Visual System Development in People with One Eye: Behaviour and Structural Neural Correlates

Postnatal monocular deprivation from the surgical removal (enucleation) of one eye in humans results in intact spatial form vision, although its consequences on motion perception development are less clear. Changes in brain structure following early monocular enucleation have either been assessed in species whose visual system is quite different from humans, or in enucleated monkeys and humans following short-term survival. In this dissertation, I sought to determine the long-term effects of enucleation on visual system development by examining behavioural visual abilities and visual system morphology in adults who have had one eye enucleated early in life due to retinoblastoma. In Chapter II, I conducted a series of speed and luminance contrast discrimination tasks not yet implemented in this group. Early monocular enucleation results in impaired speed discrimination but intact contrast perception compared to binocular and monocular viewing controls. These findings suggest differential effects of enucleation on the development of spatial form vision and motion perception. In Chapters III and IV, I obtained high-resolution structural magnetic resonance images to assess the morphological development of subcortical (Chapter III) and cortical (Chapter IV) structures in the visual pathway. Early monocular enucleation resulted in decreased optic chiasm width and volume, optic tract diameters, and lateral geniculate nuclei (LGN) volumes compared with binocularly intact controls. Surprisingly, however, optic tract diameter and LGN volume decreases were less severe contralateral to the remaining eye. Early monocular enucleation also resulted in increased grey matter surface area of visual and non-visual cortices compared with binocularly intact controls. Consistent with the LGN asymmetry, increased surface area of the primary visual cortex was restricted to the hemisphere contralateral to the remaining eye. Surprisingly, however, these increases were found for those with right- but not left-eye enucleation, suggesting different developmental time periods for each hemisphere. Possible mechanisms of altered development following early monocular enucleation include: 1) recruitment of deafferented cells by the remaining eye, 2) retention of deafferented cells due to feedback from visual cortex, and 3) a disruption in synaptic pruning. These data highlight the importance of receiving normal levels of binocular visual input during infancy for typical visual development.