Visual and Language Processing Disorders are Concurrent in Dyslexia and Continue into Adulthood

Abstract

A recent study by Slaghuis, Lovegrove and Davidson (1994) found that visual and language processing differences were concurrent in a group of preadolescent dyslexics. In the present study, two experiments are reported that investigate the concurrence and continuity of visual and language processing differences in groups of young and adult dyslexics on a measure of visual processing and a measure of phonological coding. The visual processing task in the present experiments was a measure of Ternus apparent movement which was used as an index of the duration of visible persistence. Ternus apparent movement is multistable and provides two mutually exclusive and easily distinguishable percepts for the observer, referred to as ‘element’ and ‘group’ movement, that are highly dependent on the temporal interval between frame 1 and frame 2 of the display. The language processing task in the present experiments was a test of phonological coding measured using a non-word test of 100 orthographically legal non-words. The results of the first experiment showed that in comparison to normal readers the young dyslexic participants showed a significant reduction in Ternus ‘group movement’ and a significant reduction in the pronunciation of non-words. In a second experiment, Ternus apparent movement and performance on the non-word test was measured in groups of adult dyslexic and normal readers in order to investigate whether the visual and language processing differences found in young dyslexics were also present in adult dyslexics. The results showed that adult dyslexics also have a significant reduction in Ternus ‘group movement’ and a significant reduction in the ability to pronounce non-words similar to that found in the young dyslexic group in Experiment 1. The significant reduction in Ternus ‘group movement’ in dyslexic participants was explained in terms of an increase in the duration of visible persistence and was shown to be consistent with evidence for a transient system disorder. The combined results show that visual and language processing differences are concurrent in dyslexia and continue into adulthood.