| Summary: | The present thesis was concerned with the possible constraints set by visual and attentional pre-orthographical factors on visual word recognition in dyslexic individuals.
In a first study, we investigated the visual word recognition ability of MT, a young boy with surface dyslexia, by means of a paradigm that measures performance as a function of the eye fixation position within the word, known as the "viewing position effect" paradigm. In well-achieving readers, the viewing position effect is mainly determined by factors affecting letter visibility and by lexical constraints on word recognition. We further quantified MT's sensory limitations on letter visibility by computing visual span profiles, i.e. the number of letters recognizable at a glance. Finally, in an ideal-observer's perspective, MT's performance was compared with a parameter-free model combining MT's letter visibility data with a simple lexical matching rule. The results showed that MT did not use the whole visual information available on letter identities to recognise words. These results can be best accounted for by a reduction of the number of letters processed in parallel.
Accordingly, there is growing evidence that some dyslexic children suffer from a deficit in simultaneously processing of multiple visually displayed elements. The aim of the remaining studies was to investigate possible cognitive impairments at the source of the multi-element visual processing deficit in dyslexic children. A computational model of the attentional involvement in multi-object recognition [TVA: Bundesen, C. (1990). A theory of visual attention. Psychological Review, 97(4), 523--47] served as framework for this investigation. In a second study, we used TVA to investigate multi-element processing in two young dyslexic participants, AB and PA. By combining psychophysical measurements with computational modelling, we demonstrated that this multi-element processing deficit stems from two distinct cognitive sources: a reduction of the rate of visual information uptake, and a limitation of the visual short-term memory capacity. These deficits were replicated in a third study, in which the multi-element processing was investigated in three dyslexic individuals, FA, LT and YC. The last study further demonstrated that the multi-element processing deficit observed in dyslexia is not simply due to a sluggish activation of items names, instead of visual processing difficulties. Finally, the generalisability of the multi-element processing deficit has been assessed by comparing report performance of letters vs colour patches. Unfortunately, the results were inconclusive.
Taken together, the results of these different studies point to a reduced capacity of processing visual information in parallel (at least for letters), that might constrain visual word recognition.
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