Cerebral versus Ocular Visual Impairment: The Impact on Developmental Neuroplasticity

Abstract

Cortical/cerebral visual impairment (CVI) is clinically defined as significant visual dysfunction caused by injury to visual pathways and structures occurring during early perinatal development. Depending on the location and extent of damage, children with CVI often present with a myriad of visual deficits including decreased visual acuity and impaired visual field function. Most striking, however, are impairments in visual processing and attention which have a significant impact on learning, development, and independence. Within the educational arena, current evidence suggests that strategies designed for individuals with ocular visual impairment are not effective in the case of CVI. We propose that this variance may be related to differences in compensatory neuroplasticity related to the type of visual impairment, as well as underlying alterations in brain structural connectivity. We discuss the etiology and nature of visual impairments related to CVI, and how advanced neuroimaging techniques (i.e., diffusion-based imaging) may help uncover differences between ocular and cerebral causes of visual dysfunction. Revealing these differences may help in developing future strategies for the education and rehabilitation of individuals living with visual impairment.

Keywords: cerebral; connectivity; cortical; ocular blindness; visual impairment.

FIGURE 1

White matter reconstructions (shown in sagittal view) of three main pathways involved in the processing of visual information, namely the superior longitudinal fasciculus (SLF; the neuroanatomical correlate of the dorsal visual processing stream), inferior longitudinal fasciculus (ILF; the ventral visual processing stream), and inferior fronto-occipital fasciculus (IFOF; mediating visual attention and orienting). Diffusion data was acquired using 64 direction high angular resolution diffusion tensor imaging (HARDI). The pathways were reconstructed in DSI-Studio (Yeh et al., 2010, 2013) using individual QA termination thresholds and a termination angle of 45 degrees. The three white matter pathways are reconstructed in (A) a normally sighted/developed control, (B) early ocular blind, and (C) and CVI (with associated periventricular leukomalacia) individuals. Note that all three pathways (ILF, SLF, and IFOF) are fully reconstructed in both the control and early ocular blind individuals. In contrast, the SLF and ILF are sparser, and the IFOF was could not to be reconstructed in the individual with CVI. These differences in the structural integrity along these major white matter pathways may be related to observed cognitive visual dysfunctions in CVI [Figure adapted from Bauer et al. (2014b) and Hirsch et al. (2015)].

FIGURE 2

Whole brain structural connectivity (ball and stick models; axial view) for the group averages of normally developed sighted controls (A), ocular blind (B), CVI individuals (C). White matter connectivity across the entire cortex was assessed using HARDI tractography. The number of reconstructed fibers between each of the 68 cortical regions (parcellated using the Desikan atlas; Desikan et al., 2006)) was used as a proxy for connection strength. Similar to the data shown in Figure 1, connections between each region were reconstructed in DSI-Studio. Once whole brain connectivity matrices were acquired for all subjects, they were averaged within subject groups and visualizations were rendered using BrainNetViewer (Xia et al., 2013). Each brain region is represented by a dark blue sphere, while the connection strength (i.e., number of reconstructed connections) between each region is represented by the color and diameter of the lines. Thus, a thick red line characterizes strong connections with abundant white matter fibers, while a thin blue line characterizes weak connections with minimal white matter fibers. Note the striking reduction in global structural connectivity that occurs in CVI, compared to both the control and ocular blind individuals [Figure adapted from Bauer et al. (2014a)].