We are here concerned with the horse's head movements upward, downward, to the right and to the left, and also with nodding and shaking of the head to signify "yes" and "no". We soon discovered that these experiments, also, were successful without an oral statement of the problem,--in other words, the auditory stimulus was quite superfluous. The tests with the blinders showed that Hans was lost as soon as his questioner was out of his view, but responded adequately the moment the questioner was in sight. Hans, therefore, had established no idea of any sort in connection with the terms "up", "down", etc., but in these cases, likewise, he reacted in response to certain visual stimuli. The nature of these stimuli I discovered at first in my observations of Mr. von Osten and also of myself, when working with the horse. Above all things it was necessary that the questioner, during these tests, should stand perfectly erect. If he stooped ever so slightly, the test was unsuccessful. If he carefully refrained from any movement whatsoever, and looking straight before him asked the horse, "Which direction is right?" or "Which way is upward?", Hans would execute all sorts of head movements without rhyme or reason. It was evident that he noted that a head movement of some kind was expected of him, but did not know the particular one that was wanted. But if the questioner now raised his head, Hans would begin to nod and would continue doing so until the questioner lowered his head. This reaction was interpreted as signifying "yes". Mr. von Osten had always asked Hans before each of the more difficult tests whether he had comprehended the meaning of the problem, and was reassured only upon seeing the horse's affirmative response. But contrary to Mr. von Osten's expectation, Hans also responded in this manner after a pair of ear-caps had been drawn over his ears. In the case of the tests described at the beginning of the chapter, in which the method was that of "procedure without knowledge", Mr. von Osten had always insisted that we await Hans's nod of comprehension before proceeding. We complied; Hans nodded and--regularly disgraced himself!