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Friday, May 20, 2011

"*What* is my motivation?" [Full link to blog for email clients.][FT:C44]

I will tackle the subject of motivation in two parts:  Today's blog is about the brain structures which are active to provide information which is utilized as motivation.  The second blog will discuss the psychology of motivation as it affects behavior.

The primary motivation for any behavior is "what do I gain?"  No big surprise, but there is a series of brain areas that form a circuit to assess risk, reward, gain, and the value of such gain.  We briefly touched on this in yesterday's discussion of amygdala and cingulate cortex.  However, the "brain reward circuit" as we will discuss today is much more complex.  The diagram above right by Prof. George Koob shows the circuit as a network diagram.  Another view is superimposed on the brain below left. 

Right in the center of the diagram is the "Nucleus accumbens" part of the striatum (caudate/putamen) that surrounds the thalamus at the base of the brain. Amygdala is a major component, as is hippocampus.  BNST refers to the "Bed Nuclei of the Stria Terminalis"  Stria terminalis connects the amygdala to thalamic and basal ganglia structures.  The "Bed Nuclei" simply refer to the nuclei of origination for the structure.  BNST, nucleus accumbens and ventral tegmental area (VTA) are part of the basal ganglia as described in a previous blog. 

We can see that the circuit includes hippocampus, cingulate cortex and prefrontal cortex, implying a role for memory and executive decision-making.  Orbitofrontal cortex is the particular Frontal Lobe area that matches decision-making with expectation. 

The reward circuit thus serves two main functions - prediction of expected outcomes or "Reward", and the relative value of that reward in terms of the behavior, or "Risk."  In this manner, the "reward" circuit is more appropriately termed a "Risk-Reward" circuit that predicts the payoff for a particular action in light of what kind or how much effort is required to gain that reward.  Whether the effort is "worth it" is a matter of motivation.

In animal experimentation, it is common to offer the animal one of two "payoffs" for behavior - press lever A and get fruit juice, press lever B and get a fruit-flavored food pellet.  The risk is low, reward value typically depends on whether the animal is hungry or thirsty.  "Risk" comes in if the animal has to perform an action that might involve pain (crossing over a high narrow platform with no walls, or a floor that shocks the feet), while reward value can be conveyed by sensory information (replace the food pellet with cocaine injection, and a very thirsty rat may still choose drug). 

Nucleus accumbens is involved in prediction, VTA in evaluating probability and surprise events, hippocmapus in memory of previous choices, amygdala in balancing the emotional content and past vs. future predictions, cingulate in decision between alternatives, and the prefrontal and orbitofrontal cortex provide the decision as well as updating memory and comparing the prediction to the actual outcome.  The complete circuit is involved in providing the *motivation* for actions in which a risk-reward assessment can be made. 

Such risk-reward assessments also include the judgment of whether the reward is worth the effort to be expended.  In the case of the rat at right, navigating the maze just was too much work to justify the reward gained by the cheese at the end, and an alternate pathway was found.

But all of that borders on the psychology of motivation, so we will table that until the next blog! 

Stay tuned, and don't let the LabRats steal your cheese!

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