Outside, clouds glide across the horizon. A man on the street below seems in a hurry. Two cars and a lorry drive past him. And a sparrow lands on the windowsill. As you get up to see it closer, a pencil rolls off the end of the table and falls to the floor. None of this may seem particularly eventful to you but it does take a very intricate network of cells to process the surrounding information so that you can see it all happening.