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

The Mind's Eye 263text or graphics. For the brain to use the contents of the 2 l h-D sketch, itmust employ a similar mechanism, indeed, several of them.The simplest reference frame that moves over the 2V2-D sketch is onethat stays riveted to the head. Thanks to the laws of optics, when theeyes move right, the image of the apple scoots left. But suppose theneural command to the eye muscles is cc'd to the visual field, and is usedto shift the crosshairs over by the same amount in the opposite direction.The crosshairs will stay on the apple, and so will any mental process thatfunnels information through the crosshairs. The process can happilycontinue as if nothing had happened, even though the contents of thevisual field have slid around.Here's a simple demonstration of the cc'ing. Move your eyes; theworld stands still. Now close one eye and nudge the other one with yourfinger; the world jumps. In both cases the eye moves, and in both casesthe retinal image moves, but only when the eye is moved by a finger doyou see the movement. When you move your eyes by deciding to looksomewhere, the command to the eye muscles is copied to a mechanismthat moves the reference frame together with the sliding images so as tocancel your subjective sense of motion. But when you move your eye bypoking it with your finger, the frame-shifter is bypassed, the frame is notshifted, and you interpret the jerking image as coming from a jerkingworld.There may also be reference frames that compensate for movementsof the head and body. They give each bit of surface in the visual field afixed address relative to the room or relative to the ground; the addressstays the same as the body moves. These frame shifts might be driven bycopies of commands to the neck and body muscles, though they mayalso be driven by circuitry that tracks the slippage of the contents of thevisual field.Another handy overlay would be a trapezoidal mental grid that markedout equal-sized extents in the world. A gridmark near our feet wouldcover a large stretch of the visual field; a gridmark near the horizonwould cover a smaller stretch of the visual field but the same number ofinches as measured along the ground. Since the 2V2-D sketch containsdepth values at every point, the gridmarks would be easy for the brain to