TheConversation: 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.
Video: Infra-red camera
tracking of eye movements while jogging in complete darkness. The
vestibulo-ocular reflex works by activating extra-ocular muscles to move
the eyes to compensate for head movements. The video begins with Alan
standing still (rest), then jogging (jog), then standing still again
(rest). Although the eye movements don’t appear to be big, they are
exquisitely precise.
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.