The secret of the Yawn
When a scientist began to study yawning in the 1980s, it was difficult to convince some of his research students of the merits of “yawning science.” Although it may appear quirky (诡异) his decision to study yawning was a logical extension to human beings of my research in developmental neuroscience, reported in such papers as “Wing-flapping during Development and Evolution.” As a neurobehavioral problem, there is not much difference between the wing-flapping of birds and the face- and body-flapping of human yawners.
Yawning is an ancient, primitive act. Humans do it even before they are born, opening wide in the womb (子宫) . Some snakes unhinge their jaws to do it. One species of penguins yawns as part of mating. Only now are researchers beginning to understand why we yawn, when we yawn and why we yawn back. A professor of cognitive neuroscience at Drexel University in Philadelphia, Steven Platek, studies the act of contagious yawning, something done only by people and other primates.
In his first experiment, he used a psychological test to rank people on their empathic (感情嵌入的) feelings. He found that participants who did not score high on compassion did not yawn back. “We literally had people saying, ‘Why am I looking at people yawning?” Professor Platek said. “It just had no effect.”
For his second experiment, he put 10 students in an magnetic resonance imaging machine as they watched video tapes of people yawning. When the students watched the videos, the part of the brain which reacted was the part scientists believe controls empathy – the posterior cingulate (皮层的) , in the brain’s middle rear.” I don’t know if it’s necessarily that nice people yawn more, but 1 think it’s a good indicator of a state of mind,” said Professor Platek. “It’s also a good indicator if you’re empathizing with me and paying attention.”
His third experiment is studying yawning in those with brain disorders, such as autism and schizophrenia, in which victims have difficulty connecting emotionally with others. A psychology professor at the University of Maryland, Robert Provine, is one of the few other researchers into yawning. He found the basic yawn lasts about six seconds and they come in bouts with an interval of about 68 seconds. Men and women yawn or half-yawn equally often, but men are significantly less likely to cover their mouths which may indicate complex distinction in genders.” A watched yawner never yawns，,” Professor Provine said. However, the physical root of yawning remains a mystery. Some researchers say it’s coordinated within the hypothalamus (下丘脑) of the brain, the area that also controls breathing.
Yawning and stretching also share properties and may be performed together as parts of a global motor complex. But they do not always co-occur—people usually yawn when we stretch, but we don’t always stretch when we yawn, especially before bedtime. Studies by J. I. P , G. H. A. Visser and H. F. Prechtl in the early 1980s, charting movement in the developing fetus using ultrasound, observed not just yawning but a link between yawning and stretching as early as the end of the first prenatal trimester (预产期）.
The most extraordinary demonstration of the yawn-stretch linkage occurs in many people paralyzed on one side of their body because of brain damage caused by a stroke. The prominent British neurologist Sir Francis Walshe noted in 1923 that when these hemiplegics yawn, they are startled and mystified to observe that their otherwise paralyzed arm rises and flexes automatically in what neurologists term an “associated response.” Yawning apparently activates. undamaged, unconsciously controlled connections between the brain and the cord motor system innervating the paralyzed (瘫痪的) limb. It is not known whether the associated response is a positive prognosis for recovery, nor whether yawning is therapeutic for reinnervation(再生) or prevention of muscular atrophy.
Clinical neurology offers other surprises. Some patients with “locked-in” syndrome, who are almost totally deprived of the ability to move voluntarily, can yawn normally. The neural circuits for spontaneous yawning must exist in the brain stem near other respiratory and vasomotor centers, because yawning is performed by anencephalic(无脑畸形) who possess only the medulla oblongata (脊髓延髓). The multiplicity of stimuli of contagious yawning, by contrast, implicates many higher brain regions.