For centuries, people have known that memory can be enhanced when attached to emotion. Previous research has also shown that the amygdala—almond-shaped structures located deep in the brain—facilitate the effects of emotion on various types of memory. These include learning processes that involve the nearby striatum, but little was known about exactly how this process worked.
Now a recent study by Andrei Popescu, a graduate student at the Center for Molecular & Behavioral Neuroscience at Rutgers State University, and colleagues has shown that the amygdala and the striatum sync with each other during learning, suggesting a possible mechanism of interaction between the two structures.
To parse out this phenomenon, the researchers measured local field potentials, a type of neural electromagnetic activity, in six cats. Targeted specifically were the striatum and the basolateral complex of the amygdala (BLA), which sends input into the ventral striatum. The scientists found coherence in the gamma frequency range—35 to 45 Hz—that is, when the amplitude of the gamma oscillations in the striatum increased, it also increased in the BLA. When the team measured the activity of random individual neurons in the two regions, they discovered that during increases in the power or “loudness” of the gamma frequencies relative to other frequencies, the two structures became coupled: Neurons from the BLA and neurons from the striatum began firing together.
“We wanted to see if we could find direct evidence of communication between neurons in the amygdala and those of the striatum,” Popescu says. “In a way, we now have a signature of this interaction and we know that it happens in the gamma frequency range.”
The researchers then looked at how this communication changed during learning. The cats were presented with two different tones, one that was associated with a food reward, thought to impart an emotional component to the experiment, and one that wasn’t. As the cats learned to distinguish between the two tones, researchers saw an increase in the amygdala-striatal coherence only when the tone that was paired with food appeared. The coherence increased with learning, reaching a maximum when the cats had mastered the difference between the two tones.
“This suggests that you need this interaction between the amygdala and the striatum for learning these new types of behavior,” Popescu says. “Since we know that the BLA sends projections to the striatum but not the other way round, we might say that the amygdala brings an emotional component to the information processing that goes on in the striatum”
“It’s the first step in putting a quite complicated puzzle together,” says C. Daniel Salzman, a Columbia University neuroscientist who was not involved in the study but who co-authored a positive commentary on the results.
But, he adds, there are still lots of questions to ask. “This was a fairly simple task that these animals were performing,” he says. “Obviously, tasks get much more complicated in real life. [These results] highlight the importance of trying to do this kind of study in more complex circumstances to find out if this mechanism is more broadly applicable.”
The study appeared online on May 10 and in the June 2009 issue of Nature Neuroscience. Salzman’s review, co-authored with Brian Lau, appears in the same issue.