Reference the following passage for the practice questions below:
Although the brain comprises only 2 percent of the human body’s average weight, the billions of neurons and trillions of synaptic connections that are the human brain constitute a truly impressive organ. In terms of what it can do, the human brain is in some ways unable to match the brain functioning of “lower” animals; in other ways, its capabilities are quite unrivaled. Salmon, caribou, and migrating birds, for example, have navigational abilities unparalleled in our own species, and even dogs and cats have senses of hearing and smell known only, in human form, to comic book superheroes. Yet, no other animal on the planet can communicate, solve problems, or think abstractly about itself and the future as we do. While these relative strengths and weaknesses can be attributed to the unique and sophisticated structure of the human brain, neuroscientists also have traced these characteristics to the human brain’s remarkable flexibility, or what researchers call plasticity.
Encased in a hard, protective skull that by the age of two is already 80 percent of its eventual adult size, the human brain has little room for size expansion even while the rest of the body, especially during adolescence, is experiencing significant changes in physical appearance. Th e first few years of a child’s life are a time of rapid brain growth. At birth, each neuron in the cerebral cortex has an estimated 2,500 synapses; by age three, this number blossoms to 15,000 synapses per neuron. Th e average adult, however, has about half that number of synapses. Nevertheless, the human brain’s plasticity allows for marked capacity changes because of usage, practice, and experience throughout one’s life. This idea that the human brain continues to develop and, some might say, improve over the course of one’s life is a relatively new concept. Neuroscientists, even after brain size was no longer considered a direct determiner of brain capacity, once believed that the basic structure and abilities of the adult brain are developed early in life and not subject to change. Although psychologist William James suggested as early as 1890 that “organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity,” this idea went largely ignored for many years. Then, several provocative experiments dramatically complicated conventional thinking about the human brain.
In the 1920s, researcher Karl Lashley provided evidence of changes in the neural pathways of rhesus monkeys. By the 1960s, researchers began to explore cases in which older adults who had suffered massive strokes were able to regain functioning, demonstrating that the brain was much more malleable than previously believed. Modern researchers have also found evidence that the brain is able to rewire itself following damage. One of these experiments, for example, examined the various effects an enriched environment, in this case an “amusement park” for rats, could have on brain development. Researchers kept one group of rats in an empty cage, devoid of any stimulus, while another group lived in a cage filled with ladders, platforms, boxes, and other toys. Over the course of the experiment, researchers used magnetic resonance imaging technology to observe the brain development of the two groups. Those rats that lived in the enriched environment full of stimuli developed heavier, thicker brains with more neurons and synaptic connections—the cellular activity by which the brain functions—than those that were deprived. Such results were then found to be even more noticeable in humans. Whereas it was once believed that the brain’s physical structure was permanent, this experiment and other contemporary findings show that the brain continues to create new neural pathways and alter existing ones in order to adapt to new experiences, learn new information, and create new memories. As we gain new experiences, some connections are strengthened while others are eliminated in a process called “synaptic pruning.” Frequently used neurons develop stronger connections; those rarely (or never) used eventually die. By developing new connections and pruning away weak ones, the brain is able to adapt to the changing environment, thus confirming an essential point: one’s life experiences and environment not only mold the brain’s particular architecture but can also continue tp expand its capacity to function.
C: You must have a firm grasp of the overall passage to understand the author’s central idea. If you’re uncertain, look at the first and last paragraphs. Predict that the author is emphasizing the brain’s plasticity, or flexibility. Choice (C) is correct.
A: When presented with a Detail question, use your Passage Map to locate the section that describes the issue raised by the question stem. Paragraph 2 discusses the brain’s growth and development. (“The first few . . . synapses“), the author states that three-year-olds have “15,000 synapses per neuron” and that adults have about half of that number. Use this prediction to determine that choice (A) is correct.
B: The author is describing the brain as “unrivaled” and “unique” in the first paragraph. Predict that “sophisticated” most nearly means complicated or elaborate. Choice (B) is correct.
A: In this section of the passage, the author explains the significant capacity changes that the human brain can experience, even well into adulthood. Predict that “marked” most nearly means significant or extraordinary. Choice (A) is correct.
B: Read around the cited line and word for context to determine the correct answer. The author describes the rats’ environment as being “enriched,” or enhanced, by more stimuli than the environment of the control group. Predict that “enriched” most nearly means enhanced. Choice (B) is correct.