A keystone in the understanding of how humans acquire language is the critical period theory, which states that the ability to learn to communicate verbally peaks by age six or so and declines as the child gets older. New research, however, may overturn the theory, at least in its siplest form: the ability actually may extend past the age of nine.

Understandably, evidence about the critical period is scarce, because it requires the study of children who have not learned to speak in their early years, either through strange circumstance, accident or disease. One of the first studies took place in 1797 when a "feral child" was discovered wandering in the forests ofsouthern France.TheWild Boy of Aveyron, about age 12when found, never mastered speech, despite intensive efforts by his mentor, Jean-Marc Itard. This and other cases provided the basis for the critical period theory.

Two centuries later the odyssey of an other youngster has provided contrary evidence. January's issue of Brain carries  a report about "Case Alex," derived from the study of brain-damaged children by Faraneh Vargha-Khadem,Elizabeth Isaacs and their colleagues at the Wolfson Center of the Institute of Child Health in London.

Born brain-damaged, Alex was mute until the age of nine and then rapidly learned to speak over the next two and a half years. He continued to develop increasingly complex language abilities until now, at 15, he produces well-formutated sentences conveying a knowledge of both semantics and syntax that is on a par with that of a normal 10-
year-old. As the authors of the Brain report put it: "To our knowledge, no previously reported child has acquired a first spoken language that is clearly articulated, well structured and appropriate after the age of about six."

Adding to the surprise is the fact that Alex has acquired speech without a left hemisphere, the region responsible for language in the overwhelming majority of people. That part of the brain had to be surgically removed when Alex was eight. Such a hemispherectomy is almost a routine operation for some rare neurological conditions; in Alex's case, it was Sturge-Weber syndrome, which produced a relentless succession of seizures. The epileptic activity interfered so much with the normal operation of his brain that he failed to develop language skills in any form, apart from one or two regularly used words and sounds.

For the first few months after the neurosurgical operation, Alex was kept on anticonvulsive medication. Then, a month after his medication was withdrawn, he suddenly started uttering syllables and single words. His mother recorded in her diary more than 50 words, primarily nouns but also verbs, adjectives and prepositions. Several months later he had progressed to fll sentences.

According to the researchers, if there is a critical period, Alex has raised its  upper limit to nine, a result consistent with at least one theory that su ggests that the hormonal changes of puberty put a stop to the flexibility of the brain's language areas. The next step in studying this remarkable boy is to see if reading and writing can also be learned without a left hemisphere, at least up to a level that will enable him to navigate through the everyday world of signs, forms and cereal boxes. But even before that happens, the Brain report is likely to provoke a closer look and possibly a reworking of the critical period hypothesis foreshadowed in a forest in southern France 200 years ago.