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

The Mind's Eye 261of forms with blanks to be filled in. The form for a piece of surface hasblanks for depth, for slant (how much the surface leans backward or forward),for tilt (how much it lists left or right), and for color, plus a labelfor which surface it is seen as belonging to. The form for a piece of edgehas boxes to be checked, indicating whether it is at the boundary of anobject, a groove, or a ridge, plus a dial for its orientation, which alsoshows (in the case of an object boundary) which side belongs to the surfacethat "owns" the boundary and which side is merely the backdrop. Ofcourse, we won't literally find bureaucratic forms in the head. The diagramis a composite that depicts the kinds of information in the 2V2-Dsketch. The brain presumably uses clusters of neurons and their activitiesto hold the information, and they may be distributed across differentpatches of cortex as a collection of maps that are accessed in register.Why do we see in two and a half dimensions? Why not a model in thehead? The costs and benefits of storage give part of the answer. Any computeruser knows that graphics files are voracious consumers of storagespace. Rather than agglomerating the incoming gigabytes into a compositemodel, which would be obsolete as soon as anything moved, the brainlets the world itself store the information that falls outside a glance. Ourheads crane, our eyes flit, and a new, up-to-date sketch is loaded in. As forthe second-class status of the third dimension, it is almost inevitable.Unlike the other two dimensions, which announce themselves in the rodsand cones that are currently active, depth must be painstakingly wrungout of the data. The stereo, contour, shading, and motion experts thatwork on computing depth are equipped to send along information aboutdistance, slant, tilt, and occlusion relative to the viewer, not 3-D coordinatesin the world. The best they can do is to pool their efforts to give us atwo-and-a-half-dimensional acquaintance with the surfaces in front ofour eyes. It's up to the rest of the brain to figure out how to use it.FRAMES OF REFERENCEThe 272-D sketch is the masterwork of the ingeniously designed, harmoniouslyrunning machinery of the visual system. It has only one problem.As delivered, it is useless.Information in the 2V2-D array is specified in a retinal frame of reference,a coordinate system centered on the viewer. If a particular cell