Developmental Letter Position Dyslexia in Turkish, a Morphologically Rich and Orthographically Transparent Language

Güven, Selçuk; Friedmann, Naama

doi:10.3389/fpsyg.2019.02401

ORIGINAL RESEARCH article

Front. Psychol., 05 November 2019

Sec. Psychology of Language

Volume 10 - 2019 | https://doi.org/10.3389/fpsyg.2019.02401

This article is part of the Research Topic Developmental Dyslexia: From Cross-Linguistic and Bilingual Perspectives View all 10 articles

Developmental Letter Position Dyslexia in Turkish, a Morphologically Rich and Orthographically Transparent Language

$\r\nSeluk Güven,$ Selçuk Güven^1,2

Naama Friedmann^3*

¹School of Communication Sciences and Disorders, McGill University, Montreal, QC, Canada
²Department of Speech and Language Therapy, Anadolu University, Eskişehir, Turkey
³Language and Brain Lab, Sagol School of Neuroscience and School of Education, Tel Aviv University, Tel Aviv, Israel

We present the first report of a specific type of developmental dyslexia in Turkish, letter position dyslexia (LPD). LPD affects the encoding of letter positions, leading to letter migrations within words. In a multiple case study of 24 Turkish-speaking children with developmental LPD, we examined in detail the characteristics of this dyslexia and explored its manifestation in Turkish. We used migratable words, in which a migration creates another existing word (e.g., signer-singer), which exposed the migration errors of the participants. In sharp contrast with the common assumption that dyslexics in transparent languages, including Turkish, do not make reading errors, we have shown that right stimuli can detect even up to 30% reading errors. The participants made migrations in reading aloud, comprehension, lexical decision, and same-different tasks, in both words and non-words. This indicates that their deficit is in the orthographic-visual analysis stage, a stage that precedes the orthographic input lexicon and is shared by the lexical and non-lexical routes. Their repetition of non-words and migratable words was normal, indicating that their phonological output stages are intact. They did not make digit migrations in reading numbers, indicating that the orthographic-visual analyzer deficit is orthographic-specific. The properties of Turkish allowed us to examine two issues that bear on the cognitive model of reading: consonant-consonant transpositions were far more frequent than consonant-vowel and vowel-vowel migrations. This indicates that the orthographic-visual analyzer already classifies letters into consonants and vowels, before or together with letter position encoding. Furthermore, Turkish is very rich morphologically, which has allowed us to examine the effect of the morphological structure of the target word on migrations. We found that there was no morphological effect on migrations: morphologically complex words did not yield more (nor fewer) migrations than monomorphemic ones, migrations crossed morpheme boundaries and did not preserve the morphological structure of the target word. This suggests that morphological analysis follows the letter-position encoding stage.

Introduction

Dyslexia is a general term relating to a deficit in reading. By now, more than 20 different types of dyslexia have been reported, each with different error types and different characteristics, each resulting from impairments in different stages of the word reading process (Marshall, 1984; Castles and Coltheart, 1993; Ellis and Young, 1996; Jackson and Coltheart, 2001; Castles, 2006; Coltheart and Kohnen, 2012; Castles et al., 2014; Friedmann and Haddad-Hanna, 2014; Hanley, 2017; Friedmann and Coltheart, 2018). Even though much work has been done about types of dyslexia in various languages, almost no studies have examined the way different types of dyslexia manifest themselves in Turkish. In fact, only one paper reported a specific type of dyslexia in Turkish, and it was a study of acquired dyslexia (Raman and Weekes, 2005); we could not find any study that described specific types of developmental dyslexia in Turkish.

In this study, we describe, for the first time, a specific type of developmental dyslexia in Turkish and its characteristics. We report on a multiple case study of 24 Turkish-speaking children who show a developmental dyslexia that affects their ability to encode the position of letters: letter position dyslexia (LPD).

According to the dual-route model (Coltheart, 1981, 1985, 1987; Patterson, 1981; Ellis et al., 1987; Shallice, 1988; Humphreys et al., 1990; Coltheart et al., 1993, 2001; Ellis and Young, 1996), the early stage of word reading includes the visual analysis of the letter string. It encodes the abstract identity of the letters and their relative positions (Coltheart, 1981; Humphreys et al., 1990; Ellis and Young, 1996). Letter position dyslexia is a deficit in the function that encodes the relative positions of letters within words, which leads to letter migrations within words (e.g., slime → smile, cloud → could). Studies in several languages found that LPD is mainly manifested in migratable words, i.e., words in which letter migration creates other existing words (such as flies and files, slept and spelt). Most of the migration errors affect the middle letters, whereas the first and the last letters are relatively immune to migrations. Errors in migratable words are affected by frequency so that more errors occur when the target word (spelt) is less frequent than its migration counterpart (slept) (Friedmann and Rahamim, 2007). Individuals with LPD were found to make significantly more migrations that involve only consonants than migrations of a vowel and a consonant (Khentov-Kraus and Friedmann, 2018).

