why can’t we move our eyes outwards in opposite directions?
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Biology perspective – eyes are not lone wolves. Think of two ox yoked together. They can’t go in separate directions because they will pull themselves apart. Eye muscles and nerves work the same way.
Image generation perspective – you don’t see each image from each eye separately. The images combine in the brain to make the image you see therefore they must look in the same direction or you will see double. People who don’t have both eyes pointing in the same direction either shut the image from one eye off, alternate the images from each eye or they get headaches etc if seeing both images.
Non ELI5 answer – the visual system has an extremely complex but well thought out design / evolution.
Eyes need to be in perfect alignment ie light from the target object of focus must land at the central, most light / detail sensitive tissue at the back of each eye simultaneously. Equally each eye has a blind spot and the way they work, move together counters the blind spot in the other eye.
All of this light information is converted to an electrical signal when hitting the tissue at the back of the eye. This only remains on its own individual nerve pathway for a short distance before the signals are combined and split differently and then sent to the brain.
Here the signals are separated based on type (detail or gross) and area of the eye the light touched and go through a complex processing mechanism in your brain.
The output of this mechanism is what you actually ‘see’. This arrangement allows for things such as depth perception etc. different organisms will have different setups based on need / priority. Humans have great central detail and colour vision and effectively a great overall visual balance for a variety of situations.
Further the reason your eyes can come together is because when you change the distance of fixation the eyes need to come together to maintain a single image. Ie when they are they furthest apart you are looking the furthest away and they come together as you fixate closer.
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