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

Thinking Machines 141menter asks the person to find a letter that is both green and an O, andthe letter sits somewhere in a mixed sea of green X's and red O's, the personmust consciously search the display, letter by letter, checking eachone to see if it meets the two-part criterion. The task becomes like thechildren's comic strip Where's Waldo?, in which the hero in the red-andwhite-stripedjersey hides in a throng of people -wearing red, white, orstripes.What exactly is happening? Imagine that the visual field is sprinkledwith thousands of little processors, each of which detects a color or asimple shape like a curve, an angle, or a line whenever it appears at theprocessor's location. The output of one set of processors looks like this:red red red red green red red red, and so on. The output of another setlooks like this: straight straight straight curved straight straight straight,and so on. Superimposed on these processors is a layer of odd-man-outdetectors. Each stands astride a group of line or color detectors and"marks" any spot on the visual field that differs from its neighbors incolor or in contour. The green surrounded by reds acquires a little extraflag. All it takes to see a green among reds is to spot the flag, a taskwithin the powers of even the simplest demon. An O among X's can bedetected in the same way. But the thousands of processors tiled acrossthe field are too stupid to calculate conjunctions of features: a patch thatis green and curved, or red and straight. The conjunctions are detectedonly by a programmable logic machine that looks at one part of the visualfield at a time through a narrow, movable window, and passes on itsanswer to the rest of cognition.Why is visual computation divided into an unconscious parallel stageand a conscious serial stage? Conjunctions are combinatorial. It wouldbe impossible to sprinkle conjunction detectors at every location in thevisual field because there are too many kinds of conjunctions. There area million visual locations, so the number of processors needed would bea million multiplied by the number of logically possible conjunctions: thenumber of colors we can discriminate times the number of contourstimes the number of depths times the number of directions of motiontimes the number of velocities, and so on, an astronomical number. Parallel,unconscious computation stops after it labels each location with acolor, contour, depth, and motion; the combinations then have to becomputed, consciously, at one location at a time.The theory makes a surprising prediction. If the conscious processoris focused at one location, the features at other locations should float