Introduction

Visual deprivation during different developmental periods leads to different structural and functional alterations in the brain. However, the effects of visual deprivation on the spontaneous functional organization of the brain remain largely unknown. In this study, we used voxel-based functional connectivity density (FCD) analyses to investigate the effects of visual deprivation during different developmental periods on the spontaneous functional organization of the brain^1,2. Onset of blindness and age of blindness may play a significant role in cortical thickness of occipital cortex.

Objective

To observe the structural alteration and functional connectivity based on the age differences for haptic language task versus phonological task and if any changes persist, then correlation with duration of braille reading to recognise the aspect of alteration for the plasticity in visual area.

Methodology

Forty early blind (EB) and Forty late blind (LB) subjects and thirty sighted controls (SC) in two age groups 6-12 years and 13-19 years (all right-handed) were recruited from the clinics of our institute (Table 1). Standard diagnostic and exclusion criteria were followed. Anatomical and functional data for Braille and phonological noun pair assessment test was acquired in a 3T MR scanner (Achieva 3.0T TX, Philips, Netherlands), with the following imaging parameters: slices per slab: 160, distance factor 50%, orientation: sagittal; slice thickness: 1mm; T1:1100ms; TR: 1900ms; TE: 3.37ms; averages: 1; FOV: 256mm, FOV phase: 93.8%; Base resolution: 256; Phase resolution 100; Phase encoding direction: A>>P; Band width: 130 Hz; echo spacing: 8.6ms. Data processing were carried out using SPM12 with the help of CAT12 and CONN toolbox3,4. The clusters were converted from MNI template to the Talairach and Tornoux coordinates, for estimation of anatomical areas⁵. Two-sample t-test and multiple comparison (post-hoc analysis) (p<0.001, cluster threshold 10) were used for inter-group comparisons.

Result

During phonological language task in early blind, bilateral superior frontal gyrus, left hemispheric middle frontal gyrus, para hippocampal gyrus, precuneus, superior temporal gyrus, thalamus and right cerebral cingulate gyrus, medial frontal gyrus exhibited BOLD activation. In late blind, bilateral activation in fusiform gyrus, para hippocampal gyrus, cuneus, post-central gyrus along-with right hemispheric thalamus, superior temporal gyrus, primary and auditory association cortex, dorsal posterior cingulate cortex, somatosensory association cortex, brainstem, lingual gyrus, superior frontal gyrus, and precentral gyrus, and left hemispheric inferior parietal lobule, insula, precuneus, cerebellum, primary somatosensory cortex, associative visual cortex (v3,v4,v5), premotor cortex and supplementary motor area, medial frontal gyrus were associated during the phonological task (Figure 1). During Braille reading task, in late blind bold activation was observed in right cuneus, inferior frontal gyrus, middle frontal gyrus, para hippocampal gyrus, precentral gyrus, superior frontal gyrus, superior temporal gyrus and left hemispheric fusiform gyrus, inferior parietal lobule, lingual gyrus, medial frontal gyrus, middle temporal gyrus, para hippocampal gyrus. Early blind subjects exhibited bilateral activation in superior temporal gyrus, precuneus in the two age groups. Right hemispheric lentiform nucleus, brainstem and left inferior parietal lobule, para hippocampal gyrus and left hemispheric medial frontal gyrus, para hippocampal gyrus, middle occipital gyrus, precuneus, thalamus, cerebellum, middle frontal gyrus were also active in both the groups.

Discussion

Bilateral activation of middle temporal gyrus and superior temporal gyrus (with left hemispheric dominance) was observed irrespective of age difference in all the groups (p < 0.05, FWE corrected). Visual cortical area response was higher in 13-19 years than 6-12 years of early blind and late blind groups, for the Braille reading and phonological tasks (Figure 2). The Pearson correlation analyses for the visual activation with age is positive for the non-visual language perception in both late and early blind groups (Figure 3). During the Braille reading task, the children group exhibited inferior frontal area (IFG) and Wernicke’s area, whereas adolescent group revealed in prefrontal, Wernicke’s area and Broca’s area. During the phonological language task, children group of early and late blind invoked the inferior frontal and Broca’s areas along with hippocampus and middle temporal gyrus. The same task revealed activation in IFG, prefrontal areas Wernicke’s and Broca’s area in adolescent groups. Hence early and late blind children have reduced language selectivity in prefrontal cortex relative to sighted group and adolescent group language pathway is intact and in correlation with the left visual cortex than children group.1,2 Voxel based anatomical analysis and functional connectivity revealed decreased connectivity (both short and long range) in primary visual area and somatosenso ry area in 6-12 years, than 13 -19 years (Figure 4) of both early and late blind groups⁶. For the phonological language area, a decreased connectivity was observed between visual area, somatosensory area and primary auditory cortex. These results reflect cross-modal plasticity due to adaptive compensatory mechanism in brain of visually deprived subjects. On comparing the EB with the LB, we can deduce that the age of onset of blindness and the total duration of blindness play a role in cortical thickness⁷.

Conclusion

VBM and functional connectivity revealed cross modal plasticity in visual and language areas, which are dependent on duration of blindness and duration of education.