A final source of evidence for the enactive approach comes from studies of visuomotor development in the absence of normal, reafferent visual stimulation. Held &amp; Hein 1963 performed an experiment in which pairs of kittens were harnessed to a carousel in a small, cylindrical chamber. One of the kittens was able to engage in free circumambulation while wearing a harness. The other kitten was suspended in the air in a metal gondola whose motions were driven by the first harnessed kitten. When the first kitten walked, both kittens moved and received identical visual stimulation. However, only the first kitten received reafferent visual feedback as the result of self-movement. Held and Hein reported that only mobile kittens developed normal depth perception—as evidenced by their unwillingness to step over the edge of a visual cliff, blinking reactions to looming objects, and visually guided paw placing responses. Noë (2004) argues that this experiment supports the enactive approach: in order to develop normal visual depth perception it is necessary to learn how motor outputs lead to changes to visual inputs.