For more than 60 years, scientists have known that a strip of neural tissue that runs ear-to-ear along the brain’s surface orchestrates most voluntary movement, from raising a fork to kicking a ball. A new brain-imaging study has revealed that parts of this so-called motor cortex also respond vigorously as people do nothing more than silently read words.
Not just any words get those neurons going, however. They have to be action words—active verbs.
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As volunteers read a verb referring to a face, arm, or leg action—such as lick, pick, or kick—the motor cortex areas that control the specified action exhibit high rates of blood flow, a sign of intense neural activity, say neuroscientist Friedemann Pulvermüller of the Medical Research Council in Cambridge, England, and his colleagues. For instance, reading the word lick triggers pronounced blood flow in sites of the motor cortex associated with tongue and mouth movements.
At the same time, prominent activity also occurred in so-called premotor brain regions that influence learning of new actions and in two left-brain structures—Broca’s area and Wernicke’s area—that have long been linked to understanding language.
These results challenge the theory that isolated, language-specific brain structures discern word meanings, the scientists conclude in the Jan. 22 Neuron. Instead, they propose, word understanding hinges on activation of interconnected brain areas that pull together knowledge about that particular word and its associated actions and sensations.
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“Brain areas that are used to perform an action are also needed to comprehend words related to that action,” Victor de Lafuente and Ranulfo Romo of Mexico’s National Autonomous University in Mexico City comment in an editorial in the same journal issue. “Remarkably, just the reading of feet-related action words such as dance makes [the motor cortex] move its ‘feet.'”
In its studies, Pulvermüller’s group administered functional magnetic resonance imaging scans to 14 adults, ages 20 to 30, as they viewed strings of meaningless hash marks on a computer screen and then read lists of arbitrary nouns and lists of face-, arm-, and leg-related verbs. Only the verbs elicited marked activity in the premotor cortex and relevant parts of the motor cortex.
Moreover, the same arm- and leg-related sections of motor cortex responded similarly when volunteers read verbs involving those body parts or when they responded to instructions to move their feet and fingers.
Motor cortex areas triggered by face-related words such as bite and chew showed little correspondence to those activated by voluntary tongue movements, however. That’s probably because these words refer to complex jaw and head actions that involve many movements apart from the tongue, the researchers say.
To confirm the new findings, researchers need to show that verb comprehension suffers when critical parts of the motor cortex are temporarily shut down, remark de Lafuente and Romo. This procedure can be safely accomplished with a machine that delivers brief, intense magnetic pulses to patches of brain tissue, the researchers note.