In a study of Parkinson's disease, 100 volunteers had incisions made through their skulls. The patients were randomly sorted into two groups, one of which had a new drug inserted into the brain. In the other group, the skulls were closed with no treatment given. The patients did not know who received the drug. In the weeks to follow, all 100 volunteers showed similar D improvement in physical function. What may have occurred that caused those who did not receive a drug to improve in physical function?