How did the remarkable ability to communicate in words first evolve? Researchers probing the neurological basis of language are focusing on seemingly unrelated abilities such as mimicry and movement Advances in brain imaging, neuroscience, and genetics have enabled a new contingent of researchers to go ever deeper into our brains and our biological past.
Psychologist David McNeill of the University of Chicago cites the case of a man who lost all sense of touch below the neck due to a strange virus. Although the man had to relearn the simplest movements, using cognitive and visual feedback to substitute for lost senses, he continued to gesture automatically when he spoke, even when researchers hid his hands from his own and listeners' view. "The hands are really precisely linked to speech articulation," says McNeill. "Gesture is not a behavioral fossil that was superseded by language but an indispensable part of language."
The motor-language connection continues to draw attention, in part because of a 1996 discovery. This is the so-called mirror neuron system found in monkeys' brains. Mirror neurons' link to language depends on imitation, a skill largely unique to humans and considered vital to language. Imitation is the way babies learn their first words. And it's the only way a common meaning can emerge for an abstract symbol. A team led by Giacomo Rizzolatti of the University of Parma, Italy, found what they considered a plausible antecedent for the human ability to imitate in the brains of monkeys. The team recorded electrical activity in macaques from 532 neurons in an area called F5, which is homologous to Broca's area in humans. Neurons in F5 are known to fire during monkeys' "goal-directed" hand and mouth movements--for example, when reaching for food. What intrigued the researchers is that a subset of these neurons, which they dubbed mirror neurons, also became active when a monkey merely watched another monkey (or a human) perform the action. The researchers concluded that these mirror cells form a system for matching the observation and execution of mouth and hand actions--the first steps toward imitation.
Mirror neurons provide the "neural missing link" between movement and speech control, says Arbib. They also fit well with an old theory, the "motor theory of speech perception," developed in the 1950s by the late Alvin Liberman of Yale University's Haskins Laboratories.
Psychologist Michael Studdert-Kennedy of Haskins Labs explains that when children imitate their first words, experiments have shown that they (unlike another imitator, the parrot) are guided by the "gestural" features of the sound--that is, by the actions of the mouth rather than by a sound's acoustic features. This means "you perceive speech by referring the sounds you hear to your own production mechanism," says Studdert-Kennedy. Mirror neurons, he says, "for the first time provide an example of a direct physiological hookup between input and output": the observation of an action and its imitation. He and others feel that mirror neurons offer the first concrete neurological evidence of abilities crucial to language.