Letter position dyslexia has been reported so far for Hebrew, Arabic, and English (Friedmann and Gvion, 2001, 2005; Friedmann and Rahamim, 2007, 2014; Friedmann et al., 2010; Friedmann and Haddad-Hanna, 2012, 2014; Kohnen et al., 2012), in both acquired form (Friedmann and Gvion, 2001, 2005, for Hebrew; and Friedmann and Haddad-Hanna, 2012 for Arabic) and developmental form (Friedmann and Gvion, 2005; Friedmann and Rahamim, 2007; Friedmann et al., 2010 for Hebrew; Friedmann and Haddad-Hanna, 2012, 2014, for Arabic; Kohnen et al., 2012 for English). Until now, LPD has not been reported for Turkish.

A Brief Description of the Characteristics of the Turkish Language and Orthography

Turkish is morphologically very rich, and a single Turkish word may include multiple suffixes (e.g., the word “güldüremediklerimizdensin,” which means “you are the one that we were unable to make laugh,” contains eight suffixes). The modern orthography of Turkish is composed of a 29-letter alphabet of eight vowels and 21 consonants, based on a modified Latin script. In most cases, a single phoneme is represented with a single grapheme, and the grapheme-to-phoneme correspondence is consistent and transparent (Raman, 1999). The only exceptions are words borrowed from other languages, which are usually transferred into Turkish with their original phonology. For example, the word “katip,” which is borrowed from Arabic, is written with a single a but read, like in the Arabic origin, with a long a,/kaatip/. Syllables in Turkish (except for loan words) comprise of a single vowel. Canonical syllable structure in Turkish is CV, but other structures (such as V, VC, and CVC) also exist. Syllables with consonant clusters are rare (Raman, 1999), and the length of the vowel restricts the consonants of the coda (Kabak and Vogel, 2001). Stress position is regular and final (with some exceptions, Kabak and Vogel, 2001).

Dyslexia in Turkish

Very few studies about Turkish took the neuropsychological approach to acquired and developmental dyslexia. Or, in Raman and Weekes (2008) more positive words, “Research addressing the cognitive neuropsychology of acquired language disorders in Turkish has only just begun to flourish.” The exception is a study by Raman and Weekes (2005) who studied in detail a Turkish-English bilingual stroke patient who had an acquired lexical-phonological retrieval deficit, which made him unable to read via the lexical route. This led to surface dyslexia in English, and imageability effects in reading Turkish, with good reading of non-words.

We could not find any study on types of developmental dyslexia in Turkish. The few papers that examined developmental dyslexia in Turkish did not make the distinctions between different types of developmental dyslexia and have not characterized the types of errors each of the individuals with developmental dyslexia made. Raman (2011) tested a group of students with developmental dyslexia (without identifying the type of dyslexia each of them had) and examined the effect of Age of Acquisition in word and picture naming in this group in comparison to a non-dyslexic control group. Other studies worked under the assumption that dyslexia in Turkish mainly affects reading fluency (Erden et al., 2002; Özmen, 2005; Ergül, 2012). Studies of reading in typical development focus mainly on the contribution of phonological abilities to reading and spelling acquisition both in monolingual (Öney and Durgunoğlu, 1997; Durgunoğlu, 2002; Kesikçi and Amado, 2005; Babayiğit and Stainthorp, 2010) and bilingual populations (Özdemir et al., 2012; Özata et al., 2016).

The current study, therefore, aims to start filling the gap by (1) reporting and exploring in detail a type of dyslexia that has not been reported for Turkish yet, in either the acquired or developmental form. Exploring its characteristic error types and the properties of stimuli that are more susceptible to errors. (2) Reporting for the first time a specific type of developmental dyslexia in Turkish. (3) Examining the common belief that Turkish readers with dyslexia do not make reading errors – we will demonstrate that, when the relevant stimuli are selected, they definitely do make errors. The rich morphology and the CV structure of Turkish would further allow us to ask questions about properties of LPD that have not been tested so far: (4) What is the relative order of morphological decomposition and letter position encoding. (5) How early in the reading process are letters encoded as consonants or vowel letters.

