Scientists are taking a new look at hallucinogens, which became taboo among regulators after enthusiasts like Timothy Leary promoted them in the 1960s with the slogan Turn on, tune in, drop out. Now, using rigorous protocols and safeguards, scientists have won permission to study once again the drugs potential for treating mental problems and illuminating the nature of consciousness. Researchers from around the world are gathering this week in San Jose, Calif., for the largest conference on psychedelic science held in the United States in four decades. They plan to discuss studies of psilocybin and other psychedelics for treating depression in cancer patients, obsessive-compulsive disorder, end-of-life anxiety, post-traumatic stress disorder and addiction to drugs or alcohol. Scientists are especially intrigued by the similarities between hallucinogenic experiences and the life-changing revelations reported throughout history by religious mystics and those who meditate. These similarities have been identified in neural imaging studies conducted by Swiss researchers and in experiments led by Roland Griffiths, a professor of behavioral biology at Johns Hopkins. In one of Dr. Griffithss first studies, involving 36 people with no serious physical or emotional problems, he and colleagues found that psilocybin could induce what the experimental subjects described as a profound spiritual experience with lasting positive effects for most of them.