A study was conducted on a group of 70 people from Cornell. They were given personality tests that determined whether they were extroverted or introverted. During the study, some participants took Ritalin and viewed videos in the lab.
Following this, the participants took a test to find how they reacted to the videos and environment in the lab with the rush of dopamine produced by Ritalin. The results showed that Ritalin translated to reward or motivation for extroverts, while there was no discernible association amongst introverts.
This simply was because introverts did not relate to their environment with reward while extroverts did. Researchers feel this could be because brains of introverts evaluate internal cues more heavily as compared to motivational and reward cues.
Charles Carver, a psychologist at the University of Miami, said, "The findings help link the personality trait of extraversion to a particular set of processes in the nervous system.The effect obtained was a very subtle one, because it turned on the involuntary occurrence of conditioning among some people but not others."