Materials and Methods

Participants

Dyslexic Group

The dyslexic participants in this study were 24 monolingual Turkish-speaking children in 4th grade, aged 9–10, 8 males and 16 females. All of them were right-handed. All of the participants were living in Eskişehir, Turkey. They were pupils in regular schools and regular classes. According to the reports of their parents and/or teachers, and through informal observation made by SLPs, none of them had speech and language disorders (beyond reading deficits), nor any history of brain lesion, neurological condition, or cognitive problems. None of them had been previously diagnosed as having dyslexia or learning disability. However, when we discussed their reading with their teachers, the teachers expressed concerns about the reading of almost all the participants.

To select individuals who have developmental LPD for this study, we administered the dyslexia screening task FRİGÜ (Güven and Friedmann, 2014), described in the next section. We included individuals in the LPD group according to the following inclusion criteria: significantly higher total number of errors in the screening test compared with the age-matched group, significantly more letter position errors in the screening test than the control group (and at least 6 letter position errors), and less than 10% for other types of errors.

The 24 children with LPD were identified in the following way: 16 children were identified in a school-wide reading testing in which we administered the FRİGÜ reading screening test to 299 children. The other 8 children were recruited from teachers in 6 schools (in which children from varying SES – low-middle-high– status). The teachers referred children to us who they suspected had learning or reading difficulties. We tested the reading of these children using the FRİGÜ screening test, 8 had LPD and fitted the inclusion and exclusion criteria so they were included in the study.

Control Groups

The control group for the screening task included 205 fourth graders, 111 girls and 94 boys, with no report of reading disability. The control group for the further LPD tests included 71 fourth graders aged 9 to 10 years, 39 males and 32 females who had no speech, language, hearing or other cognitive problems, based on teacher or parent reports. Additionally, the children who were examined for the control group were tested by speech-language pathologists, who reported their clinical opinion regarding each child’s language. We excluded three children who the testers suspected to have a language disorder.¹

Procedure

Each of the participants was tested individually in a quiet room in the school. All stimuli were displayed on a white page in 14 pt. font, with double vertical spacing between words. No time limit was imposed during testing, the written word lists remained in front of the participants for as long as they needed, and no response-contingent feedback was given by the experimenter. In the silent reading tasks, we instructed the participant not to read aloud. In orally presented tests, the experimenter repeated every item as many times as the participant requested. Each of the participants took part in 13 tests, which were administered in several sessions. The number of sessions and length of each session were determined by each of the participants. The research was approved by the Ethical Committee of Anadolu University, Eskişehir, Turkey. The parents of each child signed written informed consent.

General Error Coding and Analysis

In the analysis of letter transpositions, we classified the transpositions according to the letters that participated in the transposition: consonant-consonant, vowel-consonant, and vowel-vowel migrations. If the participant produced a sequence of responses to a target word, and one of these responses was an error, we counted the item as being incorrect and analyzed the erroneous response.

Statistical Analyses

We examined whether each participant with dyslexia performed significantly below his or her age-matched control group using one-tailed Crawford and Howell’s (1998) t-test. Within-participant comparisons between two conditions were conducted using chi-squared tests (two-tailed comparisons). At the group level, comparisons between two conditions were conducted using the Wilcoxon Signed-Rank test, which is the non-parametric counterpart of paired samples t-test (reported with z statistic), and more than two conditions were compared using Friedman Test. For the correlation, Spearman’s rank correlation coefficient analysis was used. Effect sizes for t-tests are reported with Hedges’ g, and for Wilcoxon’s, when there is no normal distribution, with r (Fritz et al., 2012). Comparisons at the group level between the LPD group and the large control group were done using the t-test. An alpha level of 0.05 was used in all comparisons.

The Screening Test Used to Identify Children With Lpd for This Study

The first test we administered, which we used for initial assessment and identification of individuals with LPD to include in the study, and to examine the properties of their errors, was the screening test from the FRİGÜ test battery (Güven and Friedmann, 2014), which was developed to identify types of dyslexia in Turkish. The screening part of the FRİGÜ is an oral reading test that includes three blocks: 151 single words (2–8 letters long, M = 5.12, SD = 1.29), 60 word pairs (4–5 letters long, M = 4.88, SD = 0.92), and 40 non-words (2–9 letters long, M = 5.16, SD = 1.62). The word reading block was used to identify individuals with LPD according to the criteria described above.

