Motivation in rats is influenced not only by their most recent experiences, but also by a series of recent experiences, according to a recent study by a team of Harvard biology researchers.
The study, published last April in the journal Nature Neuroscience, tested how quickly rats decided to start pursuing a predetermined reward. Researchers found that the amount of time between receiving a reward and restarting the reward-seeking process depended on the size of previously-received rewards.
“People focus too much on choices,” said molecular and cellular biology professor Naoshige Uchida, adding that his study examines instead the eagerness of a rat to make those decisions, rather than the decisions themselves.
To receive a reward, a rat first had to push a button with its nose, releasing a sweet or sour odor. A sweet odor indicated that a variable amount of water would be dispensed from a hole to one side of the rat, while a sour odor meant the water would be distributed from the other side. After the rat collected the water, the researchers measured the amount of time before the rat returned to deliver another nose poke to restart the process.
As was expected, rats who had received larger awards were more eager to restart the experiment. But interestingly, the researchers also found that the speed with which the rats returned to deliver a nose poke depended on not just the most recent reward, but on the last five rewards.
After experimenting with the rats for six weeks, the researchers applied lesions to different portions of some rats’ brains, after which the scientists found that the speed with which rats returned to restart the experiment became dependent much more on the previous reward.
The area researchers lesioned is the striatum, a part of the brain that interacts significantly with the hormone dopamine. Dysfunction of the dopamine pathway is strongly associated with Parkinson’s disease, a fatal degenerative disease that causes a breakdown of motor skills.
“Parkinson’s disease is often considered as a motor-deficient [disease] but...the new idea is that Parkinson’s disease patients may not be purely motor-deficient but more cognitive-deficient” Uchida said.
Uchida explained in an interview with the Harvard Gazette that it is possible that Parkinson’s disease may, in part, be caused by a lack of willingness to control motor function. This study, he said, could shed light on the regions of the brain controlling that motivation.
Alice Y. Wang, a research fellow at HMS and one of the authors of the study, said that she is interested in what the research indicates more generally about motivation.
“It speaks to the importance of our overall state—for our happiness in general” Wang said. “If you averaged everything in our life together, that’s what really motivates us.”