Dyslexia : challenging theories.

Experiments presented challenge theories on their ability to provide causal explanations of
the pattern of performance in dyslexia. Studies la and 1 b employed a prism adaptation
paradigm to investigate the Cerebellar Deficit Hypothesis (CDH). No group differences
were found, although unfortunately it was concluded that the paradigm could not
satisfactorily isolate cerebellar function from other compensation mechanisms. Studies 2a
and 2b exploited a sequential stereopsis technique to test the visual deficit hypothesis. No
group differences were found, although the dyslexic group did exhibit a fatigue effect on
one condition. Using an attention shifting paradigm, Study 3 found a dissociation
between focus and shift attention conditions in dyslexic children, but that they sustained
their attention as well as controls. In Study 4, supporting the Dyslexia Automatisation
Deficit (DAD) as opposed to a general resources deficit, control performance suffered
most under visually degraded conditions of the same attention paradigm. Study 5 further
investigated attention on a test thought to be sensitive to attentional lapses; dyslexic
children did make more errors, although conclusions were limited by their qualitatively
normal performance.
Deficits in dyslexia were found to be wider reaching than many theories of dyslexia
would suggest. At a cognitive level of explanation the DAD was able to account
successfully for many of the findings. However, like the Phonological Deficit Theory the
DAD specifies no neurological mechanism for the deficit; this is provided by the CDH
(for which no evidence was found here). Analyses do point towards the need for either a
very general explanation, or the identification of a smaller number of core deficits, for the
apparently disparate deficits found. The fatigue effect found only in the dyslexic group on
part of the vision experiment has further direct and immediate implications for future
research.