Did you know that more than half of people with established multiple sclerosis (pwMS) have had an episode of optic neuritis (an MS lesion in the optic nerve)? Not all episodes of optic neuritis cause symptoms and can be subclinical. We detect the latter using visual evoked potentials that show slowed conduction in a particular optic nerve.
When optic neuritis recovers the conduction speed of nerve impulses travelling through the recovered nerve rarely gets back to normal and is slower than the conduction speed in the unaffected eye. In addition, the conduction speed can change depending on fatigue or ambient temperature changes. The conduction in the affected, by remyelinated, nerves can even fail and this is can result in intermittent blurring or loss of vision called Uhthoff’s phenomenon.
The brain is usually quite good at compensating for the signals coming from the eyes at different speeds, but the one thing it is poor at doing is adjusting for depth perception. The reason we have two eyes with overlapping visual fields or binocular vision is for depth perception. This is why pwMS with previous optic neuritis may have poor depth perception, which affects their ability to judge where for example a cup or glass is on the table and as a result of this they often spill drinks. PwMS will also have difficulty playing ball sports such as tennis, table tennis or any sport that requires accurate depth perception.
One function that seems to very sensitive to this phenomenon is judging distances whilst driving at night; for example, estimating how far you are from the traffic intersection or a stop sign. Many of my patients don’t like driving at night because of this. Why? Well, the brain uses a process called parallax to compensate for the loss of depth perception; i.e. the brain uses the relative size of familiar objects to help make a judgment on how far something is away from you in the distance. By using parallax somebody who is blind in one eye can judge depth and distance relatively well. However, at night in the dark when images are not well illuminated and use of colour vision drops judgement by parallax fails and depth perception deteriorates.
I would be interested to know if any of you have any of these problems with depth perception and driving at night. If you do the chances are that you have had optic neuritis in the past.
Unfortunately, there is little we can do to treat this phenomenon, but knowing about and understanding why it occurs may help you compensate for this impairment, disability or handicap.
Parallax: Have you ever wondered why the setting-sun and full-moon on the horizon look so big compared to it being smaller when it above us in the sky? It is actually not bigger; simply having a reference on the horizon (trees, mountains, buildings, the sea, etc.) makes your brain perceive it as being larger, i.e. closer. This is how parallax affects how the brain works and judges distance from you to an object in the distance.
#T4TD = Thought for the Day
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