Hemispheric transfer and dyslexia: testing the deficit hypothesis for word and symmetry recognition using visual half-field tasks.

BACKGROUND: The interhemispheric transfer deficit theory proposes that individuals with dyslexia have impaired interhemispheric transfer, particularly affecting the integration of visual information from the left and right visual fields. This study aimed to evaluate this hypothesis by examining interhemispheric transfer in dyslexia using visual half-field tasks targeting both linguistic and visuospatial processing.

METHODS: We examined interhemispheric transfer in dyslexia using two visual half-field tasks: a lexical decision task to assess written word processing, and a symmetry decision task to examine visuospatial processing. We compared reaction times and accuracy in 90 Dutch-speaking participants (45 with dyslexia, 45 controls) across left, right, and bilateral stimulus presentations.

RESULTS: While both tasks successfully captured expected visual half-field differences in the control group, favoring the right visual field in the lexical decision task and the left visual field in the symmetry detection task, we did not observe that the dyslexia group showed increased differences between the two fields, as predicted by the interhemispheric transfer deficit theory. Furthermore, the dyslexia group benefited just as much as controls from stimuli presented simultaneously to both visual fields. Thus, no evidence of interhemispheric transfer deficits related to dyslexia was found in either task.

CONCLUSIONS: These findings challenge the broad applicability of the interhemispheric transfer deficit theory in dyslexia, suggesting that such impairments may be task-dependent rather than domain-general. Future studies should further explore the conditions under which interhemispheric transfer deficits might occur in dyslexia.