With the traditional thinking gesture, you scratch your parietal lobe. This lobe makes sense of space, both recognizing the dimensions and features of space around us and keeping track of how our body is positioned in this space. It also helps us slap coordinates on things we perceive. It’s as if the parietal lobe holds a three-dimensional graph of our surroundings, dotted with points that represent all the things we’re aware of (and many we’re not).
In fact, it holds a couple of these graphs: certainly one based on visual input and another based on auditory input, and perhaps more graphs for more senses (does our parietal hold a map of smell positions?). And based on its confidence in these maps, it can switch from one to the other like Predator switching from thermal to ultraviolet vision. In most cases our visual input most accurately captures the dimensions of our surroundings, but if sound is up and sight is down, our parietal switches to its auditory map.
MRI studies show that women tend to have proportionally greater parietal lobe gray matter than do men, while men have increased parietal surface area. This may explain males’ relative aptitude (and females’ relative ineptness) on mental rotation tasks.