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

Trie Mini's Eye | 235terns can sometimes be solved with the technique called constraint satisfactionthat we met in Chapter 2 when looking at Necker cubes andaccented speech. When the parts of a puzzle cannot be solved one at atime, the puzzle-solver can keep in mind several guesses for each one,compare the guesses for the different parts of the puzzle, and sec whichones are mutually consistent. A good analogy is working on a crosswordpuzzle with a pencil and an eraser. Often a clue for a horizontal word isso vague that several words can be penciled in, and a clue for a verticalword is so vague that several words can be penciled in. But if only one ofthe vertical guesses shares a letter with any of the horizontal guesses,that pair of words is kept and the others are erased. Imagine doing thatfor all the clues and squares at once and you have the idea of constraintsatisfaction. In the case of solving the correspondence problem in stereovision, the dots are the clues, the matchups and their depths are theguesses, and the three assumptions about the world are Jike the rulesthat say that every letter of even' word must sit in a box, every box musthave a letter in it, and all the sequences of letters must spell out words.Constraint satisfaction can sometimes be implemented in a constraintnetwork like the one I presented on page 107. Marr and the theoreticalneuroscientist Tomaso Poggio designed one for stereo vision. Theinput units stand for points, such as the black and white squares of arandom-dot stereogram. They feed into an array of units that representall of the M x 72 possible matchups of a point in the left eye with someother point in the right eye. When one of these units turns on, the networkis guessing that there is a splotch at a particular depth in the world(relative to where the eyes have converged). Here is a bird's-eye view ofone plane of the network, showing a fraction of the units.Array of processors, one for each matchupp.Processors along a 0—^0line of sight ^^\ O "0 D KI - t_L ivy x . .• \ ,• -, Neighboring processorsinhibit one another ,Q &. & O „, . „_„ J„,,U- -. .. .. y L . *• ., ^—at The same deptn~/y^ reinfoice one anotherDepth[npot from the left eyeInput from the right eye