Vision in a Clinical Rehabilitation Context
Cranial Nerves and Vision
The cranial nerves are critically important in our development and behaviour. There are 12 pairs of nerves, and they are largely responsible for the sensory and motor behaviour of the head. The brainstem nerves are very tightly integrated and tuned to enable not only very fine sensation from the core senses, but also the complex interactions between them. For example, a noise beside us will automatically trigger mechanisms to calculate required movement and to direct the eyes towards the noise. Some of these interactions can be controlled and overridden by higher cortical functions such as if we continue to read something in front of us even though there is a surprise noise to the side.
We operate between a ‘stand-by’ mode and an active seeking mode. In effect, the cranial nerves work as a team to passively scan or observe the environment, then,, spurred by external stimuli or other cortical functions, they very actively focus attention to objects of interest. The cranial nerves carry the sensory information from sensory organs to the different brain areas, but they also direct physical attention so the sense organs can do their job. The sensory information taken in is then processed in varied brain regions.
Of the 12 cranial nerves (CN), 8 are somehow related to visual function, helping to integrate touch, sound, sight, balance, and muscle control in the eyes, and head and neck in particular. The optic nerve (CN II), which carries information about vision, is one of these, the others include: oculomotor (CN III) for eye movement and focus control; trochlear (CN IV) for eye movement; trigeminal (CN V) for sensation of touch in and around the eye; abducens (CN VI) for eye movement: facial (CN VII) for movement of the facial muscles to assist in focusing and protection of the eyes; vestibulocochlear aka auditory-vestibular (CN VIII) for establishing balance and integration of auditory with visual stimuli; the spinal accessory nerve (CN XI). CN XI has a special role in vision and is used to direct the head to observe objects of interest, such as when we see a flash out of the corner of our eyes, or hear a sound, and reflexively turn to see what is happening. Also, if our eye alignment is mismatched, such as if one eye is positioned higher than the other, CN XI will help position the head to realign images on the two retinas – this is why some people walk with a turned head or neck.
This overview of cranial nerves serves to underline both the complexity of vision, but also to highlight how vision appears to be the sense of discovery preferred by our species. Put another way, we can glean more information more quickly by simply observing an object visually than by any other means. Developmentally, vision sits atop the developmental hierarchy, arriving late in development, and requiring the presence of already strong sub-skills, such as balance and bilateral coordination. For example, we cannot automatically and accurately target objects with our eyes if there is not already a strong understanding of three-dimensional space and awareness of the physical body in space and in relation to the objects in the environment. For these reasons, visual neurorehabilitation will often begin by working on these core skills.
Nearsighted Vs. Farsighted
The terms ‘nearsighted’ and ‘farsighted’ are often confused, so here are some memory boosters to remember the difference: Your vision is where the word says it is, what remains is difficult and blurry. So if you’re
- Nearsighted (aka myopia): Near targets are clear.
- The eye’s sight is set too close for distant targets.
- The eyes must try to relax to see distant targets.
- Near vision is easier relative to farsighted or neutrally sighted people.
- More likely to fail basic school sight tests.
- Farsighted (aka hyperopia): Distant target may be clear. Near targets are more likely to be blurred.
- The eye’s sight is set ‘too far’ for distant targets.
- The eyes are constantly trying to maintain focus.
- Near vision is especially difficult compared to nearsighted or neutrally sighted people. The closer the target, the more effort is required to ‘clear’ the target.
- More likely to pass school sight tests.
The greatest distinction between myopia and hyperopia in the classroom is not so much that images are at times blurred, it that there is a risk of pain and loss of vision if hyperopia is too severe for the child. The associated risks with myopia are negligible in comparison. Low to moderate nearsightedness can be a real benefit to someone who is required to work in near proximity for extended periods of time. Low to moderate farsightedness is generally uncomfortable for students and leads to many behavioural and health concerns when the child is required to work up close all day.
Astigmatism is another optical concern entirely. One can be nearsighted or farsighted in an eye, but not both. In either case, the eye can also have ‘astigmatism’, that is, it is ‘astigmatic’. Whereas myopia and hyperopia are a matter of shifting the plane of focus to clear an image, astigmatism is a condition where there are two planes of focus. The eye, therefore, struggles to know which plane of focus to shift to. (By shifting, I mean controlling tension on the ciliary body in the eye to focus or defocus light on the retina.) The result is that the eye continues to struggle to focus, and this can lead to painful headaches and eyestrain. The blur can also be strong enough to lead to reduced visual acuity, or amblyopia.
It is possible for someone to be myopic in one eye, and hyperopic in the other. In one configuration of astigmatism, one plane of focus is myopic and the other is hyperopic. While nearsightedness appears to be a problem in the classroom, farsightedness and astigmatism are by far worse obstacles to fluent and comfortable reading.