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。