Researchers have demonstrated for the first time that our 'gut instinct' has a significant impact on how we react to fear.
It is not only the brain that controls processes in our abdominal cavity; our stomach also sends signals back to the brain, researchers said.
At the heart of this dialogue between the brain and abdomen is the vagus nerve, which transmits signals in both directions - from the brain to our internal organs (via the so called efferent nerves) and from the stomach back to our brain (via the afferent nerves).
By cutting the afferent nerve fibres in rats, a team of scientists led by Urs Meyer, a researcher in the group of ETH Zurich professor Wolfgang Langhans, turned this two-way communication into a one-way street, enabling the researchers to get to the bottom of the role played by gut instinct.
In the test animals, the brain was still able to control processes in the abdomen, but no longer received any signals from the other direction.
In the behavioural studies, the researchers determined that the rats were less wary of open spaces and bright lights compared with controlled rats with an intact vagus nerve.
"The innate response to fear appears to be influenced significantly by signals sent from the stomach to the brain," said Meyer.
Nevertheless, the loss of their gut instinct did not make the rats completely fearless: the situation for learned fear behaviour looked different.
In a conditioning experiment, the rats learned to link a neutral acoustic stimulus - a sound - to an unpleasant experience.
Here, the signal path between the stomach and brain appeared to play no role, with the test animals learning the association as well as the control animals.
If, however, the researchers switched from a negative to a neutral stimulus, the rats without gut instinct required significantly longer to associate the sound with the new, neutral situation.
This also fits with the results of a recently published study conducted by other researchers, which found that stimulation of the vagus nerve facilitates relearning, said Meyer.
These findings are also of interest to the field of psychiatry, as post-traumatic stress disorder (PTSD), for example, is linked to the association of neutral stimuli with fear triggered by extreme experiences, researchers said.
Stimulation of the vagus nerve could help people with PTSD to once more associate the triggering stimuli with neutral experiences. Vagus nerve stimulation is already used today to treat epilepsy and, in some cases, depression.
