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.
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