Then, one day later, the rats were shown the same image and treated with an electric shock until they learned to connect the image with pain.
Finally, the team tested if the rats would freeze in fear in response to the sound.
The unpaired group didnt.
The rats in the paired group didit turned out human-like complex emotional models were present in rats as well.Once Johansen and Gu confirmed the capacity was there, they got busy figuring out how it worked exactly.Behaviorally, we measured freezing responses to the directly paired stimulus, which was the image, and inferred stimulus which was the sound, Johansen says.
But we also performed something we called miniscope calcium imaging.
The trick relied on injecting rats with a virus that forced their cells to produce proteins that fluoresce in response to increased levels of calcium in the cells.
Increased levels of calcium are the telltale sign of activity in neurons, meaning the team could see in real time which neurons in rats brains lit up during the experiments.It turned out that the region crucial for building these complex emotional models was not the amygdala, but the dorsomedial prefrontal cortex (dmPFC), which had a rather specialized role.
The dmPFC does not form the sensory model of the world.
It only cares about things when they have emotional relevance, Johansen explains.
He said there wasnt much change in neuronal activity during the sensory learning phase, when the animals were watching the image and listening to the sound.
The neurons became significantly more active when the rats received the electric shock.In the unpaired group, the active neurons that held the representations of the electric shock and the image started to overlap.
In the paired group, this overlap also included the neuronal representation of the sound.
There was a kind of an associative bundle that formed, Johansen says.