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opposite side of the body, the machine can be made to roll rapidly toward sources of light. Alternatively, reversing the wiring will produce a timid machine that seeks out dark corners. 4
    More sophisticated uses of paired sensors involve comparing the images that are presented to each sensor to arrive at an estimate of the location and distance of a target. When we look at an object, its image falls in slightly different locations in each of our two eyes, and our brain can compute the distance of the object based on such differences. When we listen to a sound, the differences in the qualities of sounds arriving at our two ears can be used in similar fashion to compute the location of the sound source. The power of two in this case means that animals possessing paired sensors do not need to engage in hit-and-miss games of blind man’s bluff in order to get close to the things they need. Instead, comparing the messages conveyed by each of the two sensors provides a rapid and accurate estimate of the location of a target. In simple machines built with light detectors and wheels, or in frogs and toads sitting stoically waiting for dinner to come within tongue’s reach, the use of paired sensors is a considerable advance over the simple taxic mechanisms of bacteria. In more sophisticated animals like us, many more layers of neural machinery are involved in regulating our movementswith respect to targets of interest. As preponderantly visual beasts, the story begins with our eyes.

    Spend a minute or two observing how your own eye movements contribute to your perception of the world. Find a point somewhere in the room and try hard to maintain your gaze on that location. While doing this, notice how much you can see of objects just outside your fixation point. If you hold your gaze steady, you’ll notice that your perception of the rest of your setting consists of nothing more than a few blobs of varying brightness. Notice how little can been seen clearly when the eyes are held in a stationary position. Visual details are available in a small region of space around your fixation point, but nowhere else. To build an integrated view of the layout of the space we occupy, we need to move our eyes ceaselessly.
    Working in the 1960s, when the technology for studying eye movements was primitive compared to the tools that are available to us today, Alfred Yarbus, a pioneer in the scientific study of eye movements, had participants in his experiments wear small mirrors that were attached to their eyeballs by means of small suction cups. (Yes, it was unpleasant. And, yes, Yarbus participated in his own experiments.) 5 In some experiments, participants were asked to examine paintings while Yarbus recorded the patterns of their eye movements. When the eye-movement recordings were superimposed on the paintings so that it was possible to see what the participants had been looking at, Yarbus discovered that eye movements were not scattered randomly across the paintings; nor did they seem to carry out any kind of systematic search (such as from top to bottom or from left to right, as one might imagine a machine would do). Instead, the eyes tended to seek out the parts of the picture that were most salient. For example, an inordinate amount ofattention was paid to the eyes of the human figures in a painting. Yarbus was able to show that the pattern of eye movements seen during the viewing of a painting depended on the context of the viewing. If he asked people to answer questions about what they were seeing, their eye movements would reflect the strategies that they were using to search for answers. Our eye movements are not driven by what is biggest, brightest, or flashiest in a visual scene. They reflect the purpose of our looking.
    Though Yarbus’s clever experiments stimulated legions of future researchers to use measurements of eye movements as a kind of window into our minds, he was limited by the crude technology of his day. Participants