A major tenet of the neurosciences has been that all neurons (nerve ce

Purpose and Main Idea: To suggest that current thinking about vertebrate (especially human) neurogenesis needs to be modified in light of research into canary brains. The author’s main idea is that the results of this research both defy the existing theory about vertebrate neurogenesis and open up the possibility that human brains may have the capability to regenerate themselves.

Paragraph Structure: ¶1 outlines the traditional theory that vertebrate neurogenesis basically doesn’t exist—that an adult vertebrate’s brain cannot create new nerve cells. The Contrast Keyword “However” (line 10) signals that the passage is about to provide some evidence to the contrary and, predictably, ¶s 2 and 3 go on to discuss in detail the behavioral and neurological evidence (from canaries) that throws the accuracy of the traditional theory into question. In ¶4, the author addresses the implications of these research findings for humans. The Contrast Keyword “Although” (line 48) serves to distinguish the general scientific community’s skeptical response from the author’s much more optimistic stance that human brains may be able to regenerate themselves.
The Big Picture:
A good grasp of a passage doesn’t mean assimilating all of its details (you can look them up if you have to). It means understanding what the author’s doing in the text—in this case, understanding that the author’s taking issue with the traditional theory of vertebrate neurogenesis (and its implications for the human brain) by providing evidence that contradicts it.
While the author’s specific main idea isn’t entirely clear until you’ve read through the whole passage, topic, scope, and purpose are all revealed early—by line 12.

1. (A) The only choice consistent with the author’s topic, scope, and purpose. (B) focuses on a detail. Moreover, the author suggests that there’s no difference between canary brains and the brains of other vertebrates in this respect—if he believed otherwise, why would he suggest that research findings about canary brains have implications for vertebrate neurogenesis in general? (C) The author asserts precisely the opposite. Canary research breaks with the traditional theory of vertebrate neurogenesis, and supplies clues as to how researchers may discover neurogenesis in the human brain. (D) and (E) have scope problems: To be an acceptable response, (D) should have limited itself to “ supply of vertebrate brain cells,” not the overly broad “supply of brain cells,” while (E) should have confined itself to “older hypotheses about vertebrate neurogenesis,” not the more encompassing “older hypotheses.” Both choices have other problems as well. (D) is too categorical; the author’s more cautious in his conclusions. As for (E), the author never states that vertebrates other than canaries are subject to a “yearly cycle.”

2. (D) Lines 21-24 say that the canary’s singing ability decreases during the late summer and fall. Lines 35-38 attribute this seasonal decrease to a 38% drop in the number of neurons in those parts of the brain that control singing. This is simply another way of saying that those regions of the brain decrease in size during the late summer and fall. (A) Au contraire: Lines 24-27 and 32-33 indicate that the canary’s song repertoire matures in the spring. (B) Au contraire aussi: The new song-learning neurons are generated during the winter and spring months. (C) distorts the text: Canaries do learn a new repertoire of songs (during the winter and spring), but there’s no indication that these songs are based on the songs of fellow canaries. (E) Canaries largely lose their ability to sing during the late summer and fall, after which they learn “entirely new” (line 27) songs for the next breeding season.

3. (B) The ratio of brain weight to body weight is mentioned as a possible explanation for neurogenesis in canaries only. This ratio has no direct connection per se to the larger issue of general vertebrate neurogenesis, and certainly not to human neurogenesis in particular. (A) The author’s entire argument rests on the supposition that humans, like canaries, may have the capability to generate new nerve cells; thus, it’s safe to infer that he’d consider research on neurological similarities to be important. (C) The author alludes to the importance of studying infant neurogenesis in lines 54-58. (D) The importance of understanding how long-term memory works in order to determine the possible effects of neurogenesis on long-term memory is suggested in lines 52-54. (E) Similarly, the author suggests in lines 52-54 that it’s important to understand how complex learning takes place in order to figure out the possible effects neurogenesis might have on complex learning.

4. (C) If it were true that birds similar to the canary have bigger brains, the author’s explanation for canary neurogenesis would be placed in jeopardy. In ¶3, after all, he argues that canary neurogenesis occurs because the canary needs to possess a lot of information in order to sing, yet has a small brain adapted for flight. Hence its brain, with its limited storage capacity, has to generate new nerve cells every year in order for it to relearn how to sing. (A) is consistent with the author’s explanation. He suggests that canary neurogenesis is spurred in part by the canary’s long lifetime. (B) The author’s basic explanation, which rests on the link between limited brain capacity and neurogenesis cycles, isn’t fundamentally threatened by cyclic differences from species to species. (D) has no impact on the author’s explanation, as he draws no link between the ability to sing and the ability to fly. (E) is consistent with the author’s explanation, which suggests that singing ability is directly related to brain size.

5. (D) Lines 12-13 draw a comparison between the way canaries learn to sing and the way humans learn to speak. Thus, the word “vocabulary” is meant to evoke a sense of this similarity by applying a concept taken from human speech to canary song. (A) echoes the substance of the detail rather than addressing why the author included it in the text. (B) and (E) go way too far. Indeed, the author never even discusses “patterned groupings of sounds” in canary songs or “the syllabic structures of words” (B). Nor does the text compare the level of complexity of canary song and human speech (E). (C) goes against the text, which reveals that canary songs are anything but stable and uniform over the course of the bird’s lifetime. Lines 19-27, in fact, stress just the opposite of what (C) says.

6. (A) Lines 40-42 explicitly state that a long life span may help to account for canary neurogenesis. (B)-(E) mention subjects that are connected in some way to canary neurogenesis, but none of them is ever described as a possible cause of same.

7. (C) Beginning in line 10, the author discusses “new evidence for neurogenesis.” Furthermore, this new evidence “might help uncover a mechanism” (line 50) to promote human neurogenesis, despite the fact that “neurogenesis in the adult mammalian brain is still not generally accepted” (lines 48-49). Based on these sentiments, it’s clear that the author believes that the “current understanding of neurogenesis” is “incomplete.” (A) and (B) express sentiments that are the opposite of the author’s. According to him, the traditional view of neurogenesis is neither comprehensive (A), nor forward-thinking (B). (D) and (E), on the other hand, take the author’s view to an unwarranted extreme. “Incomplete” is not the same thing as “antiquated” (D) or “incorrect” (E).