Functional Plasticity of the Visual System in the Blind during Sensory Substitution Task and at Rest

Abstract

Purpose: Blind persons may acquire visual perception indirectly by sensory substitution devices (SSDs) which convert images from a video camera to cross-modal sensory signals. To date, it remains unclear how the brain adapts to cross-modal interactions upon loss of one sensory input. This study aims to assess functional neuroplasticity in the blind by investigating the relationship between blindness duration and visual cortex (VC) activity both during an SSD task and at rest.

Methods: 12 blind and 10 age-matched sighted controls underwent task-based and resting-state blood-oxygen-level-dependent (BOLD) functional MRI (fMRI) at 3 Tesla. For task-based SSD fMRI, subjects were asked to interpret a series of soundscapes that represented moving bars across an image in one of four directions. BOLD % change representing strength of functional response was extracted from 5 regions of interest (ROIs) including Brodmann areas (BA) 17 (primary VC), 18 (secondary VC) and 19 (higher order visual areas), as well as the dorsal and ventral visual streams. For resting-state fMRI, subjects were scanned for 8 min at rest. Functional connectivity (FC) was measured by correlating time courses between brain regions. BOLD % changes and FC were compared with blindness duration.

Results: For task-based fMRI, BOLD % change was positively correlated with blindness duration in all 5 ROIs (p<0.05), and was strongest in BA19 (r=0.73) and ventral visual stream (r=0.75). While BOLD signal changes were on average positively correlated with the task in blind subjects, BOLD response in sighted subjects was either negative or not significant. No relationship was found between age and BOLD % change. For resting-state fMRI, FC between visual and somatosensory networks was decreased in blind subjects relative to controls. FC between visual networks was also negatively correlated with blindness duration. However, FC increased in blind subjects between the visual networks and the deep gray nuclei, attention network and prefrontal cortex.

Conclusions: This study shows not only altered functional activities and FC of VC due to blindness, but that these alterations are continuously related to the duration of visual deprivation. Taken together, the results suggest that over time the VCs of blind subjects become less connected with each other and other sensory brain networks, and become more strongly correlated with task-positive brain networks.

Keywords: 611 neuro-ophthalmology: cortical function/rehabilitation • 650 plasticity • 755 visual cortex