This article is rather anatomy and physiology heavy, but will be helpful for building a basic foundation (or at least make a good reference point) to better understand binocular vision disorders.
Once you have a grasp on this pathway, you can pinpoint where problems occur and hence better understand binocular vision disorders as a whole.
Anatomy of the Eye and the Visual Pathway
Vision starts at the eyes located at the front of the skull, but vision is processed in the very back of the brain in a region called the occipital cortex.
If you reach your hand to the base of your neck where it reaches your skull, you can feel a little notch of bone. The occipital cortex is located just above that little notch. That is a long way from the eyes for signals to travel!
The way vision is created is through detection of light by the eyeball, transmission of the light from the eye to the occipital lobe of the brain, and then processing of the patterned light detection by the brain to create images.
The first step to vision, therefore, is that light needs to be correctly transmitted from space through the eye so that it lands on specific light-detecting cells called photoreceptors within the retina.
We want light to land directly on the photoreceptors—if the light lands in front of or behind the photoreceptors, it creates a blurry image.
To make light land correctly on the retina, it has to be bent—to visualize this think about using a magnifying glass to focus light onto a piece of paper. Everything has to be aligned just right to work properly.
There are two main structures in the eye used to accomplish light bending—the cornea and the lens.
The cornea is the outermost structure. It is the clear part that you can physically touch with your finger, just in front of the colored part of your eye.
The lens, on the other hand, is the middle most structure. It is flexible and changes shape depending on the location of the object you are focusing on.
The flexing of the lens to change how much light is bent is called accommodation. This process is what allows us to see far away one second and up close the next.
After light is passed through both the cornea and lens, it should land accurately on the retina—the backmost layer of the eye that contains photoreceptors. If it does not, you will need a glasses or contact lens prescription to correct for this error.
The most densely concentrated area of photoreceptors on the retina is call the macula. The macula therefore is responsible for the most accurate and detailed vision.
Once photoreceptors detect light, they can convert light into electric signals that are sent to the brain.
There are two main types of photoreceptors—rods and cones. Rods are responsible for detecting light cues in dimly lit situations and are the primary players in peripheral vision. Cones are responsible for detecting light cues in brightly lit situations and are the primary players in your central “20/20” vision as well as your color vision.
The amount of light and color of light determines how many photoreceptors are “activated” by it. An activated photoreceptor detects the light stimulus and converts it into an electric signal so that it can be transmitted to the brain.
The larger an object is, the brighter an object is, and the closer the object is to you will activate more photoreceptors and therefore create a larger electric signal for the brain to interpret as sight.
The signal is then sent along a tract as axonal flow. The axons of photoreceptors across the entire retina join to create a fiber-optic tract-like structure called the optic nerve. The optic nerve is essentially a massive nerve that exits the back of the eye and carries the electric signals from the millions of photoreceptors to the brain.
The pathway of the optic nerve gets rather confusing since we have two eyes each detecting various light stimuli. The optic nerves from the two eyes meet at a midpoint called the optic chiasm where the inputs cross and intermingle, creating an X shaped crossing. At this point half of the axons from the right eye move to the left side of the tract and half of the axons of the left eye move to the right side of the tract—this is called decussation of optic nerve fibers.
From there, the fibers decussate again into superior and inferior branches called the optic radiations. The radiations are what loop all the way back to the occipital lobe of the brain to alert the brain that light has been detected by the eyes.
The right half of our vision is therefore controlled by the left side of the brain, and the left half of our vision is controlled by the right side of the brain.
At this point, the brain has the ability to fine tune vision. If it wants to focus on something a little more closely, it can tell the eye muscles to move up, down, in, out, etc. to enhance the focusing system so that the light lands on the macula.
There are several different eye movements that are controlled by the brain to accomplish this—but we’ll talk about that in more depth with the vergence eye disorders section of this series.
What you should know at this point is that the eye’s movements are controlled by 6 extraocular muscles. These muscles attach to the outer shell of the eye and push/pull the eye to get it to look in different directions.
The six extra ocular muscles are the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique muscles.
Once the macula is aligned where the brain wants it to be, information can be further processed, and now we can see what we want to see clearly!
In other words, this is a very complex process that occurs in a matter of milliseconds! Any little hiccup in the system can result in visual problems. Fortunately, vision therapy allows us to identify some of these problems and train the eye to fix them!
What Does Vision Therapy Have to Do With Any of This?
To review from the previous post—vision therapy can be used to improve problems in 3 major categories
— accommodative disorders, vergence disorders, and oculomotor disorders.
Accommodative disorders result from the lens not being able to focus light rays correctly—or problems with the accommodative ability of the lens.
The lens is controlled by a muscle called the ciliary body, like other muscles in our bodies, sometimes it needs to be trained to more accurately focus near and far.
When the accommodative system is not working correctly, you may experience symptoms of eyestrain, headaches, the need to hold reading material out further away, the feeling of your eyes “being stuck” after looking at something up close, or simply just an inability or avoidance of doing near work.
Visual therapy for accommodative disorders focuses on this area of expertise to get the lens flexing more accurately to lessen these symptoms and allow you to see clearly both at near and far away.
Vergence disorders result from the extra ocular muscles not working correctly.
If you can recall from the last article, vergence occurs when using both eyes—it involves both eyes moving inward together (convergence) or outward together (divergence).
When the vergence system is not working correctly, we again have problems looking from near to far, or far to near. This is because when we look at a far away object, our eyes naturally diverge so that the target aligns with the macula of both eyes. When we look at a near object, our eyes natural converge so that the target aligns with the macula of both eyes.
Vergence disorders present very similarly to accommodative disorders symptom-wise, however the cause behind the symptoms is very different. In vergence disorders, the extraocular muscles do not equally continue to move both eyes in or out appropriately, creating the eye strain, blur, fatigue, headaches, etc. as the two eyes are physically struggling to look within the same direction.
Vergence disorders can be a problem with one eye’s ocular muscles, or both. It can result from an overacting (too strong) muscle, or an underacting (too weak) muscle.
Your vision therapist will be able to isolate the problem(s) and create an appropriate therapy plan to try and strengthen muscles and get the eyes on an equal, level playing ground to relive some of your symptoms created from the vergence dysfunction.
Remember that our best vision is associated with the macula in the retina. We need our maculas to be aligned in order to create single, clear images. If the muscles are pulling the eyes in different directions or moving irregularly, the brain will not be able to process this information as everything becomes jumbled up.
Even the smallest eye movement or misalignment can cause problems with visual processing. It is often the small problems that go unnoticed that create the biggest problems.
This leads us to the underlying problem with oculomotor dysfunctions—eye-brain miscommunication. This is the most complex dysfunction that we will discuss.
It takes both clear vision on the macula and proper processing by the brain to eliminate oculomotor problems.
It is quite difficult, if not impossible, to edit how the brain processes visual information. It is entirely possible, however, to train the eyes muscles to work as they should to help the brain with processing—essentially “kick-starting” it to work as it should.