Auditory and visual automatic attention deficits in developmental dyslexia

Abstract

Several studies have provided evidence for a phonological deficit in developmental dyslexia. However, recent studies provide evidence for a multimodal temporal processing deficit in dyslexia. In fact, dyslexics show both auditory and visual abnormalities, which could result from a more general problem in the perceptual selection of stimuli. Here we report the results of a behavioral study showing that children with dyslexia have both auditory and visual deficits in the automatic orienting of spatial attention. These findings suggest that a deficit of selective spatial attention may distort the development of phonological and orthographic representations that is essential for learning to read.

Introduction

Developmental dyslexia is defined as a specific reading disorder despite normal intelligence and teaching, and in the absence of any manifest sensory deficit [1]. Studies have provided evidence for a phonological deficit in developmental dyslexia [4]. However, the problems of dyslexic children extend beyond the skills directly involved in reading. Indeed, recent studies provide evidence of spatial and temporal processing deficits in dyslexia [42], [22]. Also, dyslexics show visual abnormalities [19] as well as deficits in processing tactile stimuli [20], in balance and motor control [33], which could result from a more general problem in the selection of stimuli [22].

The magnocellular (M) theory of dyslexia [42] holds that the crucial disorder is a neurodevelopmental impairment of a crossmodal system responsible for processing rapid streams of stimuli. The information processed by the M system ends in the posterior parietal cortex (PPC), which is the basic area of multimodal spatial attention [11]. There is evidence that a supramodal space representation exists in the PPC with convergence of both auditory and visual inputs [17], and there is also evidence of crossmodal cells in PPC [2]. Thus, the neural pathways previously thought to be sensory-specific are in fact strongly modulated by signals from other modalities [37]. The PPC may be involved in spatial selection independently of modality [9] through a multimodal map that would be used for orienting attention in both the auditory and visual modalities [47].

In fact, the deficits in dyslexia often manifest themselves in the auditory modality with problems in speech–sound perception (phoneme discrimination) in the presence of background noise [10]. Also, dyslexic children have difficulties in discriminating between acoustically similar sounds [44] and in processing rapid sound sequences [21], [25]. That could be due to impaired ‘magnocells’ in the medial geniculate nucleus [18]. Evidence for an auditory spatial selection deficit in dyslexics was provided by Asbjornsen and Bryden [3]. These auditory perception deficits are likely related to an inability to rapidly shift auditory attention in order to discriminate properly the features of the sound [22]. In fact, several studies demonstrated that phoneme identification may be substantially influenced by the spatial distribution of auditory attention [30], [31], providing strong evidence that selective spatial attention may act to facilitate auditory perception.

In addition, it is clear that many visuospatial selection functions contribute to reading and that selective attention to words or string of words requires a filter controlled by rapid visual orienting [5]. Thus, spatial attention deficits may degrade the perception of visual elements like letters and words, during reading [19]. Difficulties with reading could be due to sluggish orienting [15], [16], [24].

Recent studies suggest that the causal link from the M deficit to reading and phonological impairments involves automatic capture of attention [22], [23].

In the present study, we measured the covert automatic capture [34] of both auditory and visual attention in 17 children diagnosed with specific reading disorder or dyslexia, and seven control children with normal reading skills, who were matched for age and IQ.