Hopkins Psychological and Brain Sciences professors Michela Gallagher and Peter Holland, along with Michael Saddoris of the University of North Carolina, have been investigating how learned cues associated with tastes can activate the nerves that encode a particular taste.
It is thought that cues associated with emotional outcomes that are both conspicuous and reinforcing can begin to act as a substitute for the actual reinforcer. Thus, a cue will predict the resulting outcome by creating a representation of it. This occurs when cells that are responsible for encoding the features of the initial reinforcer are reactivated.
Gallagher, Holland and Saddoris tested their hypothesis by investigating the role of neural arrangements in the gustatory cortex (GC) which is the region of the brain responsible not only for detecting taste but for learning and remembering smells, flavors and other new experiences associated with food.
However, little is known about how the GC actually responds to stimuli other than tastes that the animal has learned to associate with food.
The researchers looked at what occurred in the cortex of rats when receiving sucrose and water, as well as what cues were associated with such stimuli, using immediate early genes (IEGs) which respond quickly to stimuli and are responsible for activating other genes.
The researchers chose to look at the IEGs known as Arc and Homer1a, known for their plasticity, or the ability to be malleable and adapt to change.
An added perk in using these genes for research is that they are expressed in the neuronal nucleus within five to thirty minutes after the conclusion of a salient event. Thus, it can be seen how individual neurons encode the stimuli into separate behavioral occurrences.
Gallagher, Holland and Saddoris embarked on a series of experiments. The first consisted of feeding rats identical sucrose solutions which increased the activity of immediate early genes. Repeat feedings of the same sucrose solution activated overlapping regions of the GC. In comparison, rats fed tasteless water did not have increased activity of IEGs.
In the second experiment, the rats were given sucrose and exposed to a strong odor at the same time until they learned to associate the odor with the taste of sucrose.
When the rats were then exposed to only the odor, without sucrose, the activity of early immediate genes in the gustatory cortex was similar to the activity when the rats were exposed to sucrose.
The outcome of these experiments demonstrated that neural arrangements activated by the odors associated with sucrose were also activated solely by sucrose. That is, because the two were always paired together, the rat's brains learned to react in the same way to both the odor paired with sucrose, and the sucrose itself.
Overlap between the initial taste activity (the first experiment) and the associative taste activity (the latter experiment) displays the possibility that neurons in the gustatory cortex encode information about expected outcomes.
The researchers were also surprised to discover that injuries in the brain had almost no effect on the ability of the GC neurons to associate the odor cue with a taste.
