TOEFL Reading Practice #033 — The Neuron Verdict That Keeps Getting Overturned

The Neuron Verdict That Keeps Getting Overturned

¶1 For most of the twentieth century, neuroscience treated one claim as settled fact. A person is born with all the neurons, or nerve cells, that the brain will ever have, and none are added after childhood. That certainty did not survive the century intact. A finding that contradicted the rule surfaced early and was largely dismissed. Decades later, a different technique appeared to confirm that adult brains do generate new neurons after all, and the result was reported as having settled the question for good. Then, within months of each other, two more studies using comparable methods reached opposite conclusions. What began as a fixed rule about the brain has become an argument that keeps reopening every time the tools for detecting new neurons improve.

¶2 The rule traced back to Santiago Ramon y Cajal, a Spanish anatomist who studied the nervous system in the early twentieth century. Cajal described adult neurons as fixed in number, unable to divide again once the brain finished developing. His view became entrenched as the standard teaching in neuroscience for decades. In 1965, the biologist Joseph Altman challenged it directly, presenting evidence for neurogenesis, the formation of new neurons, in the brains of adult rats. Altman worked with a radioactive marker that only actively dividing cells would absorb. He injected adult rats with it and later examined thin slices of their brain tissue under a microscope. He found labeled, newly divided cells in the hippocampus, a curved structure inside the brain that is central to forming memories. The result implied that adult rat brains kept making new neurons throughout life. Most neuroscientists at the time dismissed the finding and continued teaching the older rule.

¶3 Altman's technique could not simply be repeated in people. Injecting a radioactive tracer into a healthy person for the sole purpose of studying brain tissue was not something any researcher could ethically justify. Through the 1970s and 1980s, similar labeled cells kept turning up in the brains of other adult animals, and confidence in the old rule gradually weakened. None of that evidence, however, answered the specific question of whether an adult human brain did the same thing. The dogma held in human medicine even as it eroded elsewhere, because no comparable marker could be introduced into a living person's brain.

¶4 A solution eventually came from an unrelated medical practice. Physicians treating terminally ill cancer patients sometimes injected them with bromodeoxyuridine, a synthetic marker that works the same way as Altman's radioactive tracer without the radioactivity. The point was to track how quickly their tumors were growing. In 1998, the neuroscientist Peter Eriksson and his colleagues obtained permission to examine brain tissue from patients who had since died. That tissue had already been exposed to the marker, for reasons that had nothing to do with neuroscience. Because the exposure had already happened, they did not need to inject anything new into a living human brain. In the hippocampus, the team found cells that carried the marker and also carried proteins found only in mature neurons. That combination was evidence that the cells had divided and matured into neurons after the marker was given, well into adulthood. The study was reported as finally confirming that the human brain keeps generating new neurons throughout life, and textbooks were revised accordingly.

¶5 For twenty years, that conclusion held. [A] In 2018, two research teams published detailed studies of human hippocampal tissue within months of each other, using markers similar to Eriksson's. [B] A team led by Shawn Sorrells reported that newly formed neurons became essentially undetectable in the hippocampus by adulthood, present in young children but gone by the late teenage years. [C] A team led by Maura Boldrini examined a different set of brains, preserved with different methods. They reported the opposite: cells bearing similar markers persisted into old age, suggesting the process never stops. [D] Neither paper was flawed in any obvious way, and neither has been withdrawn or corrected since. More than fifty years after Altman's original, dismissed discovery, neuroscientists still cannot agree on whether an adult human brain keeps producing new neurons, largely because the tissue itself cannot be examined without killing the very process researchers are trying to observe.