Literacy acquisition drives hemispheric lateralization of reading

Eisner, F. 1 , Kumar, U. . 2 , Mishra, R. K. 3 , Tripathi, V. N. 4 , Guleria, A. 2 , Singh, J. P. 4 & Huettig, F. 5

1 Radboud University, Netherlands
2 Centre of Biomedical Research, India
3 University of Hyderabad, India
4 University of Allahabad, India
5 Max Planck Institute for Psycholinguistics, Netherlands

Reading functions beyond early visual precessing are known to be lateralized to the left hemisphere, but how left-lateralization arises during literacy acquisition is an open question. Bilateral processing or rightward asymmetries have previously been associated with developmental dyslexia. However, it is unclear at present to what extent this lack of left-lateralization reflects differences in reading ability. In this study, a group of illiterate adults in rural India participated in a literacy training program over six months. fMRI measures during reading tasks were obtained before and after training. This group was matched to an illiterate no-training group and to low- and highly-literate control groups (total N=90). Reading ability was positively correlated with increased BOLD responses in a left-lateralized network including the dorsal and ventral visual streams for text and false fonts, but not for other types of visual stimuli, in a cross-sectional analysis before training. Lateralization of BOLD responses was further examined using a laterality index (LI) in order to analyse laterality effects in specific regions. In addition to the cross-sectional data on processing written stimuli, training-related changes in lateralization provide causal evidence that the degree of hemispheric asymmetry is related to reading ability.