 Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.In his interactions with scientists and other veterinarians, Rollin was regularly asked to prove animals are conscious and provide scientifically acceptable grounds for claiming they feel pain.Academic reviews of the topic are equivocal, noting that the argument that animals have at least simple conscious thoughts and feelings has strong support, but some critics continue to question how reliably animal mental states can be determined.