Neural Systems Affected in Developmental Dyslexia Revealed by Functional Neuroimaging

studies have employed phonological processing tasks and have clearly demonstrated that poor phonological skills are a hallmark of developmental dyslexia. While these failures in phonological processing are commonly invoked as causal factors in developmental reading failGuinevere F. Eden* and Thomas A. Zeffiro Georgetown Institute for Cognitive and Computational Sciences Georgetown University Washington, DC 20007 ure, they cannot easily account for the sensory deficits in visual and auditory processing observed in otherstudReading is a uniquely human endeavor whose impories. Similarly, these deficits in low level auditory and tance to society in this century has been heightened by visual processing do not obviously explain the equally steadily increasing demands for a more literate workwell-documented abnormalities in verbal working memforce.Consequently, there isgreat interest in uncovering ory and phonological awareness. Thus, it is possible the reasons that reading may fail to develop normally. that the language deficits in dyslexia are not causally During early schooling, individuals may exhibit difficulty related to low level sensory abnormalities but rather that in acquiring adequate reading skills, reading more slowly these perceptual and cognitive abnormalities arise from or less accurately than expected. Called developmental dysfunction of a neural system common to both. Demdyslexia, this disorder was first described over 100 years onstration that disparate behavioral deficits arise from ago. It is formally defined as an unexpected reading dysfunction of spatially colocalized cortical regions failure that cannot be explained by low intelligence quowould be evidence favoring a common pathophysiologitient (IQ) or environmental circumstances, such as cal mechanism. teaching methods or social environment. In this decade, Functional neuroimaging is providing new information estimates of the incidence of reading disability vary beconcerning the neuroanatomical localization of the systween 5% and 15%, and its familial aggregation sugtems affected indevelopmental dyslexia. Recent studies gests a genetic basis. employing positron emission tomography (PET) and Claims concerning the mechanisms responsible for functional magnetic resonance imaging (fMRI) to study dyslexia have been as numerous and varied as propossensory and language processing in dyslexia have demals for its remediation. One possibility is that the failure onstrated involvement of regions including posterior to fully understand the pathophysiology of dyslexia may temporal and inferior parietal cortical systems. Although have resulted from the complexity of its behavioral manimany sensory and cognitive processes are known to be festations. While traditionally thought of as primarily a abnormal in dyslexia, this necessarily selective review reading disorder, dyslexia’s clinical signs are varied and will emphasize those that have been examined with may include abnormal phonological awareness (Bradley functional neuroimaging. and Bryant, 1983); writing, spelling, and motor timing Behavioral Evidence: Phonological Processing (Wolff et al., 1984); verbal working memory (Hulme and Deficits in Dyslexia Roodenrys, 1995); visual processing (see Eden et al., It has long been known that the ability to isolate and 1996); and auditory discrimination (Tallal et al., 1993). In manipulate the constituent sounds of words, known as the face of this behavioral complexity, suggestions that phonemic awareness, is related to reading ability. Indithe pathophysiology of this disorder may be explained viduals with developmental dyslexia exhibit deficits in with reference to dysfunction of a single sensory or numerous measures of phonological awareness. For excognitive process have not been widely persuasive. For ample, rhyme judgement (such as, “Hat, cat, dog, mat— example, phonological skills, requiring isolation and mawhich is the odd one out?”) can measure phonological nipulation of the constituent word sounds, are good awareness and predict reading outcome (Bradley and Bryant, 1983). Another useful task is phoneme elision, predictors of reading ability. Therefore, many dyslexia