How can the nervous system make such a huge set of alternatives over the course of a lifetime?

Some essential survival behaviors appear to be hard-wired, that is, they occur without prior experience. Many prey creatures, for example, have an inherent ability to flee from perceived threats.

However, an organism’s ability to learn about the world and adapt to its ever-changing conditions is critical. How can the nervous system make such a huge set of alternatives over the course of a lifetime?

Neuroscientists at UCL’s Sainsbury Wellcome Centre investigated the adaptability of mice escape behavior.

The researchers show that, while escape behavior can be reliably evoked in a laboratory setting, mice can easily learn to suppress their escape response, effectively disregarding stimuli that are considered to pose no threat, in a new study published today in Current Biology.

“An obvious example is the domestication of cattle and pets. This clearly shows,” explains Troy Margrie, the paper’s corresponding author, “that organisms learn that things they initially interpreted as threatening may not be so. Cattle for example, were once fearful of human beings but at some point they learned that humans could become a reliable source for food, shelter and even protection from other species,”

To test this behavioral adaptability, investigators in the Margrie lab used an above expanding dark disk, known as a looming stimulus, to imitate a predator approaching them from above. They discovered that isolating mice for a few days before testing allowed them to elicit highly robust escape, and they used this robust model of escape as a starting point for assessing its flexibility.

Then, as a first step, scientists repeatedly presented this approaching stimulus to see if the mice would finally stop responding to it. Mice, on the other hand, did not consistently learn to suppress their escape behavior following multiple presentations of the stimuli.

“Funnily enough, one of the problems we faced is that under the right conditions mice react so robustly to high contrast looming stimuli,” adds Steve Lenzi, first author on the paper, “which means they run away and hide, and it can therefore take a very long time to expose mice to enough stimuli for them to reliably suppress their escape response.”

As a result, the researchers created a physical barrier prohibiting access to the neighboring shelter while also adjusting the contrast of the looming stimuli to create a gradient from low to the high threat. In mice, these changes resulted in a constant inhibition of the escape reaction.

The neuroscientists demonstrated that the suppression was strong and lasted several weeks. Furthermore, the suppression was stimulus-specific, meaning that when the mice were faced with a different scary stimulus, such as a loud noise instead of the looming stimulus, they continued to flee.

They also proved that the level of escape suppression was highly dependent on recent threat-escape history. “This suggests that escape is not simply reflexive but dependent on threat memory and is therefore under cognitive control,” adds Troy Margrie.

Although this work is primarily focused on behavior, the team thinks that the paradigm they have established in the experiment can be used to probe the neural circuitry underlying the flexibility of innate behaviors. 

“We also apply this in the search for, and study of, brain regions that are involved in the regulation of escape behaviour and we hope others will do the same,” says Steve Lenzi.

In addition to investigating how threat history influences escape behavior control, the researchers investigated the role of social context. The researchers studied the escape behavior of mice kept in groups vs mice housed individually. When mice were evaluated individually, they found that mice who resided in large groups of 20 were far less likely to flee. Mice that had been isolated and left alone for a period of time looked to be significantly more watchful or perhaps reacting.

“Initially we wanted to understand whether generic experience influences the decision to escape. Single housing or group housing is a very easy and natural way to introduce experiential differences in laboratory mice. Plus, there are many examples from field studies that show that group statistics can profoundly influence predator avoidance or surveillance behaviours. Animals that are alone need to be more vigilant, whereas in a flock they can spread the surveillance among the group,” Steve Lenzi explains.

These findings raise several unanswered concerns, and the next step for the researchers is to investigate the mechanisms underlying this type of learning.

The Margrie group intends to use this ethologically relevant procedure to begin to understand the neurological underpinnings of how animals learn to suppress escape, specifically how different brain systems interact with the escape circuitry to allow this flexibility of behavior.

Understanding this niche may help researchers begin to address the larger unknown topic of how learning interacts with our innate proclivity for particular behaviors.

Image Credit: Getty

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