Ten pairs of kittens were kept in darkness from birth, but after a few weeks Held and Hein began exposing them two at a time to light during three-hour sessions on a rotating carousel. Both kittens were connected to the same carousel, yet there was a key difference: one could otherwise move freely, while the other’s movements were restricted by a yoked gondola, or basket, inside which it had been placed and from which it could view the world. Whenever Kitten A, the mobile one, made a motion, this caused Kitten P, the immobile one, to move in response. As the sessions progressed, this setup
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