Prolonged attentional dwell time in dyslexic adults R. Hari * , M. Valta, K. Uutela Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland Received 7 June 1999; accepted 21 June 1999 Abstract Dyslexic adults have been shown to be slow in processing rapid sequences of stimuli in all sensory modalities. We now demonstrate, by means of an attentional blink task, that the attentional dwell time is prolonged by ~30% in dyslexic adults compared with normal readers. Thus a target captures attentional resources for considerably longer time in dyslexics than control subjects. The observed prolongation could signi®cantly contribute to the sluggish temporal processing of dyslexic adults. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Dyslexia; Temporal processing; Attention; Automatic attention Dyslexia, a speci®c disorder of learning to read despite normal intelligence and adequate teaching, affects at least 4% of population [5]. The disorder goes in families [7] but its etiology and pathophysiology are unknown. The temporal processing de®cits of dyslexics [14] have attracted increasing attention during the recent years and it has been suggested that they play a crucial role in the genesis of language dif®culties [18]. Dyslexics face great problems in processing sequences of rapidly presented stimuli, and this slow rate of processing lasts until adult age [8,11,16]. For example, dyslexics need a clearly longer intertone inter- val to follow a coherent stream of high and low pitch tones [11]. As de®cits of this type of temporal processing may occur in all sensory modalities [15,16] one would suspect a common cause which would unify the apparently diverse symptoms. The ®rst guess would be a general slowing and/ or increased jitter in all neuronal processing. However, dyslexic adults can detect and identify accurately simple stimuli presented in isolation [14]. They are also as good as normal readers in locating sounds on the basis of less than 1 ms interaural time and phase differences [10,19]. Further- more, they are not disproportionately worse in discriminat- ing periodicity pitch than spectral pitch although periodicity pitch is based on temporal information only [10]. It there- fore seems that rather than suffering from slowed and smeared processing at the neuronal level, the dyslexics have a prolonged short-term buffer or `cognitive integration window' within which subsequent stimuli may interfere. Here we claim that this prolongation is associated with increased attentional dwell time so that it takes longer for dyslexics than normal readers to disengage their attention from the previous target. One measure of the attentional dwell time can be obtained by quantifying the duration of the `attentional blink' in a task where the subject ®rst has to identify one target and then detect another in close temporal succession. It turns out that when attention is assigned to one target, the attentional resources are unavailable for the other target during 400± 600 ms [4]. We utlilized the attentional blink task to study 18 dyslexic subjects (mean ^ SD; age 33:6 ^ 5:9 years, 12 females, six males) and 22 control subjects (29:7 ^ 9:9 years, 14 females, eight males); all subjects were native Finnish speakers. All dyslexics had a history of reading disorders and as a group they were signi®cantly slower than the controls in reading and word recognition (P , 0:001). Fig. 1 shows the experimental setup. The subjects viewed a series of black capital letters (height 1 cm) on a computer screen at 50±60 cm distance [12]. Each letter was shown for 106±107 ms, and then immediately replaced by another so that 9.4 letters per second were presented on a greenish background of medium luminance (38 cd/m 2 ). Every sequence started with 8±17 black (5.6 cd/m 2 ) letters, followed by a white (112 cd/m 2 ) target letter (Fig. 1). The subject's task was to identify the target, which could be any letter of the English alphabet, except X or K, and also to report whether a second target `X' was present among the Neuroscience Letters 271 (1999) 202±204 0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(99)00547-9 * Corresponding author. Tel.: 1 358-9-451-2959; fax: 1 358-9- 451-2969. E-mail address: hari@neuro.hut.® (R. Hari)