Researchers have shown for the first time that the behavioral context in which communication sounds are heard affects the brain's ability to detect, discriminate and ultimately respond to them. Specifically, the researchers found that the auditory neurons of female mice that had given birth were better at detecting and discriminating vocalizations from mouse pups than the auditory neurons in virgin females.
"Although there have been many studies on communication and neurons in animals, such as in primates, birds and bats, those studies have focused on how neurons respond to sounds that were already behaviorally relevant to the animals," says Robert Liu, PhD, Emory University assistant professor of biology and lead author of the study, which appears in the June 12, 2007 issue of PLoS Biology.
"What's different about this study is that we used natural vocalizations -- a range of pup calls -- to see how well neurons in mother mice and virgin, or pup-naïve mice, detect, discriminate and act on this behaviorally important sound," says Dr. Liu. Ultrasonic calls emitted by mouse pups communicate distress and elicit a search and retrieval response from mothers.
"Our current work demonstrates that the neural code for communication sounds in adult mammals can change, either because of experience or because of hormonal mechanisms, as the significance of the signal is acquired. This means that the brain can improve information processing for specific communicative functions," says Liu.
Liu began the work as a postdoctoral fellow in the lab of senior author Christoph Schreiner, PhD, MD, professor and vice chair of otolaryngology, head and neck surgery and a member of the W.M. Keck Foundation Center for Integrative Neuroscience at the University of California, San Francisco.
In the study, the researchers determined that the neurons in the mothers' auditory cortex, an area of the brain that processes sounds, showed larger and earlier electrical spiking, or signaling, than in virgin mice, says Dr. Schreiner.
This shows, says Dr. Liu, that, "the timing plays an important role in the neural code of sounds. The idea that spike timing is important in brain processes has been around for a long time, but we're looking at it specifically in the context of natural communication. And we found that the big difference in encoding is the behavioral relevance of these sounds."
Although the pups' vocalizations vary quite a bit, Dr. Liu says the mothers can still detect the calls, understand them and take action. "What is really intriguing is that behavioral studies have shown that, if you look at vocalizations made by male adult mice, they also make very high-frequency vocalizations as do pups but the mothers don't react to them as they do to the pup calls," says Dr. Liu.
Along these lines, another behavioral study, reported by a team in Germany in 1987, revealed behavioral differences in mice in response to vocalizations. In this case, says Schreiner, there were two groups of mice: "the pup-naive mice and the mother mice who really care about these sounds." The researchers found that, "If you play these vocalizations, the mothers run over to get the pup. If it's a different sound, they don't go as often, and why should they? Likewise, the female mice who have not been mothers don't go to the source of the pup calls more often than to any other vocalization. They're the same age and same species, yet only one thing is different: one group has had pups and the other hasn't. It appears that a switch is thrown that improves sensitivity to these sounds in the mothers."
Dr. Schreiner says further research is needed to determine whether mothers recognize pup sounds immediately after they become pregnant, meaning a hormonal switch has been thrown, or after they give birth. Recognition of pup cries after birth would indicate that exposure to the cries triggers mothers' attentiveness.
Dr. Schreiner likens the improved ability of mother mice to distinguish sounds to what adult humans experience when initially learning a foreign language. "We go to a foreign country, hear what people are saying, but we can't make subtle discriminations of syllables in order to establish the border between words. With time and experience the brain is adjusting to this, our neurons are becoming more discriminative, and we can distinguish words in what initially just appeared to us as an unbroken stream of sound."
According to the authors, this study helps demonstrate how important sounds are encoded in the normal brain but also has implications for developing therapeutic strategies in children and adults who suffer from speech-perception deficits.
This study was supported by a Sloan and Swartz Foundation fellowship, the National Institutes of Health; and the Center for Behavioral Neuroscience under the Science and Technology Program of the National Science Foundation.
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