Vowel dyslexia in Turkish: A window to the complex structure of the sublexical route

Abstract

We report on developmental vowel dyslexia, a type of dyslexia that selectively affects the reading aloud of vowel letters. We identified this dyslexia in 55 Turkish-readers aged 9-10, and made an in-depth multiple-case analysis of the reading of 17 participants whose vowel dyslexia was relatively selective. These participants made significantly more vowel errors (vowel substitution, omission, migration, and addition) than age-matched controls, and significantly more errors in vowel letters than in consonants. Vowel harmony, a pivotal property of Turkish phonology, was intact and the majority of their vowel errors yielded harmonic responses. The transparent character of Turkish orthography indicates that vowel dyslexia is not related to ambiguity in vowel conversion. The dyslexia did not result from a deficit in the phonological-output stage, as the participants did not make vowel errors in nonword repetition or in repeating words they had read with a vowel error. The locus of the deficit was not in the orthographic-visual-analyzer either, as their same-different decision on words differing in vowels was intact, and so was their written-word comprehension. They made significantly more errors on nonwords than on words, indicating that their deficit was in vowel processing in the sublexical route. Given that their single-vowels conversion was intact, and that they showed an effect of the number of vowels, we conclude that their deficit is in a vowel-specific buffer in the sublexical route. They did not make vowel errors within suffixes, indicating that suffixes are converted as wholes in a separate sublexical sub-route. These results have theoretical implications for the dual-route model: they indicate that the sublexical route converts vowels and consonants separately, that the sublexical route includes a vowel buffer, and a separate morphological conversion route. The results also indicate that types of dyslexia can be detected in transparent languages given detailed error-analysis and dyslexia-relevant stimuli.

Fig 1. A dual-route reading model.

Fig 2. Reading words: %vowel errors (of the 124 words) for each vowel dyslexia participant (orange dots) compared to the control group (average in the horizontal line, with 95% confidence interval around it).

Fig 3. Reading nonwords: % vowel errors of each participant with vowel dyslexia (orange dots) compared to the control group average (horizontal grey line, and 95% CI around it).

Fig 4

Percentage vowel letter errors (red and orange) and consonant letter errors (turquoise and purple) in reading words and nonwords per participant.

Fig 5

Distribution of vowel error types in words per participant out of the target words(top) and in nonwords(bottom)per participant out of the target nonwords with a lexical potential for such errors.

Fig 6

The regression lines (blue) and LOWESS smoothing curve (red) showing the relation between vowel error rate and the number of vowels in the word (left) and between vowel error rate and the total number of letters in the word (right).

Fig 7. % errors of the participants with vowel dyslexia in the same-different task (% errors of 101 items) compared with controls (mean errors and 95% CI around it).

Fig 8. Written word comprehension task with vowel distractors–percentage errors of the participants with vowel dyslexia in comparison to controls mean and 95% CI around it.

Fig 9. The locus of impairment of our participants in the word reading model, and the evidence regarding the status of each component from the various findings in this study: Blue text and arrows signify tasks in which the participants performed well; orange signifies impaired performance.

(Good written word comprehension and good irregular word reading–for vowel dyslexics without surface dyslexia).