Whether the eyes are "the windows of the soul" is debatable; that they are intensely important in interpersonal communication is a fact. During the first two months of a baby’’s life, the stimulus that produces a smile is a pair of eyes. The eyes need not be real: a mask with two dots will produce a smile. Significantly, a real human face with eyes covered will not motivate a smile, nor will the sight of only one eye when the face is presented in profile. This attraction to eyes as opposed to the nose or mouth continues as the baby matures. In one study, when American four-year-olds were asked to draw people, 75 percent of them drew people with mouths, but 99 percent of them drew people with eyes. In Japan, however, where babies are carried on their mother’’s back, infants do not acquire as much attachment to eyes as they do in other cultures. As a result, Japanese s make little use of the face either to encode (把......编码) or decode (理解) meaning. In fact, Argyle reveals that the "proper place to focus one’’s gaze during a conversation in Japan is on the neck of one’’s conversation partner." The role of eye contact in a conversational exchange between two Americans is well defined: speakers make contact with the eyes of their listener for about one second, then glance away as they talk; in a few moments they reestablish eye contact with the listener or reassure themselves that their audience is still attentive, then shift their gaze away once more. Listeners, meanwhile, keep their eyes on the face of the speaker, allowing themselves to glance away only briefly. It is important that they be looking at the speaker at the precise moment when the speaker re-establishes eye contact: if they are not looking, the speaker assumes that they are disinterested and either will pause until eye contact is resumed or will terminate the conversation. Just how critical this eye maneuvering is to the maintenance of conversational flow becomes evident when two speakers are wearing dark glasses: there maybe a sort of traffic jam of words caused by interruption, false starts, and unpredictable pauses. According to the passage, a conversation between two Americans may break down due to______.