This emphasis on instinct put ethologists at odds with many American psychologists, who were primarily interested in learning. Using rewards and punishments experimental psychologists could drastically modify behavior. Rats and pigeons could be trained to push buttons or pull levers to receive food or avoid electrical shocks. Impressed with these results, some psychologists believed that all behavior was learned in this way. They proposed that at birth the animal's brain was like blank slate. Through experience, both painful (punishments) and pleasant (rewards), the animal learned to behave in appropriate ways.