Developmental dyslexics show deficits in the processing of temporal auditory information in German vowel length discrimination

The present study investigated auditory temporal processing in developmental dyslexia by using a vowel length discrimination task. Both temporal and phonological processing were studied in a single experiment. Seven German vowel pairs differing in vowel height were used. The vowels of each pair differed only with respect to vowel length (e.g., /a/ vs. /a:/). In German, vowel length is characterized by temporal and spectral information. Three types of differences between long versus short vowels were varied: In the phonological condition, pairs of natural vowels were used, differing in their temporal as well as in their spectral content. In two temporal conditions, in contrast, a natural vowel was always combined with a manipulated one to keep spectral content of long and short vowels identical. Thus, the only distinguishing feature between the two vowels was temporal in nature. Vowels were embedded into monosyllabic pseudo-words and presented successively in a speeded same–different task. Twenty dyslexics and twenty age-matched controls participated in the experiment. In both groups, discrimination accuracy decreased with increasing vowel height in the two temporal conditions. This result is consistent with former findings on the relevance of temporal information for vowel length identification in German and extends this topic to cover discrimination demands. In the phonological condition, groups did not differ in discrimination accuracy. In both temporal conditions, however, dyslexics performed worse than controls. These results suggest that developmental dyslexia is associated with impairments in processing basic acoustic parameters of the speech signal, in particular, with a deficit in temporal processing.