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of the torso (think of bending over and picking up an object from the ground). In order to calculate the appropriate muscle contractions, it is crucial that our brains keep careful track of the relative positions of different parts of our own bodies as well as the appearance of the visual scene in front of us. We can do some of this work by using a specialized set of sensory receptors embedded in our joints and muscles. The outputs of these so-called proprioceptors report to our brain on the position of our body. In addition, whenever our brain sends a command to our muscles to move, a copy of that command is kept at hand in a neural filing cabinet so that we can use it to keep track of the expected consequences of each movement that we make. Our brain tries to save time by predicting the consequences of a movement before it has even taken place.
    When we move our eyes, our hands, and our arms, we need only keep careful track of the relative movements of our body parts—eye relative to head, head relative to body, hand relative to shoulder, and so on. Walking changes everything. With each step,we take flight from the surface of the planet, and when we alight we are in a new location. It is no longer sufficient to measure our own muscular contractions or motor commands to determine our exact position in space. We need an entirely new set of tools.
    Carrying a full glass of beer across a crowded barroom can be tricky business. When standing still, or walking at a smooth, unchanging speed, the beer sits securely in the glass, no tidal waves of liquid threatening the floor or our clothes. But each change of direction or speed can cause the precious liquid to slosh around in the glass. Now imagine an observant and scientifically minded drinker wandering across the floor with glass in hand. She might notice that the way the beer moves in the glass is related in a very orderly way to the movements of the glass. Sudden changes of motion cause predictable reactions in the shape of the surface of the ale. In fact, a careful observer could calculate the path of the glass through space by doing nothing other than measuring and recording such changes (though she might not be the most fun person to drink beer with). To calculate accurately, she would need to note each and every movement of the surface of the fluid. If she was distracted for even a moment, or if her memory failed her, the missing data would cause her to lose track of her position completely.
    Many animals, including human beings, have a specialized set of organs that sense movement in exactly the same manner as our observant beer drinker. These structures, called the vestibular system, consist of a series of interconnected chambers and tubes within the middle ear. These wondrously shaped vestibules, looking a bit like a curvy architectural creation by Frank Gehry, are filled with a viscous fluid. Inside each of these tubes is a small chunk of gelatin, studded with tiny crystals of limestone to give it added weight. As our head accelerates and decelerates through space, the blobs of gelatin wobble around just like the beer in the glass. Tiny hairsembedded in the blobs are bent by each wobble, and these bending movements send signals to our brain.
    The vestibular system works remarkably well for controlling certain types of movement. For example, our ability to maintain fixation on a visual target as we walk around or even leap through the air is largely brought about by a precise dialogue between our vestibular system and our eye muscles. But as a device for keeping track of our movements in larger-scale space, the vestibular system has the same weakness as our beer carrier. Errors creep into the mix, and those errors accumulate over time. Without any help from other sources, the vestibular system will become lost and disoriented. One possible source of help comes from the visual system, which has specialized abilities to keep track of our position as we move through