Speech veers left in babies’ brains

In adults, the brain’s left hemisphere usually assumes primary responsibility for understanding speech. A new brain-imaging study suggests that a fledgling version of this left-brain specialization appears in 2-to-3-month-old babies as they listen to speech, even though they can’t utter a word and it’s not clear whether they understand any of what they hear.

Language acquisition may reflect the gradual expansion of a network of left-hemisphere regions that enters the neural fray within the first few months of life, propose psychologist Ghislaine Dehaene-Lambertz of the National Center of Scientific Research in Paris and her coworkers. In newborns, however, it remains unknown whether this left-brain network responds only to speech or to any series of rapidly presented sounds, the scientists note in the Dec. 6, 2002 Science.

In the new study, the scientists used a technique called functional magnetic resonance imaging (fMRI) to track neural blood flow in 20 babies as they listened to 20-second presentations of a woman’s voice reading a children’s book separated by 20-second periods of silence. Some speech segments were played backward.

Left-hemisphere areas roughly corresponding to several adult-brain areas associated with speech comprehension exhibited elevated blood flow–an indirect sign of increased neural activity–as babies listened to regular, but not backward, speech. That finding fits theories that an innate left-hemisphere mechanism underlies language.

The fMRI data also showed that part of the right frontal cortex responded to regular speech with heightened activity. This finding challenges a current theory that the frontal cortex plays no significant role in a baby’s thought processes for several months after birth.

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