Perceiving the Lengths of Rods That are Held But Not Wielded

The ambiguity inherent in the act of experimental abstraction is discussed particularly with respect to experiments that seem to prove the superfluity of active exploration in perception. For example, in the case of haptic perception of the extent of hand-held rods, the variable of the second moment of mass distribution-the moment of inertia-has been shown to predict perceived length; this variable is inherently active, identifying a system's resistance to rotational acceleration. Other sources have reported that the length of an unseen rod could be perceived even when the rods were not rotated (rendering second moment theoretically inaccessible). The first experiment of this article confirms this ability in the extreme case in which observers are instructed not to move the rod at all. Four more experiments are reported in which the relative roles of the second moment and of the first moment-the other plausible mechanical candidate-are evaluated. The first moment was a better predictor of perceived length in cases in which exploration was restricted, and the second moment was a better predictor in conditions in which exploration was not restricted, although each played some role in all conditions. These results are discussed in terms of the possibility of more than one kind of information specifying the same property.