Derrick L. Hassert, Ph. D.

Hello, my name is Derrick Hassert—welcome to my web page. I work as a postdoctoral fellow in the Behavioral Neuroscience Lab run by Dr. Cedric Williams at the University of Virginia.

My research interests center around the neurobiology of learning and memory and the role of emotion in learning processes. Currently I am involved in research examining the effects of vagus nerve stimulation on neurochemical alterations within limbic system structures, such as the amygdala and hippocampus, known to be involved in the acquisition and consolidation of learned information.When you perceive potentially dangerous or disturbing stimuli (such as when we see a snake or hear a scream), the brain activates descending pathways that prepare the body for activity. You may want to run away from the danger, or perhaps you will decide to run to someone’s aid. In either case, the body must be ready and prepared to act. You have probably experienced stressful situations like those described above. Your muscles tightened and your heart started beating faster. In such a situation, you feel (literally) the danger or anxiety provoked by the events in the environment. The brain will remember, for future reference, the events surrounding the stressful event. Information going from your heart, stomach, and lungs unite in an area called the nodose gangion. From there the information travels through a thick nerve—the vagus nerve—to the brain. Within the brain, pathways and structures involved in learning (like the amygdala, the hippocampus, and the cortex) are activated to encode the feelings associated with the important emotional event. In this manner, the visceral response helps organisms to remember what is significant.

Medial Section of a Rat Brain

As humans, we behave according to the cognitive and emotional information we are able to remember. We stay away from certain things because we have learned to do so by prior experience. Sometimes the information we recall is completely cognitive or propositional (I need to go to the doctor today at noon). In other instances we might not be able to verbalize information, yet we feel uneasy in certain environments or in the presence of specific stimuli. The frontal lobe, especially the orbital frontal cortex, is extremely important for human behavior because it is linked to structures that encode the emotional “value” of people, places, events, and things. It is an area that allows us to feel emotions based on past experience. Imagine if your ability to feel and express sadness, anger, fear, or shame at the appropriate time were taken away from you. You might laugh at funerals, get angry at inappropriate times, and make offense jokes in social setting where they aren’t welcome. This is what happens when people receive damage to their orbital frontal cortex—their personalities deteriorate. This is what happened to Phineas Gage, a railway foreman who had a steel rod sent flying through his skull in a bizarre accident in 1848. Gage lived through the event, even joking with the doctor who treated his wounds, but his personality was forever changed. In the words of one of his physicians, “Gage was no longer Gage.”

Animals have a frontal cortex as well, and when you damage it the animal often fails to learn the changing value of a stimulus. They, like their human counterparts, have a difficult time adjusting to the changing significance of the environment. Reward a rat with a food pellet for pressing a lever and it will learn to press the lever more often. If you discontinue the reward, a normal animal will learn not to press the lever as frequently. However, an animal with damage to the orbital frontal cortex will continue to press the lever as though it were still being rewarded. The rat doesn’t see that the value of the stimulus has changed. Another line of research I’m involved in focuses on training rats to approach a goalbox within a straight alley for a specific amount of a reward and then reducing the reward (please see the straight alley page for more details). Normal rats will exhibit what is often called a “frustration effect.” They will slow their approach when the reward is reduced. However, rats with frontal cortex damage (or inactivation through the infusion of certain drugs) might not be able to change their behavior—we’re in the process of looking at this question now.

Cedric Williams
The University of Virginia
Psychology Department

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