Sudden, traumatizing sounds can form lasting memories in the brain’s “flight or fight” region.
Researchers were able to heighten and improve hearing in rats by stimulating the brain region, known as the locus coeruleus.
“Our study gives us deeper insight into the functions of the locus coeruleus as a powerful amplifier in the brain, controlling how and where the brain stores and transforms sudden, traumatizing sounds and events into memories,” says senior study investigator and neuroscientist Robert C. Froemke, an assistant professor at NYU Langone Medical Center.
“Our findings, if confirmed by future studies in animals and people, should help us better understand how to improve hearing and memory abilities in those suffering from hearing loss or possibly even Alzheimer’s disease, as well as how to alter or minimize memories involved in disorders like post-traumatic stress disorder.”
The findings help explain why it only takes seconds to develop post-traumatic stress disorder, or PTSD, from a shock or sudden event—and suggest how traumatizing memories can be reshaped or dampened to lessen symptoms of PTSD.
The results also may help explain how electrical impulses, such as those produced by cochlear implants for the hearing impaired, can better be used to improve hearing.
Associate sound with food
The researchers chemically stimulated the locus coeruleus in rats while simultaneously playing them a sound paired with a food reward. After a two-week training period to ensure the rats associated the sound with food, the same sound was played much more quietly.
They found the locus coeruleus and auditory cortex still responded to the sound, even at nearly imperceptible levels, for the subsequent and remaining two weeks of the experiments.
However, chemically stimulating the locus coeruleus led to 100 percent neural activity in the auditory cortex, even in the absence of the same triggering sounds. Neural activity in the auditory cortex in response to the sounds was at least 10 times greater than when activity in the locus coeruleus was chemically suppressed.
According to Froemke, the results clearly demonstrate that the memory of the sound and its associated reward was encoded by the locus coeruleus, which helped improve the rats’ ability to perceive the sound.
In another set of experiments, the investigators paired a specific sound with a series of mild shocks to the animals and found that when the shocks stopped, but the sound continued to play, their locus coeruleus response remained the same, at 20 neuron spikes per second.
Froemke says he next plans to investigate how information is encoded within the locus coeruleus and to identify which cells are activated by cochlear implants and in animal models of PTSD. He is also planning experiments to identify patterns formed in the locus coerelus during other behavioral events involving hearing, such as mothers responding to the cries and calls of their offspring.
The National Institute on Deafness and Other Communication Disorders, the Hirschl/Weill-Caulier Career Research Award, the Sloan Research Fellowship, and the Portuguese Foundation for Science and Technology funded the study.