University of Washington researchers have located neurons behind one of the most plaguing questions of all time- how people and animals learn from experience.
By using a new imaging technique called Arc catFISH, the researchers have visualized individual neurons in the amygdalas of rat brains, which gets activated after giving the animals an associative learning task.
Ilene Bernstein, senior author of a new study and a UW professor of psychology said that associative, or Pavlovian, conditioning is a fundamental form of learning throughout the animal kingdom and is a widely researched model for studying plasticity, or how the circuits in the brain can change as a result of experience.
For the study, the researchers directly observed the convergent neurons where learning is suspected of taking place. These neurons responded to both a conditioned stimulus, in this case a novel saccharine solution, and an unconditioned stimulus, in the form of lithium chloride that made rats sick.
According to Bernstein, convergent activation is considered a key event for subsequent plasticity. However, till date, there hasn't been much of direct evidence of this activation during learning in the mammalian brain.
Using the new imaging technique, the researchers were able to visualize convergent activation that took place over a 30-minute time span. For this, they subjected animals to conditioned taste aversion training. Taste aversions have evolved in many animals to help them avoid toxic substances.
In the study thirsty rats were allowed to drink the saccharine solution for five minutes. After 25 minutes they were injected with lithium chloride, which caused nausea, and then five minutes later they were killed. Slices of the animals' brain were examined under a microscope.
Via imaging technique, researchers could see that some neurons were activated by the saccharine, or the conditioned stimulus, and the lithium chloride or the unconditioned stimulus activated others. In addition, a small number of neurons were activated by both stimuli.
"We believe that within any given learning trial the number of neurons activated by both conditioned and unconditioned stimuli is likely to be very sparse. In the area we looked at only about 4 percent of about 300 neurons show this response," said Bernstein.
For a follow-up experiment, the researchers reversed the order of the stimuli - giving the injection first and the saccharine later. Under these conditions and although animals were exposed to identical stimuli, convergent neurons were not activated.
The researchers also proposed a model that associative learning takes place when a conditioned stimulus is followed by an unconditioned stimulus, triggering convergent neurons.
In order to explore the model further, the researchers plan to use the imaging technique and fear learning.
The findings will appear online in the Early Edition of the Proceedings of the National Academy of Sciences.