Before we can discuss how the brain responds to vestibular injuries, we need to make a quick stop and review normal anatomy and physiology. Don’t worry, I’ll keep it simple. There is more to the Vestibular-Ocular Reflex (VOR) than can be covered here, but we will only go as deep as we need to in order to help understand the process of compensating from vestibular injuries.
The graphic above is a schematic of the VOR, which can also be described as the neural network between the ears and the eyes. From a functional standpoint, this is the system that allows us to have stable clear vision on a stationary object as we move our heads around. If you’ve ever watched a video that was taken by someone walking, you probably noticed the video jumping up and down with every step. That is because their camera did not have a visual motion offset. The VOR is our own personal visual motion offset.
So, what you are looking at above is a rudimentary explanation of how the ear sends the signal to the eyes to react to head movement. The red and blue circles on the bottom represent the left (blue) and right (red) inner ears. The two gray circles at the top represent the eyes, left eye on the left, right eye on the right side of the graphic. In this particular example, the response to a horizontal head movement towards the right is described, so only the horizontal semi-circular canal response is represented. That is what the two arrows at the bottom represent. Both of the ears are simultaneously rotating clockwise.
If you are a student reading this, you need to know the neural pathways. If not, don’t worry about all the squiggly lines between the ears and the eyes. What you need to know is that with every head movement a healthy inner ear sends a signal to the brain regarding what movement, or what position change, just occurred. In a healthy system, when the head is still the signal is equal and the eyes are still. In this example, when the head is turned to the right, the signal on the right increases, while simultaneously the signal on the left decreases. The brain takes that information and sends a reflexive signal to the eyes to move opposite of the head, with the goal of visually offsetting or canceling out that movement or position change.
The take-home message? When the system is healthy, the eyes are still when the head is still. When the head moves, the eyes make an equal and opposite reflexive movement to allow the eye to stay still relative to the object you are viewing.
What happens in an unhealthy system? Come back next week.
Graphic courtesy of http://www.zazzle.com/vestibulo_ocular_reflex_vor_eye_movement_stabilize_postcard-239598263501556438
Photo courtesy of http://www.neuropt.org/docs/vsig-physician-fact-sheets/peripheral-vs-central-vestibular-disorders.pdf?sfvrsn=2