New research investigates how different areas of the human brain are involved in its response to fear.
From an evolutionary perspective, fear and anxiety are quite useful. These deeply ingrained emotions used to protect our ancestors from predators, and in our times the "fight-or-flight" response is still a healthy reaction to dangerous situations.
When fear is proportionate to the danger a person is in, it is a normal, adaptive response. However, some of us have exaggerated reactions to stressful situations.
As the National Institute of Mental Health explains, when the fear response is disproportionate or lasts a lot longer than what is normally expected from the situation - to a point where it interferes with an individual's well-being and daily functioning - it is classed as an anxiety disorder.
Anxiety disorders include a wide range of conditions that reportedly affect 18 percent of the adult population in the United States.
Because we share some of the brain's architecture with our fellow mammals and we have a similar response to fear, studying animal models has provided scientists with important insights into the neuroscientific basis for fear processing.
So far, animal studies have shown that the amygdala is a key player in fear processing, and that the hippocampus also plays a significant role in forming memories of emotional events.
However, researchers from the University of California-Irvine (UCI) believe that this body of research has not sufficiently investigated how the two regions interact in the presence of a fearful stimulus.
This is why the scientists - led by Dr. Jack Lin, a professor of neurology at UCI - set out to examine the neural pathways involved in fear and anxiety processing in humans.
The findings were published in the journal Nature Communications.
Analyzing the brain's fear response in humans
Researchers surgically inserted electrodes into the amygdala and hippocampus of nine participants, who were asked to watch scenes from horror movies.
The amygdala is an almond-shaped region in the brain, situated right next to the hypothalamus, which acts as the main center for processing emotions, emotional behavior, and motivation.
The amygdala, together with the hypothalamus and the hippocampus, form the brain's limbic system, which deals with memory and emotions.
The study participants had a form of medication-resistant epilepsy. The electrode placement was done as part of the clinical evaluation of their seizure activity, and the authors reassure the readers that the electrodes were implanted solely according to the clinical needs of the patients.
Lin and team recorded the participants' neural activity. As Jie Zheng, the study's first author explains, "deep brain electrodes capture neurons firing millisecond by millisecond, revealing in real time how the brain attends to fearful stimuli."
The researchers found that the amygdala and the hippocampus directly exchange signals when an individual recognizes emotional stimuli.
First author Zheng explains the findings in more detail:
"Neurons in the amygdala fired 120 milliseconds earlier than the hippocampus," the author says. "It is truly remarkable that we can measure the brain dynamics with such precision. Further, the traffic pattern between the 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."
Lead author Lin says that the study provides "direct evidence that the amygdala first extracts emotional relevance and then sends this information to the hippocampus to be processed as a memory."
Lin also explains what this means for treatment options and how their study could impact the development of new therapies for psychiatric disorders.
"This is the first study in humans to delineate the mechanism by which our brain processes fear at the circuitry level. This has huge implications for treating neuropsychiatric disorders. For example, current drugs available to treat anxiety disorder bind to large areas of the brain, leading to unwanted side effects. Our hope is that we will one day be able to target and manipulate the precise amygdala-hippocampal circuit involved in processing negative emotions while preserving positive ones."
Dr. Jack Lin
"This study brings the promise of targeted therapy a step closer," Lin adds.