Some researchers have claimed that in certain languages (e.g., languages with transparent orthographies), dyslexia does not manifest itself in errors, only in slow reading. We think this is a misconception that partly results from not using the right stimuli to elicit reading errors. Our approach was, on the basis of the approach of the Tiltan reading battery (Friedmann and Gvion, 2003), to base the reading test on knowledge of the types of words and non-words that are most sensitive to each type of dyslexia, i.e., the type of stimuli in which individuals with this kind of dyslexia make most errors of the relevant type. Ever since the early days of the cognitive neuropsychological approach to dyslexia – [opetwcite]B42,B7[clotwcite]Marshall and Newcombe (1973); Coltheart (1981), and Patterson (1981) found that dyslexias differ with respect to the types of stimuli that are most difficult in them. So there are “dimensions of words” as Patterson called it, for example, surface dyslexia is most evident in reading irregular words, phonological dyslexia in non-words, and deep dyslexia in abstract words, function words, and morphologically complex words.

The word and non-word lists of the FRİGÜ screening test were thus constructed so that they include items that are sensitive to each of the currently known types of dyslexia; words with different stress patterns or with ambiguous grapheme-to-phoneme conversion for identifying surface dyslexia; function words and morphologically complex words to identify phonological output buffer dyslexia, orthographic input buffer dyslexia, and deep dyslexia; abstract words for deep dyslexia; words (and non-words) with many orthographic neighbors for identifying orthographic analyser-output visual dyslexia, orthographic input buffer dyslexia, and letter identity dyslexia; words (and non-words) that can be read as other words by neglecting one side of the word, for identifying neglect dyslexia; and words in which vowel letter omissions, additions, migrations, or substitutions create other existing words, for the identification of vowel letter dyslexia.

The non-words were included for identifying phonological and deep dyslexia as well as various peripheral dyslexias; the word pairs were constructed such that between-word migrations create other existing words, to enable the detection of attentional dyslexia.

Importantly, the screening part of FRİGÜ was designed to also detect LPD. The list of 151 words contains 121 migratable words: 91 words in which a middle letter migration would create another existing word, and 54 words in which migration of exterior letters creates a word (24 of these words allowed for both interior and exterior migrations).

Results: Reading Screening Test

The participants with LPD made between 6 and 19 letter position errors in the single word reading block, with an average of 10.3 letter position errors (SD = 4.1). The control participants, on the other hand, made only 1.8 letter position errors on the average in this task (0–5 errors, SD = 2). Each of the LPD children performed significantly poorer than the control group [t(204) > 1.94, p < 0.02, for each of the participants].

Oral Reading of Migratable and Non-Migratable Words

Now that the screening task identified 24 children who had LPD, we continued with a line of tests that were developed to examine the nature of this dyslexia, and the way it is manifested in Turkish. We created a list of 183 migratable words to allow for the in-depth assessment of the effect of morphology on migrations (see section “Does Morphological Analysis Precede Letter Position Encoding? Assessing the Interaction of Morphology and Migrations”); the effect of the consonant-vowel status on migration (see section “Is Letter Position Encoding Sensitive to the Consonant-Vowel Status of the Target Letters?”); the position of the migrating letters within the word (assessing middle-exterior, adjacent-non-adjacent, within-across syllable, and length effect, see section “Further Analyses of the Properties of Letter Migrations in Turkish”); and frequency effect (see section “What Is the Nature of the Letter Position Encoding Deficit? Incorrect Underspecified Encoding? The Effect of Frequency on Migrations and Its Theoretical Implication”).

Experimental Stimuli

The migratable word list included 183 migratable words, 4-to-8 letters long (M = 5.2, SD = 0.9). Each of these words was such that at least one letter migration within the word results in an existing word (see examples for various types of migrations in Table 1). Each of the words in the list was also such that a letter identity error could create another existing word.

TABLE 1

Table 1. Examples for migration errors of various types that the LPD participants made.

Results

Migrations in Reading Migratable Words

The LPD participants made a total of 433 migration errors in reading the migratable word list. Figure 1 summarizes the letter position error rates the children with LPD made. Each of the 24 children with LPD made significantly more migrations than the control group (p < 0.001, using Crawford and Howell’s, 1998, t-test for the comparison of an individual to a control group). The difference was also significant at the group level, where the LPD group made significantly more letter migrations (10% migrations) than the control group (who made only 1% migrations on this list, SD = 1.27), t(24) = 10.71, p < 0.0001, with a very large effect size (g = 2.53). See Table 1 for examples for the various types of migrations that the participants with LPD made.

