Quick detection of social, emotional, or threatening stimuli is critical for adaptive fitness in humans. These deeply ingrained emotions are useful in protecting us, and the “fight-or-flight” response is still a healthy response to dangerous situations. When fear is proportionate to the danger someone is in, it is a normal adaptive response. However, some have exaggerated reactions to stressful situations and so over-interpretation of fear may lead to anxiety and other mental disorders.
Understanding how the human brain processes fear and fearful information has been a topic of immense research, and up until now the brain circuit underlying fear has only been mapped out in rodents. Since we share some of the brain’s architecture with mammals and we have a similar response to fear, studying animal models has provided researchers with key insights into the neuroscientific basis for fear processing. Up until this point, animal studies have shown that the amygdala is important in fear processing, and that the hippocampus also has a significant part in forming memories of emotional events. But researchers at the University of California did not believe that this body of research had adequately investigated how the amygdala and hippocampus interact when a fearful stimulus is present. And so, these researchers set out to identify a key neural pathway in humans that explains how the brain processes feelings of fear and anxiety, a finding that could help scientists unlock new ways to treat mental health disorders.
The researchers of this study recorded the neuronal activity by surgically inserting electrodes into the amygdala and hippocampus of nine individuals with medication-resistant epilepsy as part of an assessment of their seizure activity; the participants were instructed to watch scenes from horror movies to stimulate the recognition of fear. As stated by one of the researchers, “Deep brain electrodes capture neurons firing millisecond by millisecond, revealing in real time how the brain attends to fearful stimuli.”, and the researchers were able to discover that the amygdala and hippocampus directly exchange signals when a person recognises emotional stimuli. Neurons in the amygdala fired 120 milliseconds earlier that the hippocampus, and the traffic pattern between these two brain regions are controlled by the emotion of the movie; a unidirectional flow of information from the amygdala to the hippocampus only occurred when people were watching fearful movie clips but not while watching peaceful scenes. The study provided direct evidence that the amygdala initially extracts emotional relevance and then sends this information to the hippocampus to be processed as a memory.
The results of this study and understanding the activation of the exact brain network in processing fearful stimuli is vital to the development of new therapies for psychiatric disorders especially in the age of personalised medicine, for example being able to one day target and manipulate the precise amygdala-hippocampal circuit involved in processing negative emotions while preserving positive ones.
Original Source: Zheng J, Anderson K, Leal S, et al. (2017) “Amygdala-hippocampal dynamics during salient information processing.” Nature Communications 8: 14413.