Passage IV

You cannot see what is going on inside your new- born’s brain. You cannot see the electrical activity as her eyes lock onto yours and, almost instantaneously, a neuron in her retina makes a connection to one in her 5brain’s visual cortex that will last all her life. The image of your face has become an enduring memory in her mind. And you cannot see the explosive release of a neurotransmitter—brain chemical—as a neuron from your baby’s ear, carrying the electrically encoded 10sound of “ma,” connects to a neuron in her auditory cortex. “Ma” has now commandeered a cluster of cells in the infant’s brain that will, as long as the child lives, respond to no other sound.

You cannot see any of this. But Dr. Harry Chugani 15can come close. With positron-emission tomography PET), Chugani, a pediatric neurobiologist, watches the regions of a baby’s brain turn on, one after another, like city neighborhoods having their electricity restored after a blackout. He can measure activity in the primi- 20tive brain stem and sensory cortex from the moment the baby is born. He can observe the visual cortex burn with activity in the second and third months of life. He can see the frontal cortex light up at 6 to 8 months. He can see, in other words, that the brain of a baby is still 25forming long after the child has left the womb—not merely growing bigger, but forming the microscopic connections responsible for feeling, learning and remembering.

Scientists are just now realizing how experiences 30after birth, rather than something innate, determine the actual wiring of the human brain. Only 15 years ago neuroscientists assumed that by the time babies are born, the structure of their brains had been genetically determined. But by 1996, researchers knew that was 35wrong. Instead, early-childhood experiences exert a dramatic and precise impact, physically determining how the intricate neural circuits of the brain are wired. Since then they have been learning how those experi- ences shape the brain’s circuits.

40At birth, the brain’s 100 billion or so neurons form more than 50 trillion connections (synapses). The genes the baby carries have already determined his brain’s basic wiring. They have formed the connections in the brain stem that will make the heart beat and the lungs 45respire. But that’s all. Of a human’s 80,000 different genes, fully half are believed to be involved in forming and running the central nervous system. Yet even that doesn’t come close to what the brain needs. In the first months of life, the number of synapses will increase 20- 50fold—to more than 1,000 trillion. There simply are not enough genes in the human species to specify so many connections.

That leaves experience—all the signals that a baby receives from the world. Experience seems to exert its 55effects by strengthening synapses. Just as a memory will fade if it is not accessed from time to time, so synapses that are not used will also wither away in a process called pruning. The way to reinforce these wispy connections has come to be known as stimula- 60tion. Contrary to the claims of entrepreneurs preying on the anxieties of new parents, stimulation does not mean subjecting a toddler to flashcards. Rather, it is some- thing much simpler—sorting socks by color or listening to the soothing cadences of a fairy tale. In the most 65extensive study yet of what makes a difference, Craig Ramey of the University of Alabama found that it was blocks, beads, peekaboo and other old-fashioned mea- sures that enhance cognitive, motor and language development—and, absent traumas, enhance them per- 70manently.

The formation of synapses (synaptogenesis) and their pruning occurs at different times in different parts of the brain. The sequence seems to coincide with the emergence of various skills. Synaptogenesis begins in 75the motor cortex at about 2 months. Around then, infants lose their “startle” and “rooting” reflexes and begin to master purposeful movements. At 3 months, synapse formation in the visual cortex peaks; the brain is fine-tuning connections allowing the eyes to focus on 80an object. At 8 or 9 months the hippocampus, which indexes and files memories, becomes fully functional; only now can babies form explicit memories of, say, how to move a mobile. In the second half of the first year, finds Chugani, the prefrontal cortex, the seat of 85forethought and logic, forms synapses at such a rate that it consumes twice as much energy as an adult brain. That furious pace continues for the child’s first decade of life.

• The main point of this passage is to:
  • A. illustrate the importance of genetics in the formation of a baby’s brain.
  • B. illustrate the importance of stimulation and experience in the formation of a baby’s brain.
  • C. indicate the great need for conducting further research on babies’ brains.
  • D. compare the latest research on babies’ brains with similar research conducted fifteen years ago.
• The main point made in the second, third, and fourth paragraphs (lines 14–52) is that the structure of a baby’s brain:
  • F. is genetically determined before the child is born.
  • G. can be seen through positron-emission tomography.
  • H. can be altered through a process known as pruning.
  • J. is still developing after the child is born.
• According to the passage, one thing PET allows neurobiologists to do is:
  • A. observe activity in the frontal cortex of a baby’s brain.
  • B. determine the number of genes involved in the formation of a baby’s brain.
  • C. control the release of neurotransmitters in a baby’s auditory cortex.
  • D. restore microscopic connections in a baby’s brain.
• When she compares a baby’s brain to city neighborhoods, the author is most nearly illustrating her point that:
  • F. neurotransmitters are actually brain chemicals.
  • G. regions of the brain are awakened through experience.
  • H. the visual cortex allows a baby to recognize specific images.
  • J. a baby’s brain has about 1,000 trillion synapses.
• Which of the following would the author of the passage be LEAST likely to recommend as a way to strengthen the synapses of a baby’s brain?
  • A. Reading to a baby
  • B. Playing peekaboo with a baby
  • C. Teaching a baby with flashcards
  • D. Showing a baby how to distinguish red socks from blue blocks
• The last paragraph suggests that the formation of synapses occurs most rapidly:
  • F. during the first two months of a child’s life.
  • G. during the first nine months of a child’s life.
  • H. from the time a child is about six months old until that child is about ten years old.
  • J. from the time a child is about one year old until that child is well into adolescence.
• As it is used in line 30, the phrase something innate most nearly means:
  • A. a memory.
  • B. learned behavior.
  • C. physical immaturity.
  • D. an inherited trait.
• The fifth paragraph (lines 53–70) suggests that one of the main causes of pruning is:
  • F. a lack of stimulation.
  • G. an insufficient number of genes.
  • H. the use of flashcards.
  • J. the strengthening of synapses.
• When the author refers to “entrepreneurs preying on the anxieties of new parents” (lines 60–61), she is most likely suggesting that new parents should:
  • A. give their babies products such as flashcards only if they have examined these products carefully.
  • B. not be deceived by advertising that claims certain products will increase a baby’s intelligence.
  • C. not worry if their babies’ development is slightly behind that suggested by neurobiologists.
  • D. take their pediatrician’s advice before they listen to the advice given by other family members.
• The passage states that, in terms of development, the average baby should be able to:
  • F. focus his or her eyes on an object at two months of age.
  • G. develop a “startle” reflex at about two months of age.
  • H. make logical connections between ideas at about four months of age.
  • J. form explicit memories at about nine months of age.