The Science of Learning & Speaking Languages | Dr. Eddie Chang

KEYPOINTS

• 01

  The sounds of early life shape the hearing brain

  As a medical student working with Mike Merzenich at UCSF, Chang raised rat pups in continuous, moderate white noise that masked natural sounds. The critical period of the auditory cortex stayed open months longer than normal. That sounds like a benefit, but it actually meant delayed maturation. Humans have a similar window: within a couple of years, infants lose sensitivity to speech sounds outside their native language.

• 02

  White noise machines for babies are an open question

  Asked whether white noise harms infants' hearing development, Chang says the studies haven't been done. He has used soothing tools himself with three kids. His caution is that continuous white noise doesn't occur in nature, and the developing brain seems to need structured, meaningful sounds.

• 03

  Electrical stimulation of brain areas can switch anxiety on or off

  During awake brain mapping, stimulating the orbitofrontal cortex reduced stress, but only in patients who were already anxious. Stimulating the amygdala could cause sudden anxiety, and stimulating the insula could cause disgust. Chang suspects some psychiatric conditions reflect imbalances in electrical activity across these areas.

• 04

  Epilepsy often resists medication

  About a third of people with epilepsy (defined here as three or more seizures) aren't controlled by modern drugs. If two or three medications have failed, a fourth or fifth rarely helps. A subset can benefit from surgery to remove the seizure source or from implanted stimulators.

• 05

  The textbook map of language areas is partly wrong

  The textbooks still assign speaking to Broca's area (based on Broca's 19th-century patient 'Tan') and understanding to Wernicke's area in the left temporal lobe. In hundreds of patients, Chang's team found that surgery on the posterior frontal lobe often leaves speech intact. The speech-motor areas of the precentral gyrus, which control the lips, jaw and larynx, matter more for producing words. He estimates that about 50% of what's taught about the brain is accurate and the rest oversimplified.

• 06

  Which side of the brain handles language depends partly on handedness

  In right-handed people, language is on the left side 99% of the time. In left-handed people it's about 70%, with 20–30% using both sides or the right. After a left-sided stroke, nearby tissue or even the right hemisphere can take over. Chang thinks both sides have the machinery but the brain specializes one.

• 07

  Speech cortex has a 'salt-and-pepper' map of sound features

  Recording directly from the brain surface at millimeter and millisecond resolution, his lab finds sites tuned to features such as plosives (b, d, g, made by closing and releasing the mouth) and fricatives (sh, th, made by air turbulence). These sites are scattered rather than laid out in an orderly map. Speech may also reach the speech cortex by its own route, bypassing the primary auditory cortex, which is ordered by frequency.

• 08

  About a dozen movements build every word

  Speech production comes down to roughly 12 movement features of the tongue, jaw, lips and larynx. Each is meaningless alone, but combined in sequence they generate every word, which Chang compares to DNA's four bases encoding life.

• 09

  Reading is borrowed from older brain systems

  Writing is too recent to have evolved its own brain area. Instead, a visual word form area at the back of the temporal lobe is recruited and linked to the speech-sound system. Many cases of dyslexia stem from weak phonological awareness, a mismatch between seen words and their sounds, rather than purely a visual problem.

• 10

  Bilingual speakers use overlapping circuits

  In Chang's ongoing lab study, the same speech areas respond to both languages, and the 'English' cortex even tries to parse an unfamiliar language. What differs is the stored memory of sound sequences that make up each language's words.