FIGURE 1

Figure 1. Reading migratable words: % letter position errors of each LPD participant (orange circles) compared to the control group average (horizontal blue line) in reading the list of 183 migratable words.

Refuting a Misconception: Turkish Dyslexics Do Make Errors When the Appropriate Stimuli Are Presented: Migratable vs. Non-migratable Words

We asked whether the participants make more migration errors when the target words are migratable, i.e., words in which a migration error can create another existing word (like the English word form, in which a migration error can create from) than on non-migratable words.

We compared the migrations in the list of 183 migratable words to the rate of migrations in reading a list of non-migratable words, which included words in which no single transposition created an existing word (e.g., the target word gözlük [glasses], for which all possible migrations result in non-lexical responses). This non-migratable word list included 32 words, 4-to-7 letters long (M = 4.9, SD = 0.7), with a relatively high frequency (all were among the 5000 most frequent words in Turkish, and more than half of them were among the top 2000 most frequent words, according to Aksan et al. (2016) frequency data.

This analysis showed a striking difference between migratable and non-migratable words: the children with LPD made no migrations in non-migratable words, whereas they made an average of 10% migrations in reading the migratable words (Table 2).

TABLE 2

Table 2. % migration errors in migratable and non-migratable words.

Relatedly, to examine whether the letter position errors of the participants with LPD tended to create existing words, we analyzed their migration responses in reading migratable words. For each migratable word, there was at least one option for a migration that yields an existing word, and at least one other option for migration response that yields a non-word. We examined whether the participants’ migrations tended to create existing words.

This analysis showed that when they read a migratable word with a migration error, most of their responses were lexical (76.6% of their migration responses were lexical, SD = 31.2%). The same was true in reading migratable non-words, in which, as we report below in Section “What Is the Locus of LPD in the Reading Model: Nonword Reading and Silent Reading Tasks – Results”, most errors were lexical as well.

The tendency to produce lexical responses guided us in the analyses of the characteristics of the participants’ migrations: for each analysis, we calculated the rate of errors out of the number of target words in which such errors would create an existing word, i.e., words that have a lexical potential for the relevant error type (for example, we calculated adjacent migrations not out of all words, but rather only out of the target words in which a migration of adjacent letters creates an existing word).

Does Morphological Analysis Precede Letter Position Encoding? Assessing the Interaction of Morphology and Migrations

Turkish has a very rich morphology, so studying LPD in Turkish readers allowed us to examine a theoretical question about the interaction between letter position encoding and morphological analysis. Specifically, we were interested in the relative order in which letter position encoding and morphological analysis take place. If morphological analysis precedes letter position encoding, then the morphological structure of the target word should affect letter position errors, and migrations should occur only within a morpheme. If, however, letter position encoding precedes morphological analysis, then the morphological structure of the target word should not affect migration errors.

To examine this question, we used three kinds of analysis. We examined whether morphologically complex words yielded a different rate of letter position errors than mono-morphemic words. We also tested whether letter migrations changed the morphological structure of the target word and whether migrations occurred only within morpheme or also across morphemes.

Analyses and Results

The first analysis examined whether morphologically complex words yield a different rate of letter position errors than morphologically simple words. For this analysis, we compared 44 morphologically-complex words from the migratable word list (which includes words with derivational and words with inflectional morphemes) to 78 monomorphemic migratable words. We selected these words so that they would be matched on length, which led us to include 4–6 letters long morphologically complex words (M = 5.39, SD = 0.61) and 5–7 letters long monomorphemic words (M = 5.22, SD = 0.47), so the word lengths in the two groups did not differ significantly.

The results were such that the children with LPD had very similar rates of migrations on the morphologically complex words (8.9%) and on the morphologically simple words (9.5%), and this difference was not significant (Wilcoxon z = 0.24, p = 0.81).

In the second analysis, we examined whether the migrations changed the morphological structure of the target word. This analysis indicated that there were quite a few migrations that changed the morphological structure of the target word: 21% of the migrations changed the morphological structure of the target word (SD = 19%). For example, the monomorphemic target word akran (peer) was read with a transposition as “arkan”, a morphologically complex word, constructed from arka (back), and the suffix –n (singular 2nd person possessor). Or the morphologically simple target word eskiz (sketch), which was read with a transposition as the morphologically complex “eksiz” in which the stem ek means “supplement” (or, to confuse us, “a morpheme”), and the suffix siz means “without” (so this morphologically complex word actually means “without a morpheme” i.e., a monomorphemic word).

