Brain. 2003 Apr;126(Pt 4):841-65.
Theories of developmental dyslexia: insights from a multiple case study of
dyslexic adults.
Ramus F, Rosen S, Dakin SC, Day BL, Castellote JM, White S, Frith U.
Institute of Cognitive Neuroscience, University College London, London, UK.
ramus@lscp.ehess.fr
A multiple case study was conducted in order to assess three leading theories of
developmental dyslexia: (i) the phonological theory, (ii) the magnocellular
(auditory and visual) theory and (iii) the cerebellar theory. Sixteen dyslexic
and 16 control university students were administered a full battery of
psychometric, phonological, auditory, visual and cerebellar tests. Individual
data reveal that all 16 dyslexics suffer from a phonological deficit, 10 from an
auditory deficit, four from a motor deficit and two from a visual magnocellular
deficit. Results suggest that a phonological deficit can appear in the absence
of any other sensory or motor disorder, and is sufficient to cause a literacy
impairment, as demonstrated by five of the dyslexics. Auditory disorders, when
present, aggravate the phonological deficit, hence the literacy impairment.
However, auditory deficits cannot be characterized simply as rapid auditory
processing problems, as would be predicted by the magnocellular theory. Nor are
they restricted to speech. Contrary to the cerebellar theory, we find little
support for the notion that motor impairments, when found, have a cerebellar
origin or reflect an automaticity deficit. Overall, the present data support the
phonological theory of dyslexia, while acknowledging the presence of additional
sensory and motor disorders in certain individuals.