All the world's a three-dimensional stage, or so it seems. While it feels real, it is in fact, a cunning illusion.
At any given moment, you would need to process a prodigious amount of information to fully model yourself within your surroundings. A crude analogy is that super hi-res video can run at over 700 megabytes per second.
As you read this, there's the text in front of you, and perhaps you're sitting on a chair in a room with a myriad of details around. Clearly it isn't possible to take all this in, so your brain is finely tuned to absorb only as much as necessary. For example, your eye has a tiny area of acute vision and outside that, it's less detailed. The myth that we use only 10 per cent of our brains is silly because the brain is a large, expensive organ.
There are a few facets of how you perceive yourself in space. Proprioception is an awareness of the location of your body. It uses neurons in tendons, muscles and joints to indicate things such as the movement, load and location of limbs.
Your hearing is another important cue because the separation of the ears with skull in between, sounds will be louder on one side. There's also a tiny difference in timing.
That works well for sounds left-right, but not so well for sounds up or down. To help with those, the oddly shaped folds in your outer ear change sound according to the direction it comes from. It can help to tilt your head, which partly explains why your dog does that when you talk to it. Being social creatures, it's also a way of their way of saying "I'm actively listening to you".
Since humans are visually oriented, perhaps the most important input is vision. With eyes about 7 centimetres apart, there's a slight difference in angles. This is similar to parallax used to measure the distance to planets and nearby stars.
Even with one eye you have some sense of depth using other clues, although not as rich. For example, near objects obscure those behind. When moving, things nearer to your move more quickly than things further away.
