Instinctive behaviors are essential for animal survival, as are reflexes. Escaping from a dangerous situation is a good example of an instinctive survival behavior. But how do we know where safety is to run towards it?
Is it a hard-wired reflex that causes us to flee from danger as fast as we can, or do our brains calculate the best course of action, modifying our response depending on the situation we find ourselves in? For example, if you found yourself face-to-face with a Grizzly Bear, would you run or play dead? Would your choice depend on how close you were to safety?
A recent study from Tiago Branco’s group at the Sainsbury Wellcome Center for Neural Circuits and Behavior, University College London, elegantly explores the decision making that underlies a survival escape behavior.
Instead of Grizzly Bears and unsuspecting undergraduate students, the team tested the survival instincts of untrained mice by placing them in a specially constructed arena and exposing them to sound and light stimuli that recreated natural threats.
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The arena designed by first author Ruben Vale is a modified Barnes Maze, a large flat disc around which are 20 equally-spaced holes. Each hole has the potential to be a shelter for the mouse. The modification means the Barnes Maze is in two portions, an inner ring portion and an outer ring. The shelter holes exist in the outer ring, which is moveable. This enables the shelter site to be moved, by rotating the outer ring as the mouse explores the inner ring.
A mouse explores the Barnes Maze. The gray circle in the center of the image is the expanding spot projected from above. The black circles around the edge of the arena are the shelter sites, only one of which leads to shelter, the others are blocked. Credit: Ruben Vale, Sainsbury Wellcome Center for Neural Circuits and Behavior, UCL.
To investigate flight behavior, Ruben blocked 19 of the 20 holes, meaning only one could be the actual shelter. He then placed a mouse inside the maze and let it explore. The mouse quickly found its shelter and settled inside for several minutes. When it was ready to explore, the mouse stepped out of the shelter and began to explore the maze. As the mouse entered the centre of the maze it was subjected to either an ultrasound stimulus or an expanding black spot overhead, simulating a bird of prey swooping down from above.
Every mouse trialed quickly ran back to the shelter in response to either stimuli. The survival response was so great that even if the expanding spot, generated using an overhead projector, was initiated between the mouse and its shelter, the mouse would still run straight to the shelter, not simply in a direction away from the spot. This suggested escape behavior is a goal-directed action, not a reflexive move away from danger.
Ruben elaborated, “The drive to get to safety is really strong. What was interesting for us is how the mice behaved in the first few seconds of receiving the simulated threat.” explaining, “We zoomed the camera in on each mouse’s head and slowed the video down. When the simulated threat was perceived by the mice, their first action was to turn towards the shelter. The head always moved through the shortest angle, so the mice weren’t searching for the direction of their shelter, they knew exactly in which direction it was.”
Navigating to safety
Mice aren’t known for their good eyesight. But were the mice using directional clues from the maze, such as the labels on the maze walls or smells to orientate themselves towards the shelter, ready to flee? Or, were they integrating information about the path they had taken away from the shelter?
To test this, Ruben rotated the outer ring and walls, moving the shelter, when the mice entered the center portion of the maze. When an ultrasound was given the mice invariably ran towards the original shelter site.
Mice navigate to safety by path integration. Mice explore the Barnes Maze and respond to an ultrasound stimulus by running for their shelter (blue 'S'), or where their shelter used to be. Credit: Ruben Vale, Sainsbury Wellcome Center for Neural Circuits and Behavior, UCL.
Tiago explained, “This evidence suggests mice assimilate information about their path as they explore and use this information to direct their flight when they are scared.” adding, “The speed and certainty with which the animal headed towards the nest also depended on how long it had spent in the shelter prior to the stimulus, those that spent longer in the shelter had a faster reaction time than those that had spent less time in the shelter.”
Finally, to prove that mice learn where safety is and direct their escape behavior accordingly, the scientists blocked the shelter hole. The exploring animal visited the site of the blocked shelter hole but had no access to it. When the mouse was exposed to the threatening stimulus it froze rather than tried to escape, suggesting it had updated its memory about the environment and changed its survival strategy. There was no shelter to escape to, so the best course of action was to freeze.
“Now we want to investigate the brain circuitry underlying these escape behaviors. Where is the memory of the shelter formed in the brain? How does the circuitry compute and adapt to changes in the environment? And how do we decide which is the best course of action to take to maximize our chances of survival?” said Tiago, adding, “Understanding decision-making processes in general, especially in stressful situations, could help with understanding maladaptive decision making during depression, for example.”
Vale, R., Evans, D. and Branco, T. (2017). Rapid Spatial Learning Controls Instinctive Defensive Behavior in Mice. Current Biology.