You know that feeling when everything suddenly goes quiet? Researchers have identified a novel neural circuit that plays a critical role in processing sound cues of danger to trigger defense responses in rats when silence falls. The study publishing May 12, 2020 in the open-access journal PLOS Biology by Marta Moita of the Champalimaud Centre for the Unknown in Portugal, and colleagues, sheds light on how the brain processes sensory cues and uses this information to generate survival behaviors.
Animals use sound cues produced by others to detect impending danger. Most research has focused on actively emitted signals, such as alarm calls and foot stamping. But rats use a passive signal - silence. When rats feel threatened, they often stop moving, or freeze, in fear.
The silence resulting from this freezing behavior is noticed by other rats, which themselves respond by freezing because they associate the silence of others with danger. But little is known about the neural mechanisms by which natural sounds (or silence) trigger defensive responses. To address this gap in knowledge, Moita and colleagues set out to identify brain regions that are necessary for rats to behave defensively (i.e., freeze) in response to silence, which was induced by the freezing of other rats faced with a threatening situation.
When the researchers separately inactivated three different brain regions, rats became less likely to freeze in response to the silence of other rats exposed to foot shocks. This neural circuit consists of well-established sound-processing brain regions, including the dorsal sub-nucleus of the medial geniculate body, and the ventral area of auditory cortex. In addition, the circuit includes a brain region called the lateral amygdala, which is involved in emotional responses such as fear. This circuit includes brain regions that have not been previously implicated in defense responses. The study provides new insight into the neural mechanisms by which prey animals use the sound of others (or lack thereof) to infer danger and respond defensively.
Pereira AG, Farias M, Moita MA (2020) Thalamic, cortical, and amygdala involvement in the processing of a natural sound cue of danger. PLoS Bio l 18(5): e3000674. https://doi.org/10.1371/journal.pbio.3000674
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