Ocular Stability in Three-Dimensional Space

abstract

During natural movement, we visually orient ourselves continuously to objects in our environment that are located in different directions and at different viewing distances from our body. Since humans have their eyes placed frontally, we have the ability to look at these objects using combined images of two eyes. This is called binocular vision. Visual information that enters each eye remains segregated in the primary pathways up to the visual cortex. In the primary visual cortex the information of the two eyes are merged, which enables humans to perceive depth. There is a caveat in having binocular vision. To obtain sharp vision, objects of interest have to be projected on the fovea of both retinas, where photoreceptor density is largest. During movement, it is essential to keep track of the objects around us and to maintain clear and sharp binocular vision. The most essential tool in accomplishing these goals is making eye movements. Eye movements are necessary to compensate for motion and direct the lines of sight of both eyes to new objects of interest. This interaction between processing of visual information and making eye movements is an ongoing process, which allows us to continuously update all changes in our visual environment. Thus, eye movements can be regarded as ‘the motor system that sees the world’. There are two types of eye movements; stabilizing and voluntary eye movements. Stabilizing eye movements, especially in association with the vestibular system, are necessary to maintain sharp vision during movement. Voluntary eye movements are used to shift gaze to new objects of interest. Voluntary eye movements can be triggered from visual, auditory or tactile stimuli, or from memory. This thesis focuses on the accuracy and dynamics of binocular eye movements to objects at varying distances in three-dimensional space. The interaction between the direction of gaze of both eyes (version) and the angle between both eyes (vergence) was studied under different visual conditions, under memory guided conditions, when eye movements interact with the vestibular system and when whole body movements intervene with visual spatial memory. In the next chapters, an overview is presented of the characteristics of binocular vision, binocular eye movements, vestibulo-ocular reflex (VOR) and visual spatial memory.