The final analysis tested whether migrations occurred only within-morpheme or also across morphemes. This analysis indicated that, in reading the morphologically complex words, the participants transposed letters of the stem with letters of the non-stem morpheme (e.g., konulu: konu-lu, themed →kolunu: kolu-nu, your arm). Such cross-morpheme migrations occurred on 6.3% of the morphologically complex words (SD = 3.7). In fact, migrations within the stem, which involved two letters of the stem (yenile: yeni-le, renew → yinele: yine-le, repeat) and migrations within the non-stem morpheme (kirli: kir-li, dirty → kiril, Cyrillic) occurred less frequently than across-morpheme migrations (within the stem: 2% of the morphologically complex words, SD = 2.3, within the non-stem morpheme: 0.7%, SD = 1.1).

These three kinds of evidence for the lack of sensitivity of letter position errors to the morphological structure of the target word suggest that LPD affects a stage that precedes morphological analysis, and hence, it is not sensitive to the morphological structure of the target words.

Is Letter Position Encoding Sensitive to the Consonant-Vowel Status of the Target Letters?

To examine the theoretical question of when the consonant-vowel distinction becomes accessible during the process of single word reading, we examined the effect of the consonant-vowel status of a letter on the rate of migrations.

We asked two main questions: whether consonant and vowel letters are differentially susceptible to migrations, and whether they tend to migrate more within their class (consonants transpose with consonants and vowels with vowels) than across class.

Analyses

For this sake, we compared four types of migration:

(1) V₁CV₂: a transposition of two vowels across a consonant [e.g., eli (his/her hand) > “ile” (with)]

(2) C₁VC₂: a transposition of two consonants across a vowel [e.g., tak (attach) > “kat” (floor)]

(3) CC: a transposition of two adjacent consonants: [e.g., atlı (horseman) > “altı” (six)]

(4) CV: a transposition of adjacent consonant and vowel [e.g., atlı (horseman) > “atıl” (idle)]

We did this analysis only for the participants who did not have vowel dyslexia (Khentov-Kraus and Friedmann, 2018) in addition to LPD, because a higher rate of migrations that involve vowels may, in their case, be a result of their vowel dyslexia.

Within the 183 migratable words list, there were 98 words in which a consonant-consonant migration creates another word (51 words allowing for Adjacent CC migrations and 67 allowing for CC migration across a vowel); 63 words that allow for adjacent consonant-vowel migration, and 74 words that allow for a vowel-vowel transposition across a consonant.

Results

The results, summarized in Table 3, indicated a clear difference between the different kinds of migration. Migrations that involved only consonants (either adjacent CC transposition or CVC- transposition of two consonants across a vowel) were the most common type of migration (14%), whereas migrations that involved only vowel letters, or migrations that involved a consonant and a vowel occurred less often (5% each). A Friedman’s test indicated that the difference between the three types of migration (C with C, C with V, and V with V) was significant, χ²(2) = 30.45, p < 0.001, with consonant-only migrations being the most frequent migrations.

TABLE 3

Table 3. Consonant and vowel letter migrations of the participants with LPD in migratable word reading (% migrations of each type out of the number of migratable words with a lexical potential for such error).

Consonants migrated across vowels (C₁ V C₂ → C₂ V C₁) significantly more frequently than vowels across a consonant (V₁ C V₂ → V₂ C V₁), Wilcoxon z = 4.42, p < 0.0001, r = 0.64. Consonants transposed with an adjacent consonant (CC) significantly more often than with adjacent vowels (CV), Wilcoxon z = 3.85, p < 0.0001, r = 0.56.

Another type of analysis that points in exactly the same direction is the analysis of the “preferred migration type” in target words that allow for several types of migrations (C-C, V-V, V-C). There were 64 such target words, and the participants showed the same tendency toward consonant migrations: when they read a word in which several types of migrations were possible, they most often made a C-C migration (67% of the migrations on these words) and had much fewer V-V migrations (17%) or C-V migrations (16%). The C-C migrations were significantly more frequent than the other migration types, Friedman’s test χ² = 21.62, p < 0.001.

