Related to the startle reflex, and to the auditory system’s exquisite sensitivity to change, is the habituation circuit. If your refrigerator has a hum, you get so used to it that you no longer notice it—that is habituation. A rat sleeping in his hole in the ground hears a loud noise above. This could be the footstep of a predator, and he should rightly startle. But it could also be the sound of a branch blowing in the wind, hitting the ground above him more or less rhythmically. If, after one or two dozen taps of the branch against the roof of his house, he finds he is in no danger, he should
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