Electrophysiological and behavioural correlates of dyslexia in perceptual and cognitive tasks

Despite recent extensive research in the field of dyslexia, the causal links between various behavioural symptoms and underlying neural mechanisms of this developmental disorder proposed by different theories are still hotly debated. In this project I aimed to combine behavioural and neurophysiological tests of global coherent motion (magnocellular), visual word from recognition and lexical decision (phonological), as well as attention deficits in English speaking dyslexic adolescents. Three studies are described in this thesis. In the first study 10 dyslexic participants
(ages 15.5-17.4) and 10 control participants (ages 14.4-18.3) were tested during the
Continuous Performance Task (CPT), an established test o f attentional
performance. In the second study the 9 dyslexic and 10 control participants (from
the same set as in the first study) were tested for the magnocellular deficit
hypothesis, with low contrast and low coherence level random dot kinematograms
(RDK) presented in a coherent motion detection test. In the final study the
participants (9 dyslexics, 9 controls) had to decide whether they saw a word or a
pseudoword (lexical decision task). In all three studies the event related potentials
(ERPs) were recorded simultaneously with the behavioural measures, such as
reaction time (RT) and error rate.
According to the results o f the first study, no between-group differences in
behavioural performance on CPT were found, whereas the late ERP components
were delayed, attenuated and atypically symmetrical in the dyslexic group. The
results o f the second study showed magnocellular impairment only in one dyslexic
participant, according to both ERP and behavioural data. Thus, the latency o f the
N1 and P2 ERP components was delayed and sensitivity o f the performance was
poorer in this participant when compared to the rest o f the group average. In the
third study, the lexical decision task, dyslexics performed significantly worse than
controls in terms o f accuracy and response latency. The early ERPs related to the
pre-lexical visual word form recognition were atypically symmetrical, and the later
ERP peaks were significantly delayed and attenuated for the dyslexic group.
The behavioural and electrophysiological results o f these studies suggest that
abnormal attentional performance is not a ‘core’ feature o f dyslexia, as well as
confirm previous findings o f impaired magnocellular function in a small subset o f
dyslexic population. The atypically symmetrical early and later ERP components
highlight the potential explanatory value o f altered interhemispheric function in
dyslexia, whereas the attenuated and delayed later ERP components highlight the
deficits at later, cognitive stages o f processing among dyslexics. Brain-behaviour
cross-study correlational analysis showed that speed and amplitude o f the early and
late ERP components consistently associated across the tasks, the poor literacy
scores and larger error rates associated with attenuated and delayed ERPs, whereas
individual participant effect sizes showed that magnocellular impairment associated
with larger error rate and delayed ERPs in the CPT and lexical decision tasks.