http://teddysratlab.blogspot.com [Full link to blog for email clients.][FT:C44]
In the previous blog, I discussed the brain areas that process motivation, and mentioned that they operate by assessing risk, reward, prediction and outcome. So how does that work?
First, neurons in the striatum (caudate, nucleus accumbens - NA, ventral tegmental area - VTA) fire at different rates when an subject performs different tasks for relative types of reward. Water/juice reward might invoke only a small response in a hungry subject, while generating a large response in a thirsty subject. Neurons in hippocampus encode information into, and recall from, memory regarding prior actions, memory of the task, and memory of the conditions leading to the current reward value. Amygdala, cingulate and orbitofrontal cortex are involved in prediction and matching that prediction to the real outcome. Neurons in VTA and substantia nigra (SN) are activated by "surprise" when the outcome does *not* match prediction, and this all feeds back into hippocampus, Parietal Lobe and Temporal Lobe to become the next iteration of task memory.
Next we will examine how reward *value* gets into the game, but it involves some of the psychological measurements that are commonly performed in lab test animals. However, modern noninvasive imaging techniques have shown that similar processes exist in humans, even though we do not and can not test in the same way.
But first, a slight side track on motivation...
Commonly abused drugs such as cocaine short-cut the whole process of reward assessment. Many of the neurons in the reward circuit either produce dopamine (VTA, NA, SN) or have receptors for dopamine inputs (amygdala, hippocampus, neocortex). A simple explanation of the action of many *stimulant* drugs (cocaine, methamphetamine, etc.) is that they directly affect dopamine neurotransmission by *prolonging* it - by blocking metabolism or reuptake back into the neurons to break it down. We now know that there is more to the mechanism, but suffice it to say that by acting directly on the neurons, instead of through the normal sensory inputs, drugs "take over" the reward pathway and produce the maximum *value* reward compared to any other outcome.
"Value" of a reward is relative, of course. Water has low value to a hungry subject, but high value to a thirsty subject. Fear of shock, pain or anxiety can have a higher value than thirst, until the subject gets thirsty enough. In psychology studies, researchers often look for whether the *work* or effort a subject puts forth is greater or less than the reward. A mouse may run a maze for cheese, but not water. A lab rat may press a lever 20 times for water, but not 21 times; may press only 10 times for food, but over 100 times for cocaine. Likewise a rat may risk stepping onto a grid that produces a mild shock to the feet in order to escape a brightly lit chamber and retreat to a dark, covered corner.
An interesting task given to human subjects is called the "Gambling Task" (or "IOWA Gambling Task" since it was developed at the Univ. of Iowa). Subjects can draw a card from any one of four decks. From any deck, they may draw a win or loss - two decks have small wins, but also small losses - the other decks have large wins, but even larger losses. The strategy that a subject uses to choose cards - slow and steady win, big win / big loss, or a mix - tells a lot about how that person assesses risk and reward. It is telling that persons who are big risk-takers - as well as drug abusers - tend to go for the chance of a big win, even though the big loss is even more likely. They also tend to choices that are impulsive, rather than informed by history and calculation of likelihood of reward.
Sop motivation is based on many things - the sensory information that yields information about reward value, predcition / expectation of outcome and the historical confirmation or denial of that expectation, the risk-reward assessment, and directly manipulation of the neurons that serve the motivational circuit. I don't want to downplay the role of informed decision-making and executive function, but that is material for the next few blogs on identity, personality, and thought.
So until next time, take care of your brain, and don't let it get "psyched out."