The results show clearly that consonant letters are more susceptible to transpositions than vowel letters, and that consonant-only transpositions occur more frequently than transpositions that involve a consonant and a vowel letter. Beyond its bearing on the characterization of LPD, this finding that indicates that the classification of letters to consonants and vowels happens very early in the process of orthographic-visual analysis, before or together with letter position encoding, and that consonant letters are processed separately and differently from vowel letters.² It is also interesting to note that the two children who had vowel dyslexia in addition to letter position dyslexia (SS and TEK, presented in the bottom of Table 3) made more transpositions that involve a vowel than transpositions that include only consonants.

Further Analyses of the Properties of Letter Migrations in Turkish

Middle vs. Exterior Letter Migrations

Studies of LPD in Hebrew, Arabic, and English report that individuals with LPD make more migrations that involved only middle letters than migrations that involve an exterior letter (Friedmann and Gvion, 2001; Friedmann and Rahamim, 2007; Friedmann and Haddad-Hanna, 2012, 2014; Kohnen et al., 2012). To examine whether this was also the case for Turkish LPD, we compared the rates of migrations that involved only middle letters and migrations that also involved an exterior (first or final) letter.

The 183 migratable words list included 91 words that have at least one possibility for middle migration, 34 words with a possibility for a migration that involves an exterior letter, and 58 words with a potential for both middle and exterior migrations.

The results, presented in Table 4, show that the Turkish-readers with LPD made both middle letter migrations and exterior letter migrations, but they, like LPD participants in the other languages tested, made significantly more migrations of middle letters (9%) than migrations that involved an exterior letter (6%), Wilcoxon z = 2.73, p = 0.006, r = 0.39.

TABLE 4

Table 4. Percentages migrations of the various kinds that the participants with LPD made in reading migratable words.

This predominance of middle migrations can also be seen in another type of analysis that assesses the “preferred migration type” in target words in which both middle and exterior migrations create other existing words. There were 55 such target words, and the participants showed the same tendency toward middle migrations: when they read a word in which both a middle migration and an exterior migration would create existing words, they made almost three times more middle migrations (73%) than exterior ones (27%), a difference that was significant, Wilcoxon z = 5.06 p < 0.0001.

Migrations of Adjacent and Non-adjacent Letters

Hebrew readers with LPD make more migrations in adjacent letters than non-adjacent letters. We tested whether this is the case also for Turkish children with LPD.

Within the 183 migratable words list, in 61 words only adjacent letter migrations created other existing words, 95 words allowed only for non-adjacent letter migration, and 27 words had a lexical potential for both adjacent and non-adjacent letter migration.

The results, summarized in Table 4, show that the Turkish LPD participants made significantly more migrations in adjacent letters (12%) than in non-adjacent letters (7%), Wilcoxon z = 3.09, p = 0.002, r = 0.45. However, as we report in Section “Is Letter Position Encoding Sensitive to the Consonant-Vowel Status of the Target Letters?,” the consonant-vowel status of the letter affects migration considerably; once the consonant-vowel status is kept constant (analyzing only consonant-consonant migrations), the size of the difference between adjacent and non-adjacent letters shrinks, Wilcoxon z = 2.02, p = 0.04.

Migrations Across and Within Syllables

We also investigated whether more migrations occur within- or across syllables. The results, presented in Table 4, show that there were significantly more across-syllable migrations (8%) than within-syllable migrations (5%), Wilcoxon z = 3.07, p = 0.002, r = 0.44.

Length Effect

To examine the effect of word length on the rate of migrations, we analyzed the participants’ migration rates in reading aloud the list of 183 4–8 letter migratable words (see section “Experimental Stimuli”).

We conducted Spearman’s Rho correlation coefficient analysis to see if there is a relationship between length and migration errors. A correlation analysis of the results (presented in Table 5) indicated that the correlation coefficient was low and non-significant (R = −0.11, p = 0.19).

TABLE 5

Table 5. The relationship between the number of letters and LPD error rate.

What Is the Locus of Lpd in the Reading Model: Non-Word Reading and Silent Reading Tasks

The next question was where in the word-reading model is the impairment that gives rise to LPD. For this sake, we examine the participants’ non-word reading and compare it to their word reading (section “Non-word Reading” below): if they make migration errors in non-words as well, this would mean that the deficit is not in lexical components and that it is rather in a component that is shared by lexical and non-lexical processes. We then test the participants’ silent reading using various tasks to examine whether the locus is in the orthographic-visual analysis stage. If it is, a deficit in the orthographic-visual analysis should cause migrations not only in reading aloud but also in lexical decision and comprehension of written words (section “Silent Reading Tasks” below). If the deficit is indeed in the orthographic input, phonological output should not show migrations when the input does not involve reading. This we tested in Section “Assessing Phonological Output Using Non-word and Word Repetition,” which examined the participants’ phonological output using non-word and word repetition.

