Listen to the following passage. Write in English a short summary of 150-200 wor

Research on the bilingual brain has gone through several stages over the years as scientists unlock more of the neurological secrets of the bilingual brain. They’re learning that speaking more than one language may have cognitive benefits that extend from childhood into old age.
    Until fairly recently, parents and educators feared that exposing children to a second language at too early an age might not only delay their language skills but harm their intellectual growth. New research, however, has found that bilingual children reach language milestones, such as first word and first fifty words, at the same age as monolingual children. Nor do they show any evidence of being "language confused". Indeed, young infants are able to use rhythmical cues to keep their two languages distinct, and do so from the first days of life.
    In fact, being bilingual may give children an advantage at school. Bilingual preschoolers have been found to be better able than their monolingual peers at focusing on a task while tuning out distractions. A similar enhanced ability to concentrate, a sign of a well-functioning working memory, has been found in bilingual adults, particularly those who became fluent in two languages at an early age. It may be that managing two languages helps the brain sharpen and retain its ability to focus while ignoring irrelevant information.
    Other research suggests that bilingualism may delay the onset of age-related dementia, including Alzheimer’s disease, by up to four years. Although scientists don’t know why bilingualism creates this "cognitive reserve", some theorize that speaking two languages may increase blood and oxygen flow to the brain and keep nerve connections healthy—factors thought to help ward off dementia.
    More recently, scientists have discovered that bilingual adults have denser gray matter, which is brain tissue packed with information-processing nerve cells and fibers, especially in the brain’s left hemisphere, where most language and communication skills are controlled. The effect is strongest in people who learned a second language before the age of five and in those who are most proficient at their second language. This finding suggests that being bilingual from an early age significantly alters the brain’s structure.
    Finally, neuroscience research is showing promise for evaluating and treating bilingual patients who lose the ability to produce or understand speech after a brain injury. Research is showing that rehabilitation efforts that use both languages, not just one—even a patient’s native language—hold the greatest promise for recovery. How exactly it works remains a research interest requiring more advanced technology to probe into.