Anatomo-functional interpretations of developmental dyslexia: one or more deficits?

Introduction: Developmental dyslexia is usually defined as a significant difficulty in reading, writing and spelling despite normal intellectual capacity and educational resources, adequate socio-cultural opportunity and in the absence of macroscopical acquired lesions. Given the complexity of cognitive processes involved in reading, a wide range of possible explanations have been suggested for dyslexia, such as (A) a specific linguistic problem due to a deficit in phonological processing (1-3); (B) an impairment of the magnocellular pathway (4-5) (C) a deficit of the cerebellar system (6-8). Each of these theories has found support in behavioural and/or imaging experiments. However, the majority of these studies have investigated only one explanatory variable on the same sample of dyslexics (1-8). Recently, multiple single-case studies have been performed to assess to what extent phonological, magnocellular and cerebellar deficits co-exist in the same sample of dyslexic subjects. Ramus and collegues (2) concluded that dyslexia is best explained by the presence of a phonological deficit. On the contrary, Reid and collegues (9) concluded that different sub-types of developmental dyslexia with different underlying causes may exist. However, these studies lack explicit testing of some of the hypotheses which are anatomical in nature (e.g. the cerebellar hypothesis). The aim of this study was therefore to re-assess the co-morbidity issue in dyslexia in the same sample of subjects using both behavioural and imaging indexes to challenge the three aforementioned hypotheses. Methods: 23 right-handed controls and 8 right-handed subjects with diagnose of dyslexia participated in the study. The two groups were matched for age, education and I.Q. Before scanning, in all subjects, we assessed reading and behavioural tasks to evaluate reading ability, phonological skills, auditory and visual magnocellular skills and motor-cerebellar skills. During fMRI session, subjects performed 4 tasks: non-word reading, auditory rhyming task (phonological task), movement detection of a high spatial frequency stimulus (magnocellular task) and a motor sequence learning task (motor task known to involve the cerebellum). Each task corresponded to an fMRI session that involved 60 fMRI scans. A block design was used. Simple effects and comparisons between dyslexics and controls were computed for each task. Behavioural results: There was a significant between-group difference for words and non-words reading, spoonerism task, stress assigment task, digit naming and sentence dictation. However, the single-subject analyses showed that reduced performance in reading and in phonological tasks was occasionally associated with a reduced performance in tasks designed to test magnocellular visual or the motor “cerebellar” systems. fMRI results: No significant between-group differences emerged from the analyses of the behavioural data during fMRI scans. Normal controls showed the well known pattern of left hemisphere activation for reading, in the inferior frontal gyrus, in the temporo-parietal areas and in ventral occipito-temporal areas. Dyslexics showed reduced activation in the left inferior frontal gyrus and in the left temporal and occipito-temporal areas for non-word reading, a task where dyslexics are slower than controls when tested in time constrained protocols outside the scanner. However, the dyslexics also had reduced activations for tasks in which they did not show systematic behavioural deficits. In particular, in the left anterior occipito-temporal cortex for the simple rhyming task adopted here, in the right posterior occipitotemporal region for motion detection, and in the cerebellum for the motor learning task. Discussion: Although at the behavioural level, the vast majority of the dyslexic subjects did not show a reduced performance in “magnocellular” and motor tasks, functional data showed physiological deficits that involved all systems under investigation (phonological, visual magnocellular and motor-cerebellar systems). On the basis of these results we propose that a disorder of phonological, magnocellular and cerebellar systems co-occurs in developmental dyslexia. However, the phonological deficit together with the reading speed deficit stands above other difficulties being evident at both behavioral and physiological level.Bibliography 1. Frith U. (1999) Dyslexia 5(4): 192-214. 2. Ramus F. (2003) Curr Opin Neurobiol 13(2): 212-8. 3. Snowling M.J. (2001) Dyslexia 7(1): 37-46. 4. Stein J. and Walsh V. (1997) Trends Neurosci 20(4): 147-52. 5. Demb J.B. et al. (1998) J Neurosci 18(17): 6939-51. 6. Nicolson R.I. and Fawcett A.J.(1990) Cognition 35(2): 159-82. 7. Nicolson R. I., Fawcett A. J., et al. (1999) Lancet 353(9165): 1662-7. 8. Nicolson R. I., Fawcett A. J., et al. (2001) Trends Neurosci 24(9): 508-11. 9. Reid A., Szczerbinski M., et al. (2007) Dyslexia 13(1): 1-24.