November 17, 2016
The Eyes Have It: The Basics
In the first post of this series (The Eyes Have It: The Foundation) we discussed the development of functional vision, it’s impact on movement and it’s intimate connection to the vestibular system. Today we’re going to discuss terms that have to do with functional vision: visual efficiency, visual processing and visualmotor integration 1.
There are several components to visual efficiency. Visual acuity is a term we commonly hear. It refers to the sharpness of our vision – how well we can see objects at various distances, either close up or far away. That lovely 20/20 vision indicates from 20 feet away, a person can read the line of letters 4th from the bottom on a standard (= Snellen) eye chart. It is one part of visual efficiency.
Other components of visual efficiency have to do with how the eyes work together. Each eye is controlled by six extra-ocular muscles. These muscles control all the movements of the eye. The different movements the eyes must perform are:
Fixation is the ability to maintain gaze on an object. It involves keeping an object in the center of the fovea (=clearest point of vision). It is important to understand if the child can fixate on an object when they are at rest, but also if they can fixate on an object when their head is moving so they can see objects when they are moving through space.
There are 2 types of vergence skills – divergence and convergence. Divergence indicates that the eyes move away from each other when focusing on/tracking an object. Convergence indicates that the eyes move towards each other when focusing on/tracking an object. Both eyes must aim at the same point, in order for the brain to get the identical images from each eye. When we are visually tracking a ball that is coming towards us or moving away from us, we require vergence skills.
Here is a video of a girl tracking an object as it moves towards her nose (convergence of the eyes) and as it moves away from her nose (divergence of the eyes).
These are rapid movements of the eyes between fixation points. We have reflexive saccades (micro-movements) of our eyes at all times but when we assess this movement we are looking at voluntary saccades. We need saccades when we shift our gaze quickly from one object/point of interest to another. When we are deciding to cross a road, we may need to shift our gaze quickly between several cars to judge how quickly they are approaching and whether it is safe to cross.
Pursuit occurs when the eye closely follows a moving target. We use smooth pursuits to follow a ball as it is moving through space.
The eyes must learn to perform all these movements in a precise, coordinated way, something we call eye teaming or binocular vision. This allows the brain to create an accurate 3-D representation of the world.
Here’s a video of a baby displaying excellent visual efficiency skills.
Visual processing is the processing of visual information by the brain. It involves our ability to understand:
Up/down, in front/behind, top/bottom, above/below (directionality)
Form of objects
Ability to visualize a complete picture when we see only a partial picture (visual closure). Below is an example of a visual closure task.
It has been estimated that > 60% of the brain is involved in visual processing!
This is the formal term for eye-hand-body coordination. Sometimes this gets shortened to eye-hand coordination. But because postural control provides the stable base for distal motor function no matter what motor function we are discussing (fine motor, oral motor, gross motor, visual motor), I prefer eye-hand-body coordination. I have seen many children in clinical practice who improve their eye-hand-body coordination dramatically when we work towards mature postural control.
The eyes, head and body often share common orientations, but just as often they do not and the foundation of this flexibility is postural control 2. Much has been made of the retention of postural reflexes and their impact on visual skills. To be sure, reflexes contribute a great deal to the sensory and motor wiring that is required for mature postural control. However, they are a symptom of poor central stability and need to be understood/treated in the context of the development of postural control, which is incredibly complex and not separated out and treated in isolation. Vision itself, is dependent on the dynamic interaction of systems.
The Vestibular System (again)
The most basic control of the eye is created at the reflex level; the vestibular system provides the very fast and efficient movements of the eye. If there is any difficulty with vestibular function (registration, processing or output) then the eyes will not have an adequate foundation for the most basic of visual efficiency skills. However our perception of where we are in relation to gravity also serves as a foundation for visual processing skills (how do you develop directionality if you don’t have a solid gravitational reference?3). Therefore a disruption of the vestibular system also impacts visual processing. Finally, the vestibular system is foundational to all the other sensory systems and to the motor system as well (if you wonder about this last connection, you can read more here). One can’t develop good coordination without a robust vestibular system.
All of these skills contribute to posture, balance and the ability to move through space safely and efficiently 2. As PTs we work on crawling, walking, going up and down stairs, negotiating obstacle courses/classrooms/busy hallways – each of these is intimately connected with visual skills.
Next in this series: suggestions for assessment and treatment of functional vision and visual-motor integration.
1. Paudula WV, Munitz R, Magrun WM. Neuro-Visual Processing Rehabilitation: An Interdisciplinary Approach. Santa Ana, Optometric Extension Program Foundation, 2012.
2. Land MF, Tatler BW. Looking and Acting. Vision and eye movements in natural behaviour. New York, Oxford University Press, 2009.
3. Riley SS. Blanche EI, Schaaf RC. Understanding the Nature of Sensory Integration with Diverse Populations. Therapy Skill Builders, 2001.