MIT scientists, including one of Indian-origin, have identified two populations of neurons in the amygdala that process positive and negative emotions.
These neurons relay the information to other brain regions that initiate the appropriate behavioural response, said neuroscientists from Massachusetts Institute of Technology's Picower Institute for Learning and Memory.
"How do we tell if something is good or bad? Even though that seems like a very simple question, we really don't know how that process works," said senior study author Kay Tye, the Whitehead Career Development Assistant Professor in the Department of Brain and Cognitive Sciences.
"This study tells us that streams of information are hard-wired and are separated into good and bad at the level of the amygdala," Tye added.
The findings could also help scientists to better understand how mental illnesses such as depression arise, she said.
The neurons of the basolateral amygdala are intermingled, making it difficult to distinguish which populations might be involved in different functions.
Tye and colleagues suspected they might be able to distinguish populations of neurons that respond to different emotions based on their targets elsewhere in the brain.
Previous studies had suggested that some of these neurons project to the nucleus accumbens, which plays a role in reward learning, while others send information to another part of the amygdala known as the centromedial amygdala.
To identify these populations, the researchers delivered green and red fluorescent microspheres called retrobeads to the target cells in the nucleus accumbens and centromedial amygdala, respectively.
These spheres travelled backwards until they reached the neurons of the basolateral amygdala, clearly marking two distinct populations.
After labelling these neurons, the researchers analysed amygdala activity as the mice learned either a fear-conditioning task or a reward task.
In the fear-conditioning task, the mice learned to associate a tone with a foot shock, and in the reward task the tone was paired with a drink of sugary water.
The next day, the researchers measured the strength of the connections coming into the two populations, which carry sensory information to the amygdala.
They found that basolateral amygdala neurons that connect to the nucleus accumbens receive stronger input after reward learning, but their inputs are weakened after fear learning. Neurons that connect to the centromedial amygdala show the opposite response.
The results suggest that these two populations essentially function as a gate for sensory information coming into the amygdala, said Graduate student Praneeth Namburi, paper's co-lead author.
The researchers then found that by shutting down the pathway to the fear circuit, they not only impaired fear learning, but also enhanced reward learning.