Patterns of de¢cit in auditory temporal processing among dyslexic adults Gal Ben-Yehudah, 1, w, * Karen Banai 1, * and Merav Ahissar 2,CA Departments of 1 Neurobiology; 2 Psychology, Hebrew University, Jerusalem 91905, Israel w Present address: 608 LRDC, Pittsburgh University, Pittsburgh, PA 15260, USA CA Corresponding Author: msmerava@mscc.huji.ac.il * G. B.-Y. and K. B. contributed equally to this work Received10 December 2003; accepted 6 January 2004 DOI: 10.1097/01.wnr.0000117647.40277.f8 We characterized temporal psychoacoustic performance in a group of adult dyslexics with excellent cognitive abilities. These in- dividuals performed adequately on a two-tone frequency discrimi- nation task when inter-stimulus intervals were long (Z1s or more). However, their performance gradually deteriorated as in- terval duration was shortened (0.7^ 0.1s). Dyslexics with similar reading abilities, but with average cognitive abilities, performed poorly at all intervals. These ¢ndings show that, though both groups su¡er from non-phonological de¢cits in auditory discrimi- nation, the nature of their de¢cits di¡ers. Dyslexics with excellent cognitive abilities seem to have slow, yet adequate, stimulus identi- ¢cation and comparison mechanisms.The fundamental impairment of the other dyslexic group is di/cult to determine because their performance was poor across all intervals. NeuroReport 15:627^ 631  c 2004 Lippincott Williams & Wilkins. Key words: Auditory processing; Dyslexia; Frequency discrimination; Inter-stimulus interval; Sluggish attention; Temporal processing INTRODUCTION Developmental dyslexia is a specific reading disability affecting 5–10% of the population, in which proficient reading skills are not acquired despite sufficient cognitive abilities and education. Although it is commonly accepted that the majority of dyslexics have impaired phonological awareness [1], dyslexia is a heterogeneous disorder. Dys- lexic individuals vary in types of reading errors, presence of other developmental disorders (e.g. language delay [2]) and the extent of perceptual difficulties [3]. Perceptual deficits in dyslexia have been found on both visual [4] and auditory [5–7] tasks. The relationship between perceptual deficits and subtypes of dyslexia is still an open question. A prominent perceptual theory proposes that dyslexics suffer from a fundamental deficit in processing rapidly presented brief signals. This theory is based on Tallal’s [8] finding that reading-disabled children had a difficulty in judging the temporal order of complex tones when separated by short intervals. Since this difficulty was highly correlated with decoding ability (reading non-words), Tallal suggested that impaired acoustic processing leads to inadequate perception of brief phonemes, which degrades phonological awareness and impedes reading acquisition. According to this theory, dyslexics should have difficulties in auditory processing whenever stimuli appear in fast succession, but not when presentation rate is slower or stimuli are longer. There is substantial evidence for dyslexics’ impaired performance with brief stimuli (see [6,9] for reviews); however deficits with longer stimuli were also reported [5–7,9]. For example, in frequency discrimina- tion tasks, using a variety of procedures, significantly higher thresholds were found in disabled compared with age- matched normal readers [4,5,7,10]. Ahissar et al. [5] measured two-tone frequency discrimination with both temporal order judgments of brief tones and intervals, and same/different judgments of longer tones and intervals. Under both procedures frequency discrimination was impaired among disabled readers and thresholds were similarly correlated with decoding abilities. Frequency discrimination findings vary extensively. Although many studies found poorer performance in dyslexics than in normal readers [4,7,10], others did not [11]. Two inter-study differences may underlie this variance. First, studies vary in the experimental design used, such as tone duration, inter-stimulus interval (ISI) and behavioral paradigm, which could affect dyslexics’ relative difficulties. Second, studies vary in their recruitment procedure, which affects the heterogeneity of the dyslexic group and potentially their perceptual discrimination abilities. Although dyslexics form a highly heterogeneous popula- tion, the relevant subtyping to groups differing in their etiology of reading deficits is still debated. One classification to subtypes is based on patterns of reading and spelling difficulties. However, it is difficult to divide dyslexics to those with and without phonetic deficits, since most have both [1]. Another classification is based on co-morbidity of reading difficulties with other learning disorders, such as an oral language delay [12]. Indeed, several studies suggested that temporal processing deficits are found only among dyslexics with concomitant language disorders [12]. These AUDITORYAND VESTIBULAR SYSTEMS NEUROREPORT 0959-4965  c Lippincott Williams & Wilkins Vol 15 No 4 22 March 2004 627 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.