A new neural mechanism for learning

Synapse 19: New research on how learning occurs in the brain

If you've been following the past few weeks of Synapse, you know that we've been on a journey learning about all of the different factors that affect our behavior in fascinating ways. One of those factors is, of course, the neuron connections in our brain. If you've heard of the adage "neurons that fire together, wire together" then you're familiar with the high-level theory that, in the brain, learning occurs by the strengthening of connections between neurons that are firing at the same time.

Except when it doesn't. 

New research out of Stony Brook has provided, for the first time, evidence of a new neural mechanism for learning that could have important implications in research attempting to help people break addictions to harmful substances. 

To study this new mechanism for learning, the researchers used a taste aversion test in which they fed rats sugar water and then immediately induced a stomach ache. The next day, they presented two bottles of water, one with sugar and one without. Despite preferring the sugar the day before, the rats induced with a stomach ache avoided the sugar water when compared to the control. If you've ever gotten sick after eating a particular meal and repulsively avoided it from then on, you know how powerful taste aversion learning can be. 

It has been known for years that taste aversion learning involves both the basolateral amygdala (a "threat center" in the brain) and the gustatory cortex. Based on our general understanding of learning, we might expect that the neural mechanism for this learning occurs by the strengthening of the connections between our "threat center" and the "taste center" thus teaching the rat to avoid the threat of the sugar water. 

However, the researchers surprisingly found the exact opposite. They found that after the stomach ache was induced, there was actually reduced synaptic activity between these two brain regions. That is, there were less powerful signals sent from the amygdala fear center to the taste center gustatory cortex. 

Even further, the researchers used optogenetics—a powerful technique in which researchers can manipulate the firing capacity of neurons using light—to manipulate the connection between the threat center and taste center. Remarkably, depressing the signal from the threat center to the taste center while the rats drank sugar water was sufficient for taste aversion the next day. 

What's so interesting about this finding is that it shows a case of an associative memory that was formed not by strengthening the connection between two brain areas, but actually weakening it. From one of the researchers Melissa Haley: 

"The findings also suggest reducing the activity between two brain centers can be an important way in which animals learn other behaviors, which adds to our understanding of the processes that enable the formation of memories and lead to changes in behavior."

Modulating learned behaviors that are harmful, such as drug abuse, is an important clinical goal and this research sheds light on a new strategy for this field. 

🔗 Links

🧠 'Is Anybody in There?' Life on the Inside as a Locked-In Patient

This story from the guardian is a really interesting case study of the experience of Jake Haendel who suffered from “locked-in syndrome” following brain damage from heroin abuse.

“I could do nothing except listen and I could only see the direct area in front of me, based on how the staff would position me in bed,” Jake later wrote

💉 The AstraZeneca Covid Vaccine Data Isn't Up to Snuff

While there has been a lot of good COVID-19 vaccine news from Pfizer and Moderna, this Wired article does a good job outlining the shaky science underlying the AstraZeneca vaccine data that we have so far. While the phase-3 clinical trials put forth by Pfizer and Moderna have been exemplary as a case-study in how to design an effective vaccine trial during a pandemic, the AstraZeneca trial has been anything but.

🧬 Are RNA vaccines safe?

An excellent Twitter thread on the safety profile of a vaccine based on RNA by immunologist Shane Crotty. It’s accessible to everyone.

🐦 Tweet of the week

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