Learning to read alters cortico-subcortical cross-talk in the visual system of illiterates

Abstract

Learning to read is known to result in a reorganization of the developing cerebral cortex. In this longitudinal resting-state functional magnetic resonance imaging study in illiterate adults, we show that only 6 months of literacy training can lead to neuroplastic changes in the mature brain. We observed that literacy-induced neuroplasticity is not confined to the cortex but increases the functional connectivity between the occipital lobe and subcortical areas in the midbrain and the thalamus. Individual rates of connectivity increase were significantly related to the individual decoding skill gains. These findings crucially complement current neurobiological concepts of normal and impaired literacy acquisition.

Keywords: dyslexia; functional connectivity; illiterates; pulvinar nucleus; reading; resting-state fMRI; superior colliculus; visual system.

Fig. 1. Learning to read modifies subcortical network centrality.

Whole-brain degree centrality map thresholded at z = 2.58 (P < 0.005, corrected for cluster size) with corresponding color bar indicating the range of z scores. The effect of literacy instruction is depicted as a group (reading-trained individuals versus untrained illiterates) by time (before versus after intervention) interaction. The significant cluster stretches from the right superior colliculus of the brainstem (MNI coordinates: +6, −30, −3) to the bilateral pulvinar nuclei of the thalamus (MNI coordinates: +6, −18, −3; −6, −21, −3). The box plot resolves the interaction by separately showing the individual mean z values for each factor level. Mean degree centrality values of the untrained group did not differ significantly from zero (time 1: t_1,8 = 1.76, P = 0.116; time 2: t_1,8 = 1.10, P = 0.302; one-sample t tests).

Fig. 2. Learning to read strengthens cortico-subcortical functional connectivity.

(A) Voxel-wise functional connectivity map derived from seeding in the significant degree centrality cluster. The image is thresholded at z = 2.58 (P < 0.005, corrected for cluster size). The color bar indicates the range of z scores. Becoming literate goes along with increased coupling of BOLD signal time courses between mesencephalic/diencephalic visual nuclei and a single cluster spanning the areas V1, V2, V3, and V4 of the right occipital cortex (MNI coordinates: +24, −81, +15; +24, −93, +12; +33, −90, +3). (B) The group (reading-trained individuals versus untrained illiterates) by time (before versus after intervention) interaction becomes evident from the box plot, indicating that the functional connectivity is strongly and specifically enhanced in the group that underwent reading instruction. (C) Line graphs depicting the coefficients of the correlations between the hemodynamic time series separately for each individual subject, each group, and each time. (D) Mean time series of the BOLD signal for each group and each time.