Their results supported the idea that the rats' ability to remember a learned response was influenced by their state.[4] The phenomenon continued to be studied more than thirty years later. In 2004, Mohammad-Reza Zarrindast and Ameneh Rezayof studied mice, to see how memory and learning were affected by morphine. They found that when mice learned a response under the influence of morphine, they later performed it most efficiently under the influence of morphine.