The Fear Factor

Humans are predisposed to react with fear to loud, sudden noises or fast-approaching objects.

However, with experience, they learn to suppress instinctive fear of perceived threats, and even enjoy them, for example, fireworks.

Now, a new study shows the hows and whys.

“Our results challenge traditional views about learning and memory,” said Sonja Hofer, senior author of the study.

Researchers from the Sainsbury Wellcome Centre stuck silicon probes into mice’s brains and observed their behaviors while an overhead expanding shadow mimicking an aerial predator approached them.

During their first encounters with the shadow, the mice ran away seeking shelter. However, with repeated exposure and no real danger, the mice learned to stay calm, offering researchers a model to investigate how the brain can suppress fear responses.

Based on previous studies, researchers knew that the brain’s ventrolateral geniculate nucleus (vLGN) area can suppress fear reactions by linking them to previous experiences involving a threat.

The vLGN is situated between the neocortex, responsible for detecting threats, and the brain stem, responsible for activating the body’s instinctive response, the Washington Post explained.

While the vLGN also receives inputs from visual areas in the cerebral cortex, essential in learning whether to fear a visual threat, the researchers discovered that the vLGN is the area responsible for storing these learning-induced memories, too.

Co-lead author Sara Mederos explained that when specific cortical visual areas were not activated, animals failed to learn to suppress their fear responses. However, once the mice had learned when to remain calm, the involvement of the cerebral cortex was no longer essential.

Sonja Hofer, a co-lead author, noted that these findings challenge traditional views on learning and memory. While the cerebral cortex has long been seen as the main driver of these processes, the new study shows it is the subcortical vLGN that actually stores the crucial memories necessary to learn – and unlearn – fear.

The release of endocannabinoids – neurotransmitters that regulate mood and memory – increases neural activity in specific vLGN neurons. This reduces inhibitory input, leading to heightened activity in the vLGN when the subject encounters a visual threat, which in turn suppresses the fear response.

These findings could provide insight into what happens when this mechanism is impaired – when fear response regulation fails in conditions like phobias, anxiety, and post-traumatic stress disorder – and help identify brain areas to target for treatment.