Steven Pinker -- How the Mind Works - Hampshire High Italian ...

264 | HOW THE MIND WORKScalculate. This world-aligned reference frame would allow us to judgethe genuine angles and extents of the matter outside our skin. The perceptualpsychologist J. J. Gibson argued that we do have this sense ofreal-world scale superimposed on the retinal projection, and we canmentally flip between not using it and using it. Standing between therailroad tracks, we can assume one frame of mind in which we see thetracks converge, or another in which we see them as parallel. These twoattitudes, which Gibson called "the visual field" and "the visual world,"come from accessing the same information by either the retinal frame ora world-aligned frame.Yet another invisible frame is the direction of gravity. The mentalplumb bob comes from the vestibular system of the inner ear, a labyrinthof chambers that includes three semicircular canals oriented at rightangles to each other. If anyone doubts that natural selection uses principlesof engineering rediscovered by humans, let them behold the XYZCartesian coordinate axes etched into the bones of the skull! As the headpitches, rolls, and yaws, fluid in the canals sloshes around and triggersneural signals registering the motion. A heavy mass of grit pressing downon other membranes registers linear motion and the direction of gravity.These signals can be used to rotate the mental crosshairs so they arealways correctly pointing "up." That is why the world does not seem tolist even though people's heads are seldom plumb perpendicular. (Theeyes themselves tilt clockwise and counterclockwise in the head, butonly enough to undo small head tilts.) Oddly enough, our brains do notcompensate for gravity very much. If the compensation were perfect, theworld would look normal when we are lying sideways or even standing onour heads. Of course, it does not. It's hard to watch television lyingon your side unless you prop your head on your hand, and it's impossibleto read unless you hold the book sideways. Perhaps because we are terrestrialcreatures, we use the gravity signal mostly to keep our bodiesupright rather than to compensate for out-of-kilter visual input whenthey are not.The coordination of the retina's frame with the inner ear's frameaffects our lives in a surprising way: it causes motion sickness. Ordinarily,when you move about, two signals work in synchrony: the swoops oftexture and color in the visual field, and the messages about gravity andinertia sent by the inner ear. But if you are moving inside a container likea car, a boat, or a sedan chair—evolutionarily unprecedented ways to getaround—the inner ear says, "You're moving," but the walls and floor say,