Balance is the vital sense that gives much-needed stability to our teetering, upright bodies. Good balance is usually associated with having stable posture, but it also has a lot to do with visual stability.
The importance of the balance system is shown by the vast number of connections it makes with the brain. These connections reveal that the forces of motion we create and encounter in the environment can go on to affect many parts of the brain, including those that control vision, hearing, sleep, digestion, and even learning and memory.
How does balance work?
Every sensory system uses detectors or receptors outside the brain to gather information about the environment. For example, the visual system uses light-sensitive receptors in the retina to detect visible light. The balance system relies on specialised motion-sensitive receptor cells in the inner ear.
While obviously associated with hearing, the inner ear is also the shelter for balance. It has a labyrinthine structure, made up of a series of fluid-filled canals and ducts. Within this labyrinth are five balance receptors that are ideally placed to detect different types of movement. There are three receptors for head rotation, another for horizontal acceleration, and one for vertical acceleration (or gravity).
Each balance receptor is an organ made up of thousands of cells with long hair-like projections. As a result of head movement, these so-called hair cells are excited when their projections are pushed in a particular direction by fluid called endolymph.
Movement of endolymph within the inner ear is complex. When you spin a bowl of water, for instance, the water takes time to “catch up” with the turning bowl. This lag is due to inertia and applies to all fluids, including endolymph.
When the head begins to move, the endolymph initially stays still. This actually translates to a fast relative movement of the endolymph in the opposite direction to the head. This relative movement excites the hair cells that are aligned to detect that particular head movement.
So in an elegant and precise way, endolymph and hair cells work in concert to provide a constant stream of information about head movement to the brain.
The inner ear balance organs are remarkable in their ability to detect head movements both small and large, fast and slow, and in any direction. The brain uses signals from the organs to orchestrate a suite of balance reflexes that control our muscles, right down to our toes!
However these reflexes not only control our muscles of posture but our eye muscles as well. Together, these reflexes underlie our ability to remain upright with stable vision in an ever-changing and ever-moving physical environment.
Why doesn’t our vision bounce up and down when we jog?
Maintaining our upright posture is an obvious job for our exquisitely sensitive and responsive balance system. However, it also has a profound effect on the control of our eye movements. The up-down movement generated when walking or jogging would have a destabilising effect on our vision.
Like footage from a hand-held camera, even a simple jog along a flat path or a smooth road would result in unstable and shaky images. When watching hand-held camera footage, it can be unpleasant and difficult to focus on stationary objects like trees because they are moving too violently.
But what about our eyes? Thankfully, our visual field is remarkably stable when we jog. This is due to a reflex most of us take for granted, called the vestibulo-ocular reflex.
The vestibulo-ocular reflex is one of the fastest and most active reflexes in the human body. It uses head movements detected by the inner ear to generate compensatory eye movements that are equal – but opposite in direction – to head motion. This subconscious, ongoing adjustment of eye position results in a stable visual field, despite significant movement of the head.
What happens when balance goes wrong?
For many, the idea of suddenly losing a sense like vision or hearing is terrifying (and rightly so), and a sudden loss of your sense of balance would be similarly catastrophic.
Initially, a debilitating and frightening dizziness would prevent you from completing even simple daily tasks without falling over. The worst symptoms would subside with time as you begin to rely more heavily on other senses such as vision. But even a partial loss of the vestibulo-ocular reflex would mean stopping and standing still every time you wanted to recognise a face or read the price of a grocery item.
The fact that we are almost totally unaware of this elegant reflex is evidence of the superb, undercover work the balance system does for us. It not only allows us to walk without falling over, but also gives us a constant and reliable view of a beautifully changing world.
Lauren Poppi receives funding from National Health and Medical Research Council of Australia and the Neuro-Otological Society of Australia.
Alan Brichta receives funding from National Health And Medical Research Council of Australia and the Garnett Passe and Rodney Williams Memorial Foundation.
Authors: Lauren Poppi, PhD candidate in Anatomy, University of Newcastle