Over the past 25 years, Jack Fletcher at the University of Texas in Houston and his colleagues have been visiting Costa Rica to test the mental skills of very heavy users.

• 在过去的25年里，休斯敦德克萨斯大学的杰克·弗莱彻和他的同事访问过哥斯达黎加，测试超大剂量吸食者的智力。