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Phonological decoding ability, spatial attention, and event-related potentials


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Matthews, Allison J 2006 , 'Phonological decoding ability, spatial attention, and event-related potentials', PhD thesis, University of Tasmania.

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Evidence for a selective spatial attention deficit among children and adults with
developmental dyslexia has been interpreted to reflect impairment in the posterior attentional
network including the magnocellular-mediated posterior parietal cortex, and particularly the
right hemisphere. According to cognitive interpretations, dyslexia stems from a core
phonological deficit and it has been shown that phonological decoding is essential for normal
reading development. Thus, the aim of the present research was to investigate the relationship
between selective spatial attention and reading ability among adults with good and poor
phonological decoding ability as assessed by nonword reading ability. Five experiments were
conducted to investigate this relationship.
The results of Experiment 1 indicated that poor phonological decoders are slower to
search for feature conjunctions, particularly for searches defined by the features of form and
motion. These findings are consistent with previous research in populations with dyslexia and
suggest differences between good and poor phonological decoders in terms of the guidance of
spatial attention. Experiments 2 to 5 aimed to examine differences in attentional processing in
more detail by examining the specific mechanisms involved in both sustained and transient
spatial attention tasks and their relationship to early event-related potential components.
Research has demonstrated that the early sensory components of the event-related potential
waveform (P1 and N1) index early visual processing and are modulated during tasks in which
attention is manipulated. However few previous studies have investigated the
electrophysiological correlates of spatial attention in good and poor readers.
In Experiment 2 the mechanisms of attentional focussing and inhibition were
investigated using a task in which a focussing cue preceded a target that was flanked by
compatible or incompatible flankers. Poor decoders showed a greater reaction time cost for
incongruent stimuli preceded by large cues, which was suggested to indicate difficulty in
focussing attention and suppressing information at unattended locations. This finding was accompanied by a reduction in the modulation of N1 amplitude by both cue-size and flanker
compatibility for poor decoders, and a reduction in the modulation of the frontal N2 component
by flanker compatibility. Together, these findings suggested hemispheric differences in the
functioning of the posterior attentional network as well as differences in inhibitory processing
within the frontal attentional network.
Experiment 3 aimed to examine differences between good and poor phonological
decoders in the allocation of attention to global and local levels of hierarchical stimuli. Poor
phonological decoders were slower than good phonological decoders when attention was
directed to both the global and local processing levels. This was accompanied by a lack of task
related modulation of the posterior N1 and N2 components, and an overall increase in N2
amplitude among poor decoders. Together, these findings suggested differences in the early
allocation of spatial attention and compensatory processing at later perceptual stages.
Dyslexia has also been associated with performance differences on covert orienting
tasks involving valid and invalid spatial cues. These differences are often greater for stimuli
presented in the left visual field which is suggestive of a right hemisphere parietal deficit. The
aim of Experiment 4 was to investigate covert orienting in good and poor phonological decoders.
Poor phonological decoders showed fewer reaction time benefits of valid spatial cues relative to
good decoders, particularly for left visual field trials. This effect was greatest for male
phonological decoders, who also showed a lack of N1 modulation in the right hemisphere for left
visual field trials and an overall lack of attentional modulation of N1 latency. In comparison,
female poor decoders showed a greater involvement of the right hemisphere which may reflect
compensatory processing due to a left hemisphere deficit.
The aim of Experiment 5 was to investigate the effect of valid and invalid spatial cues
on the performance of orthographic and phonological decision tasks. Consistent with early
selection models of attention in word recognition, good decoders showed consistent behavioural
effects such that reaction time benefits were observed when words were preceded by valid spatial cues. However, poor phonological decoders showed fewer reaction time benefits for
words preceded by valid spatial cues, particularly for words presented in the left visual field
when processing was biased towards phonological processing. These behavioural differences
were accompanied by an absence of the attentional modulation of both PI and N1 latency in
poor decoders, and this was explained by differences in early perceptual and attentional
processing in the posterior attentional network.
Together, the findings of the present series of experiments provide evidence that the
spatial attention difficulties observed in developmental dyslexia are also observed in adults who
are poor phonological decoders. The poor phonological decoding group generally showed less
attentional modulation of the early posterior N1 and P1 components which is consistent with
differences in the functioning of the posterior attentional network. The findings of the present
research are broadly consistent with the proposal that the phonological decoding deficits
observed in developmental dyslexia are associated with attentional processing differences in the
posterior parietal cortex. This research also provided preliminary evidence for sex differences in
the lateralisat ion of ERP components which require further investigation.

Item Type: Thesis - PhD
Authors/Creators:Matthews, Allison J
Keywords: Dyslexia, Reading disability
Copyright Holders: The Author
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Copyright 2006 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2008. Includes bibliographical references. Ch. 1. Reading development, word recognition and developmental dyslexia -- Ch. 2. Neurobiological explanations of dyslexia -- Ch. 3. Attention and reading ability -- Ch. 4. Electrophysiology, attention and dyslexia -- Ch. 5. General rationale & aims -- Ch. 6. Experiment 1: visual search for feature conjunctions and phonological decoding ability -- Ch. 7. Experiment 2: ERP correlates of flanker and cue-size effects among good and poor adult phonological decoders -- Ch. 8. Experiment 3: electrophysiological indices of global/local processing in good and poor phonological decoders -- Ch. 9. Experiment 4: electrophysiological indices of covert orienting in good and poor phonological decoders -- Ch. 10. Experiment 5: phonological decoding ability and electrophysiological indices of spatial attention during cued phonologicval and orthographic decision tasks -- Ch. 11. General discussion and conclusions

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