After reading the article, I get the impression that they were given very low doses of dopamine a single time - which would not be expected to lead to dyskinesias under any circumstance. It seemed to me that they were trying to investigate the effects of natural levels of dopamine, and would therefore have stayed within the typical range of dopamine levels. As people age, they naturally produce lower levels of dopamine. This would explain why in the study they use elderly people. They could compare lower levels of dopamine to higher levels of dopamine while still remaining in the typical range dopamine levels in the average person.