Monkey Do, Monkey See ... Pre-Human Say?

Human speech derives from a gestural communication system used by proto-humans and facilitated by a neural circuit found in both monkeys and humans, neuroscientists at the University of Southern California and the University of Parma (Italy) propose.

In an article published in the May issue of the journal Trends in Neuroscience, these researchers describe a group of neurons in the brains of monkeys--neurons that discharge when the monkey grasps or manipulates an object and when it sees the experimenter making similar actions.

USC's Michael A. Arbib, Ph.D., says "the neurons, located in the premotor cortex just in front of the motor cortex, are a mechanism for recognizing the meaning of actions made by others."

The University of Parma's Giacomo Rizzolatti, M.D., called the cells "mirror neurons" when his group first noticed them several years ago, in an experiment performed to determine which neurons "fire" whenever a monkey grasps an object.

While many neurons in the premotor cortex were found to be active when the monkey performed a single type of action--such as manipulating, tearing or grasping an object--the mirror neurons were special because they also fired whenever a monkey saw another monkey, or even a human, making the same moves.

"This mechanism provides the neural prerequisite for development of inter-individual communication, and finally of speech," Dr. Arbib says.

"For communication to succeed, both the individual sending a message and the individual receiving it must recognize the significance of the sender's signal. Mirror neurons are thus the missing link in the evolution of language. They provide a mechanism for the sharing of meaning."

Do humans have a similar apparatus? Using positron emission tomography (PET) scans of people, the researchers found a similar neural circuit for the execution and observation of hand movements in Broca's area, the part of the human brain believed responsible for speech.

These findings support the theory that human speech evolved from an ancient gestural communication system (rather than vocal communication evolving directly, as some scientists have speculated).

Analysis of fossil skulls suggests that, as the primate brain evolved, it was building a language mechanism even before there was a larynx capable of articulating speech. While monkeys and other non-human primates continue to lack the vocal equipment needed to articulate speech, they do have the brain circuits needed for simple communication.

"Monkey vocalizations are limited to calls and screeches, but they are able to communicate via a repertoire of oral-facial actions (such as tooth chattering and lip smacking)," Arbib says. "The interesting point is that it is the oro-facial gestures, not the vocalizations, that link to the monkey's 'pre-Broca' area."

Broca's area in the human brain and its counterpart in the monkey brain both have "the neural structures for controlling oro-laringeal, facial and hand-arm movements," Arbib explains, and "both are endowed with mechanisms linking action perceiving and action producing."

Arbib and Rizzolatti are now charting the evolutionary steps that led from mirror neurons to human language.

Arbib is a professor of computer science and biological sciences and also holds appointments in the USC departments of biomedical engineering, electrical engineering and psychology. His research on the brain and language is funded by grants from the National Institute of Mental Health, the National Science Foundation and the Human Frontier Science Program.