Non-word Reading

To examine how the LPD participants read non-words, and to further test whether their deficit was pre-lexical or at a lexical stage, we presented them with an additional list of 60 non-words. The non-words in the list were 4-to-6 letters long (M = 4.92, SD = 0.42). Half of the non-words (30) were migratable, i.e., non-words in which a letter migration creates an existing word (e.g., the non-word “bakrı” is migratable because the migration of the two middle consonants creates the word “barkı,” his/her home). The other 30 non-words were non-migratable so that no migration created another word (e.g., “solik” or “bikeş”).

The 30 migratable non-words were selected to allow the examination of the characteristics of migrations also in non-words. To compare middle and exterior letter migrations, 27 of the non-words were such that at least one migration of an exterior letter would create an existing word, and 10 non-words were such that migration of middle letters would create an existing word (7 of these words had both middle and exterior migration potential). There were 15 words that had a potential of consonant-consonant transposition, 3 words with a potential of vowel-vowel transposition, and 25 words with vowel-consonant transposition potential.

Results

The 24 participants with LPD made an average of 18% migrations when reading the migratable non-words. The rate is significantly higher than that of the control group (2%), t(25) = 7.63, p < 0.0001, g = 1.8. On the individual level (see Figure 2), 21 children with LPD made significantly more migration errors than the control group (14 children p < 0.001, and 7 children p < 0.05, using Crawford and Howell’s (1998), t-test).

FIGURE 2

Figure 2. Migratable non-word reading: % letter position errors of each LPD participant orange circles) compared to the control group (horizontal blue line).

The participants made significantly more migrations in reading the migratable non-words (18%) than in reading the non-migratable non-words, where they made 7% migrations (Wilcoxon z = 4.41, p = 0.0001, r = 0.64). Still, the rate of migrations that the children with LPD made in non-migratable non-words was significantly larger than that of the control group, who only made 0.3% migrations in non-migratable non-words, t(23) = 4.92, p < 0.0001, g = 1.16. This finding is important because it indicates that the deficit that underlies the migration errors in LPD is not in the orthographic input lexicon but rather in an earlier stage that affects words, non-words, and non-migratable non-words: the orthographic-visual analyzer.

When we analyzed the errors that the LPD participants made in reading the migratable non-words (presented in detail in Table 6), we see that, like in their reading of the existing migratable words, they made significantly more migrations in middle letters (31%) than migrations that involved an exterior letter (9%; Wilcoxon z = 4.51, p = 0.0001, r = 0.65). They made significantly more migrations that involved only consonants (22%) than migrations of a consonant and a vowel (11%), Wilcoxon z = 3.01, p = 0.001, r = 0.44, and significantly more migrations across syllables (24%) than within a syllable (9%), Wilcoxon z = 4.30, p = 0.0001, r = 0.62.

TABLE 6

Table 6. Characteristics of migration errors in reading migratable non-words (% errors in each of the stimulus types).

Like in the word reading, most of the error responses the LPD participants made in reading the migratable non-words were lexical (M = 81.3% of the total errors in migratable non-words, SD = 17.1%), with significantly more lexical errors than non-lexical errors, Wilcoxon z = 5.36, p < 0.001, r = 0.77. When we only look at migration responses in reading the migratable non-word list, the picture remains the same: most of their migration responses were lexical (M = 88.6% of their transposition errors in migratable non-words, SD = 44.3%; significantly more lexical than non-lexical migration responses, Wilcoxon z = 5.63, p < 0.001, r = 0.82).

Not surprisingly, when they made migration errors in reading non-migratable non-words, where migrations could not yield an existing word, they produced mainly non-lexical responses (M = 33.6% of the total errors in non-migratable non-words, SD = 35.1%, Wilcoxon z = 2.87, p = 0.002, r = 0.42).

Silent Reading Tasks

If the source of letter migrations is indeed in a deficit in the orthographic-visual analysis stage, we would expect migrations to occur in silent reading tasks that do not involve reading aloud. To examine this, we ran 3 reading tasks that did not involve reading aloud: lexical decision, same-different decision, and comprehension.

Lexical Decision

The word list for lexical decision included 59 items: 29 words and 30 non-words, 4–5 letters long (M = 4.89, SD = 0.37). All the words and non-words were migratable. We asked the participants to read the list silently and mark only the existing words.