(B) Neurobiologists at the University of Pittsburgh School of Medicine give new meaning tothe term “motor mouth” in a study published today in the Proceedings of the NationalAcademy of Sciences. By carefully mapping neural networks in marmoset and macaquemonkeys, they determined that multiple areas in the brain's frontal lobe control themuscles of vocalization and could provide a foundation for complex speech. The findings—which could lead to a better understanding of speech disorders— refutea long-existing presumption that only the primary motor cortex, nicknamed M1, directlyinfluences the larynx or voice box, said principal investigator Peter L. Strick, Ph.D.,Thomas Detre Professor and chair of neurobiology at Pitt. Instead, several corticalregions send signals to laryngeal muscles to create greater vocal finesse in somenonhuman primates.  “This kind of parallel processing in our neural wiring might explain why humans arecapable of highly sophisticated language that allows us to share information, expressand perceive emotion, and tell memorable stories,” said Strick, who also is scientificdirector of Pitt's Brain Institute. “Our remarkable speech skills are due to more evolvedbrains, not better muscles.” Led by Christina M. Cerkevich, Ph.D., the research assistant professor of neurobiology,the investigators compared in marmosets and macaques neural networks that are theorigin of descending command signals to control monkey vocalizations. “We selected these two monkey species because of the striking differences in their vocalbehavior,” Cerkevich explained. “Marmosets readily vocalize in ways that are akin tohumans by taking turns to speak and altering the volume, timing and pitch of their callsto each other. Macaques, on the other hand, make mostly simple, spontaneous calls.” The researchers injected a transneuronal tracer made from rabies virus into thecricothyroid muscle of the monkeys' larynxes. The tracer infects nerve cells and has theunique property of moving from one neuron to another only at synapses, which are thespecial sites where neurons interact with each other. This makes it possible to trackneuronal circuits from the muscle back to the areas of the cerebral cortex that control it. In addition to M1, both kinds of monkeys had multiple premotor areas in the frontal lobethat send descending command signals to the cricothyroid muscle. But two of thepremotor areas provided a substantially larger source of descending output in marmosets,leading the researchers to propose that the enhanced vocal motor skills of marmosets aredue, in part, to the expansion of neural signaling from these premotor areas. “This result challenges the long-held view that improvements in motor skills ofvocalization are due largely to changes in the output from M1, the primary motorcortex," Strick said. "It appears there is no single control center, but rather parallelprocessing sites that enable complex vocalization and, ultimately, speech.”  Next steps include studying other nodes in the vocal motor network and to understandhow alterations in this network contribute to or result in vocal disorders, includingstuttering and speech apraxia.

【題組】78. What is the main idea of the passage？
(A) The author refutes the assumption that only M1 controls the larynx.
(B) The author explains how different cortical regions can affect the larynx.
(C) The author contrasts the experiments on two monkey species.
(D) The author suggests future research on the vocal motor network.