The eyes don't see

A little-known truth about vision

The eyes don't see

A commonly held myth - even amongst eye care professionals believe it or not - is that if one eye is bad, and the other eye is good, the good eye will be ‘strained’ and ‘overworked’. Although a perfectly logical thing to assume, it is not strictly true. Why is this the case? The eyes don't see - the brain sees.

The eyes are an extension of the brain. Light captured by the eyes is transmitted through the optic nerve (think of it as an ultra-HD video cable) which sends the signals to the visual cortex all the way at the back of the head. Here, these signals are interpreted and transmitted to the conscious mind. Click the image to continue reading.

When one eye sees poorly, it continues to send information to the brain. In effect, both eyes continue to ‘work’ as much as they always have. The brain receives the images, takes whatever useful information available from each eye, and ignores the rest. For the better eye to do ‘more work’ would require that the brain send a signal back to the eye to ask it to do twice the work. This does not happen.

Case and point is those people who are born either with one ‘lazy eye’ or suffer an eye injury or disease in their youth. These people rarely if ever report any symptoms of ‘eye strain’ in their remaining good eye. 

It’s not all in the detail

Fine detail is only one type of vision. We also have:

  • Peripheral vision
  • Movement detection
  • Colour vision
  • Brightness detection

Even if the 'weaker' of your two eyes doesn’t see fine detail doesn’t mean that it’s useless. It can often still see things coming at you from the sides/above/below. It can still see movement and often colour. Depth perception also doesn’t require 100% detail from each eye.

The brain can adapt

Let’s be clear, however, that sudden deterioration or loss of vision in one eye can, in the short term, take some adjustment. Patients who lose vision in their dominant eye will find full adaptation more difficult. As traumatic as loss of vision can be, overthinking the situation will make it harder for the brain to adapt.

Covering each eye in turn?

A natural desire to cover each eye in turn can exacerbate symptoms and prevent the brain from adapting to a new visual situation. You likely never covered each eye in turn before, so why start now? You see with both eyes open and uncovered. If you have no symptoms in this natural state, then you have nothing to worry about. If you remain symptomatic despite time and your best efforts, there are often things that can be done to help. Remember that any intervention in an asymptomatic patient is not always a good idea.

About the Author

Marcello Leucci

Marcello Leucci

In addition to working with Dan, Marcello is the lead optometrist of the Keratoconus service at Moorfields Eye Hospital. He is responsible for development of protocols on keratoconus monitoring and progression criteria and is actively involved in research in corneal tomography, and the effectiveness of both new and existing treatments for the condition. 

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