At-Bristol Science Centre

Question of the fortnight: When we look at things how do we judge how far away they are?

Blogging science to life

Wed 17 November 2010, Written by: Nicole

Every fortnight, we are going to answer your science questions about the human body. It's all part of finding out how amazing you are, in the run up to the opening of our new exhibition All About Us in February 2011.

Question asked via twitter @Walksnap

Judging how far away things are is a complex process which involves your eyes, brains, muscles and nerves all working together and communicating with each other. Distance and depth perception is something which is learn through infancy and childhood. Depth perception relies on how our two eyes work together and our knowledge of the world around us.

 How do we judge distance?

Human eyes see in binocular or stereovision because we have two eyes which are in different positions on our head. This means they each receive information from a slightly different field of view. Binocular vision also means that we can see three spatial dimensions; width, height and depth. It is our binocular vision which allows us to perceive depth and judge distance.

 Most of the information received by the brain from each eye is the same, however a small amount of this information is slightly different and it is these small differences which the brain uses to perceive depth. The brain combines the two visual images it receives, matches up the similarities and adds in the small differences to create a 3D image.

The slightly different fields of vision we get from seeing in binocular (also called stereopsis) work with other visual cues which are learnt through infancy. Have you ever noticed how babies often seem uncoordinated? That is because they are still learning the binocular clues that we quickly learn to rely on.

Visual cues such as parallax, how nearer objects appear to be larger than further away objects is one of the cues used to judge depth. Size constancy, which is how we can accurately judge real size of object regardless of its distance, is also essential. Perspective and motion parallax, the way objects that are moving towards us appear to be getting bigger and those moving away from us get smaller, also help us determine how far away things are. All these cues explain why things may appear to differ in size, but it is this which helps us determine depth and distance.

eye eye!So judging distance is something our brain works out by receiving information from our eyes and then deciphering it by relying on the rules and expectations about the world. Optical illusions, such as Ames room, show us how the assumptions and expectations our brain relies upon and what we ‘see’ is not always what is actually happening!

For more optical illusions click here or visit At-Bristol - we have Richard Gregory's famous 'cafe wall illusion' which was discovered by Richard Gregory.

Finally did you know that of all the information that comes through the eye and into our brains only 10% of  that information is what we use to see? The other information that factors into seeing is received from our body and senses.

Hope this answers you question! If you want to know more about tthe human eye watch out for the Lens Changer exhibit when All About Us opens in March 2011.

Do you have a science question about the human body that you’d like answering? Tweet your question to @atbristol using the hashtag #QF and we’ll answer one a fortnight!

Find out more information about our new exhibition All About Us opening in March 2011

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