单项选择题

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A blind baby is doubly handicapped. Not only is it unable to see, but because it cannot receive the visual stimulus from its environment that a sighted child does, it is likely to be slow in intellectual development. Now the ten-month old son of Dr. and Mrs Dennis Daughters is the subject of an unusual psychological experiment designed to prevent a lag in the learning process. With the aid of a sonar-type electronic that he wears on his head, infant Dennis is learning to identify the people and objects in the world around him by means of echoes.
Dennis and a twin brother, Daniel, were born last September almost three months too early. Daniel died after five days, and Dennis developed retrolental fibroplasias, an eye disorder usually caused by overexposure to oxygen in an incubator. He went blind, but through a paediatrician at the premature unit where he was treated, the Daughterses were contacted by Dr. Tom Bower, a psychologist from the University of Edinburgh then serving a fellowship at the Stanford University Centre for Advanced Study in the Behavioural Sciences. Bower wanted to see how a blind infant might respond if given an echo-sounding device to help him cope with his surroundings and the Daughterses agreed to help.
By the time the child was six weeks old, his parents noticed that he continuously uttered sharp clicking sounds with his tongue. Bower explained that blind people often use echoes to orient themselves, and that the clicking sounds were the boy’s way of creating echoes. This, Bower believd, made the child an ideal subject for testing with an electronic echosounding device.
Signals: The device used in the study is a refinement of the "Sonicguide", an instrument produced by Telesensory Systems, Inc., of Palo Alto, Calif., and used by blind adults in addition to sane or guide dog. As adapted for Dennis, it consists of a battery-powered system about the size of a half dollar that is on a headgear. A transmitter emits an ultrasonic pulse that creates an 80degree cone of sound at 6 feet. Echoes from objects within the cone are perceived as sounds that vary in pitch and volume with the size and distance of the object.
The closer an object is, the lower the pitch, and the larger the object, the louder the signal. Hard surfaces produce a sharp ping, while soft ones send back signals with a slightly fuzzy quality. An object slightly to the right of Denny’s sends back a louder sound to his right ear than to the left. Thus, by simply moving his head right and left and up and down, he can not only locate an object but also get some notion of its shape and size, thanks to the varying qualities of sounds reaching his ears as the cone of ultrasound passes its edges. Dennis likes to use the device to play a kind of peek-a-boo with his mother. Standing on her knee and facing her directly, he receives a strong signal in both ears. By turning his head away, he makes her seem to disappear. "From the first time he wore it," says Mrs. Daughters, "it was like a light going on in his head."
The boy also learned to identify many objects, including his favourite toy, a rubber caterpillar with six antenna-like projections coming out of its body. And at six-and-a-half months, when a teething biscuit was held in front of Dennis, the child immediately grabbed it with both hands and put it into his mouth.
So far, the study has shown that a normal blind baby can employ echoes as well as, or even better than, an unsighted adult can. What remains to be determined is how well the device will help Dennis cope with his surroundings as he begins to walk and venture further into his environment. Meanwhile, Telesensory, Inc., is working on the development of a sonar device with somewhat the same sensitivity as Dennis’s for use by school-age children.
Bower believed Dennis would be the ideal subject for his experiment because ______.