The Contribution of the Magnocellular Visual Pathway to the Process of Visual Word Recognition
Previous research on visual word recognition has uncovered a variety of factors which influence how easily this process is achieved. Some factors are intrinsic to the word itself (e.g., length, frequency, regularity) and some are environmental factors (e.g., stimuli contrast or visual field position). Any proposed account of visual word recognition must consider not only the properties of the word itself, but also the properties of the visual system that processes the words. This thesis tested the hypothesis that the magnocellular visual pathway contributes to the processing of words and that this contribution is most evident when words are presented in parafoveal vision. Experiments 1 and 2 investigated the effect on the recognition of isolated words of limiting input to the visual system by occluding one eye. We looked at the effect of visual field presentation position and word length. Previous research using binocular viewing had shown a large length effect in the left visual field. We found that occluding the right eye reduced the left visual field length effect. Experiments 3, 4 and 5 looked at the impact of varying presentation position on competent readers and dyslexics. Numerous studies in sentence processing have shown that phonological information can be extracted during parafoveal preview. We asked whether dyslexics’ well attested phonological impairment will hinder their ability to extract phonological information in parafoveal vision. Experiments 3 and 4 demonstrated that only the dyslexic group showed an effect of word regularity. Experiment 5 used a rhyme-matching task to show that only dyslexic readers have a problem in extracting phonological information from word pairs presented to the right visual field. We relate this to magnocellular functioning. Experiments 6, 7 and 8 used isoluminant stimuli to directly test the consequences of inhibiting the magnocellular visual pathway on the recognition of words presented both foveally and parafoveally. The results of these experiments show that blocking the magnocellular pathway affects parafoveal areas of the visual field more than the foveal area and that words are affected by this whereas non-words are not. In conclusion, we demonstrated that the magnocellular pathway does contribute significantly to the recognition of words and that the parafoveal area of the retina is more heavily dependent on the magnocellular pathway compared to the foveal area of the retina. We go on to propose plans for future research looking at the role of the magnocellular pathway in parafoveal preview in sentence reading.