A major tenet of the neurosciences has been that all neurons (nerve cells) in the brains of vertebrate animals are formed early in development. An adult vertebrate, it was believed, must make do with (make do with: v.设法应付) a fixed number of neurons: those lost through disease or injury are not replaced, and adult learning takes place not through generation of new cells but through modification of connections among existing ones.
However, new evidence for neurogenesis (the birth of new neurons) has come from the study of canary song. Young canaries and other songbirds learn to sing much as humans learn to speak, by imitating models provided by their elders. Several weeks after birth, a young bird produces its first rudimentary attempts at singing; over the next few months the song becomes more structured and stable, reaching a fully developed state by the time the bird approaches its first breeding season (breeding season: 生育期). But this repertoire of song is not permanently learned. After each breeding season, during late summer and fall, the bird loses mastery of its developed “vocabulary,” and its song becomes as unstable as that of a juvenile bird. During the following winter and spring, however, the canary acquires new songs, and by the next breeding season it has developed an entirely new repertoire.
Recent neurological research into this learning and relearning process has shown that the two most important regions of the canary’s brain related to the learning of songs actually vary in size at different times of the year. In the spring, when the bird’s song is highly developed and uniform, the regions are roughly twice as large as they are in the fall. Further experiments tracing individual nerve cells within these regions have shown that the number of neurons drops by about 38 percent after the breeding season, but by the following breeding season, new ones have been generated to replace them. A possible explanation for this continual replacement of nerve cells may have to do with the canary’s relatively long life span and the requirements of flight. Its brain would have to be substantially larger and heavier than might be feasible for flying if it had to carry all the brain cells needed to process and retain all the information gathered over a lifetime.
Although the idea of neurogenesis in the adult mammalian brain is still not generally accepted, these findings might help uncover a mechanism that would enable the human brain to repair itself through neurogenesis. Whether such replacement of neurons would disrupt complex learning processes or long-term memory is not known, but songbird research challenges (to demand as due or deserved: REQUIRE) scientists to identify the genes or hormones that orchestrate (特地安排) neurogenesis in the young human brain and to learn how to activate them in the adult brain.
1. Which one of the following best expresses the main idea of the passage?
(A) New evidence of neurogenesis in canaries challenges an established neurological theory concerning brain cells in vertebrates and suggests the possibility that human brains may repair themselves.
(B) The brains of canaries differ from the brains of other vertebrate animals in that the brains of adult canaries are able to generate neurons.
(C) Recent studies of neurogenesis in canaries, building on established theories of vertebrate neurology, provide important clues as to why researchers are not likely to discover neurogenesis in adult humans.
(D) Recent research into neurogenesis in canaries refutes a long-held belief about the limited supply of brain cells and provides new information about neurogenesis in the adult human brain.（A）
(E) New information about neurogenesis in canaries challenges older hypotheses and clarifies the importance of the yearly cycle in learning processes and neurological replacement among vertebrates
2.According to the passage, which one of the following is true of the typical adult canary during the late summer and fall?
(A) The canary’s song repertoire takes on a fully structured and stable quality.
(B) A process of neurogenesis replaces the song-learning neurons that were lost during the preceding months.
(C) The canary begins to learn an entirely new repertoire of songs based on the models of other canaries.
(D) The regions in the canary’s brain that are central to the learning of song decrease in size.（D）
(E) The canary performs slightly modified versions of the songs it learned during the preceding breeding season
3.Information in the passage suggests that the author would most likely regard which one of the following as LEAST important in future research on neurogenesis in humans?
(A) research on possible similarities between the neurological structures of humans and canaries
(B) studies that compare the ratio of brain weight to body weight in canaries to that in humans
(C) neurological research on the genes or hormones that activate neurogenesis in the brain of human infants
(D) studies about the ways in which long-term memory functions in the human brain（B）
(E) research concerning the processes by which humans learn complicated tasks
4. Which one of the following, if true, would most seriously undermine the explanation proposed by the author in the third paragraph?
(A) A number of songbird species related to the canary have a shorter life span than the canary and do not experience neurogenesis.
(B) The brain size of several types of airborne birds with life spans similar to those of canaries has been shown to vary according to a two-year cycle of neurogenesis.
(C) Several species of airborne birds similar to canaries in size are known to have brains that are substantially heavier than the canary’s brain.
(D) Individual canaries that have larger-than-average repertoires of songs tend to have better developed muscles for flying.（C）
(E) Individual canaries with smaller and lighter brains than the average tend to retain a smaller-than-average repertoire of songs.
5. The use of the word “vocabulary” (line 23) serves primarily to
(A) demonstrate the presence of a rudimentary grammatical structure in canary song
(B) point out a similarity between the patterned groupings of sounds in a canary’s song and the syllabic structures of words
(C) stress the stability and uniformity of canary’s song throughout its lifetime
(D) suggest a similarity between the possession of a repertoire of words among humans and a repertoire of songs among canaries（D）
(E) imply that the complexity of the canary’s song repertoire is equal to that of human languag
6. According to the passage, which one of the following factors may help account for the occurrence of neuro genesis in canaries?
(A) the life span of the average canary
(B) the process by which canaries learn songs
(C) the frequency of canary breeding seasons
(D) the number of regions in the canary brain related to song learning（A）
(E) the amount of time an average canary needs to learn a repertoire of songs
7. Which one of the following best describes the organization of the third paragraph?
(A) A theory is presented, analyzed, and modified, and a justification for the modification is offer.
(B) Research results are advanced and reconciled with results from other studies, and a shared principle is described.
(C) Research results are presented, further details are provided, and a hypothesis is offered to explain the results.
(D) Research results are reported, their implications are explained, and an application to a related field is proposed.（C）
(E) Research results are reported, their significance is clarified, and they are reconciled with previously established neurological tenets.
8.It can be inferred from the passage that the author would most likely describe the current understanding of neurogenesis as