can activate this interactive process. In the case of pseudowords, the absence of top-down constrains makes the processing via the slower fine-grained route the only way to get disambiguating information on the letter string. Present findings highlight that the global factor explaining dyslexic’s deficit is independent from pronounceability and lexicality of the stimulus. Then, according to the dual-route model (Grainger and Ziegler, 2011), it appears to indicate a deficit at an early stage of processing, i.e., when the initial mapping of visual features onto abstract letter identities is performed. In the subsequent stages of processing, children with dyslexia do not appreciably differ from control readers, as highlighted by the absence of the group by pronounceability or the group by lexicality interactions, once over-additivity was controlled for.
It is interesting to speculate on which mechanism may underlie the selective deficit in processing letter strings shown by children with dyslexia. As stated above, the deficit is confined to the simultaneous processing of several letters while it is much smaller or absent when the task regards single letters or bigrams (e.g., Bosse et al., 2007; Martelli et al., 2009; De Luca et al., 2010) or when the target letters are presented sequentially (Lassus-Sangosse et al., 2008). The present results indicate that children with dyslexia can actually use information from a letter string provided that responding is limited to a single letter presented subsequently to the letter string prime. Thus, the requirements for targets to be multiple and input to be simultaneous seem at the core of the group difference. One reasonable candidate to accommodate for these characteristics is visual crowding. Crowding refers to the decrease in recognizability of a letter surrounded by other letters placed closer than a critical distance (e.g., Pelli et al., 2004, 2007). In the case of letter strings, crowding affects the central letters much more than the initial or final ones (Bouma, 1970); thus, it seems to explain well the single-multiple dimension, as crowding between letters is only expected in the case of multiple displays and not with isolated letters. Further, as a perceptual mechanism, crowding can also easily account for the simultaneity requirement. Early evidence that children with dyslexia show enhanced sensitivity to crowding was presented by Bouma and Legein (1977). In the last years, several studies have shown results compatible with this interpretation (Spinelli et al., 2002; Martelli et al., 2009; Callens et al., 2013; Collis et al., 2013). For example, Martelli et al. (2009) found critical spacing to increase as a function of eccentricity with a greater proportionality for children with dyslexia than typically developing readers. Furthermore, particularly in the dyslexic group, degree of crowding appears to correlate significantly with reading (Martelli et al., 2009; Callens et al., 2013).
It is important to keep in mind that we examined the reading performance of children speaking a very regular language. It is well-known that orthographic consistency modulates the reliance on holistic reading processes (e.g., Ziegler et al., 2001; Ziegler and Goswami, 2006). For this reason, the present findings cannot be directly generalized to inconsistent orthographies, such as English or Hebrew. At any rate, it is interesting that several investigations based on children speaking French, a moderately irregular language, are in keeping with a visual-orthographic, as compared to a visual-to-phonology, impairment (e.g., Lobier et al., 2012a,b; Valdois et al., 2012). For example, Lobier et al. (2012a) reported that, in a visual categorization task with verbal and non-verbal stimuli, children with dyslexia were impaired independently of stimulus type, in keeping with the idea that the impairment was visual and not verbal. These findings suggest that a deficit in pre-lexical graphemic analysis may be present also in inconsistent orthographies, although this possibility certainly deserves further examination.
Overall, children with dyslexia were impaired when they had to process strings, not only of pronounceable stimuli but also of unpronounceable stimuli, a deficit well accounted for by a single global factor. By contrast, they were much less affected when they had to recognize an isolated letter (and no global factor was present) and could take advantage of a pronounceable context, effectively using the ortho-phono-tactic information derived from a previously seen letter string. Therefore, the present findings are in keeping with the proposal that an impairment in pre-lexical graphemic analysis is a core deficit in developmental dyslexia at least in a regular orthography (such as Italian) while they are inconsistent with the alternative view that orthographic–phonological binding may represent a proximal cause of dyslexia.