Sentence:  Thus, a tap that precedes the final one by a tenth of a second will likely appear to be midway between the contactors, whereas a tap preceding the final one by only 20 ms will appear to be virtually superimposed on it. Because the overall experience is of a stimulus jumping from place to place, Geldard called the phenomenon <WSD>"saltation</WSD>"....First, <WSD>saltation</WSD> can occur only over a limited distance: A tap on the shoulder will not be drawn toward a later one delivered to the foot. 
Meaning of the word marked with <WSD> tags:  Any abrupt transition.