According to conventionaltheory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallowbreathing that accompanies lack of sleep or boredom. Unfortunately, the few scientific investigations of yawning have failed to find any connection between how often someone yawns and how much sleep they have had or how tired they are. About the closest any research has come to supporting the tiredness theory is to confirm that adults yawn more often on weekdays than at weekends, and that school children yawn more frequently in their first year at primary school than they do in kindergarten.
Another flaw of the tiredness theory is that yawning does not raise alertness or physiologicalactivity, as the theory would predict. When researchers measured the heart rate, muscletension and skin conductance of people before, during and after yawning, they did detect some changes in skin conductance following yawning, indicating a slight increase in physiologicalactivity. However, similar changes occurred when the subjects were asked simply to open their mouths or to breathe deeply. Yawning did nothing special to their state of physiologicalactivity. Experiments have also cast serious doubt on the belief that yawning is triggered by a drop in blood oxygen or a rise in blood carbondioxide. Volunteers were told to think about yawning while they breathed either normal air, pureoxygen, or an air mixture with an above-normal level of carbondioxide. If the theory was correct, breathingair with extracarbondioxide should have triggered yawning, while breathingpureoxygen should have suppressed yawning. In fact, neither condition made any difference to the frequency of yawning, which remained constant at about 24 yawns per hour. Another experiment demonstrated that physicalexercise, which was sufficientlyvigorous to double the rate of breathing, had no effect on the frequency of yawning. Again the implication is that yawning has little or nothing to do with oxygen.
A completely different theory holds that yawning assists in the physicaldevelopment of the lungs early in life, but has no remaining biological function in adults. It has been suggested that yawning and hiccupping might serve to clear out the fetuses airways. The lungs of a fetussecrete a liquid that mixes with its mother's amniotic fluid. Babies with congenital blockages that prevent this fluid from escaping from their lungs are sometimes born with deformed lungs. It might be that yawning helps to clear out the lungs by periodically lowering the pressure in them. According to this theory, yawning in adults is just a developmental fossil with no biological function. But, while accepting that not everything in life can be explained by Darwinian evolution, there are sound reasons for being skeptical of theories like this one, which avoid the issue of what yawning does for adults. Yawning is distracting, consumes energy and takes time. It is almost certainly doing something significant in adults as well as in fetuses. What could it be?
The empiricalevidence, such as it is, suggests an altogether different function for yawning—namely, that yawning prepares us for a change in activitylevel. Support for this theory came from a study of yawning behavior in everyday life. Volunteers wore wrist-mounted devices that automatically recorded their physicalactivity for up to two weeks: the volunteers also recorded their yawns by pressing a button on the device each time they yawned. The data showed that yawning tended to occur about 15 minutes before a period of increased behavioralactivity. Yawning bore no relationship to sleep patterns, however. This accords with anecdotalevidence that people often yawn in situations where they are neither tired nor bored, but are preparing for impendingmental and physicalactivity. Such yawning is often referred to as "incongruous" because it seems out of place, at least on the tiredness view: soldiers yawning before combat, musicians yawning before performing, and athletes yawning before competing. Their yawning seems to have nothing to do with sleepiness or boredom—quite the reverse—but it does precede a change in activitylevel.
4.In the paragraph 2, why does the author note that there were physiological changes when subjects opened their mouths or breathed deeply?
C 修辞目的题,首先找到修辞点所在的第三句,本句只是在重复这个例子,没有观点性的东西,往前看。前句说测到了一些生理上的变化,还是在说例子,所以看中心句,也就是第一句,说打呵欠根本不会提高清醒程度,所以答案是 C。A 和 D 反了,B 说质疑实验的可靠性,作者不是质疑实验本身,而是质疑结论,并用实验支持这个质疑。